Skip Headers
Oracle® Streams Concepts and Administration
11g Release 1 (11.1)

B28321-03
Go to Documentation Home
Home
Go to Book List
Book List
Go to Table of Contents
Contents
Go to Index
Index
Go to Master Index
Master Index
Go to Feedback page
Contact Us

Go to previous page
Previous
Go to next page
Next
PDF · Mobi · ePub

2 Oracle Streams Information Capture

Capturing information with Oracle Streams means creating a message that contains the information and enqueuing the message into a queue. The captured information can describe a database change, or it can be any other type of information.

The following topics contain conceptual information about capturing information with Oracle Streams:

See Also:

Ways to Capture Information with Oracle Streams

There are two ways to capture information with Oracle Streams: implicit capture and explicit capture.

Implicit Capture

With implicit capture, data definition language (DDL) and data manipulation language (DML) changes are captured automatically either by a capture process or by synchronous capture. A specific type of message called logical change record (LCR) describes these database changes. Both a capture process and synchronous capture can filter database changes with user-defined rules. Therefore, only changes to specified objects are captured.

The following topics describe capture processes and synchronous captures:

Capture Processes

A capture process retrieves change data from the redo log, either by mining the online redo log or, if necessary, by mining archived log files. After retrieving the data, the capture process formats it into an LCR and enqueues it for further processing.

A capture process enqueues information about database changes in the form of messages containing LCRs. A message containing an LCR that was originally captured and enqueued by a capture process is called a captured LCR. A capture process always enqueues messages into a buffered queue. A buffered queue is the portion of a queue that uses the Oracle Streams pool to store messages in memory and a queue table to store messages that have spilled from memory.

A capture process is useful in the following situations:

  • When you want to capture changes to a relatively large number of tables

  • When you want to capture changes to schemas or to an entire database

  • When you want to capture DDL changes

  • When you want to capture changes at a database other than the source database using downstream capture

Synchronous Captures

Synchronous capture uses an internal mechanism to capture DML changes immediately after they happen. Synchronous capture enqueues information about DML changes in the form of messages containing row LCRs. Synchronous capture enqueues these LCRs into a persistent queue. Synchronous capture always enqueues messages into a persistent queue. A persistent queue is the portion of a queue that only stores messages on hard disk in a queue table, not in memory. The messages captured by a synchronous capture are persistent LCRs.

Synchronous capture is useful in the following situations:

  • For the best performance, when you want to capture DML changes to a relatively small number of tables

  • When you want to capture DML changes to a table immediately after these changes are made

Explicit Capture

With explicit capture, applications generate messages and enqueue them. These messages can be formatted as LCRs, or they can be formatted into different types of messages for consumption by other applications. Messages can also be enqueued explicitly by anapply process or by an apply handler for an apply process.

Explicit capture is useful in the following situations:

  • When applications generate messages that must be processed by other applications.

  • When you have a heterogeneous replication environment in which an apply process in an Oracle database applies changes that originated at a non-Oracle database. In this case, an application captures LCRs based on the changes at the non-Oracle database, and these LCRs are processed by an apply process at an Oracle database.

See Also:

Types of Information Captured with Oracle Streams

The following types of information can be captured with Oracle Streams:

Logical Change Records (LCRs)

An LCR is a message with a specific format that describes a database change. There are two types of LCRs: row LCRs and DDL LCRs. A capture process, synchronous capture, or an application can capture LCRs.

You can capture the following types of LCRs with Oracle Streams:

  • A captured LCR is an LCR that is captured implicitly by a capture process and enqueued into the buffered queue portion of an ANYDATA queue.

  • A persistent LCR is an LCR that is enqueued into the persistent queue portion of an ANYDATA queue. A persistent LCR can be enqueued in one of the following ways:

    • Captured implicitly by a synchronous capture and enqueued

    • Constructed explicitly by an application and enqueued

    • Dequeued by an apply process and enqueued by the same apply process using the SET_ENQUEUE_DESTINATION procedure in the DBMS_APPLY_ADM package

    There are no differences between persistent LCRs that were enqueued in any of these ways. That is, a persistent LCR that was captured by a synchronous capture is the same as a persistent LCR that was constructed and enqueued by an application. Further, both of these persistent LCRs are the same as a persistent LCR enqueued by an apply process.

  • A buffered LCR is and LCR that is constructed explicitly by an application and enqueued into the buffered queue portion of an ANYDATA queue.

The following sections contain information about LCRs:

See Also:

Row LCRs

A row LCR describes a change to the data in a single row or a change to a single LONG column, LONG RAW column, LOB column, or XMLType stored as CLOB column in a row. The change results from a data manipulation language (DML) statement or a piecewise update to a LOB. For example, a single DML statement can insert or merge multiple rows into a table, can update multiple rows in a table, or can delete multiple rows from a table. Applications can also construct LCRs that are enqueued for further processing.

A single DML statement can produce multiple row LCRs. That is, a capture process creates an LCR for each row that is changed by the DML statement. In addition, an update to a LONG, LONG RAW, LOB, or XMLType stored as CLOB column in a single row can result in more than one row LCR.

Each row LCR is encapsulated in an object of LCR$_ROW_RECORD type and contains the following attributes:

  • source_database_name: The name of the source database where the row change occurred.

  • command_type: The type of DML statement that produced the change, either INSERT, UPDATE, DELETE, LOB ERASE, LOB WRITE, or LOB TRIM.

  • object_owner: The schema name that contains the table with the changed row.

  • object_name: The name of the table that contains the changed row.

  • tag: A raw tag that can be used to track the LCR.

  • transaction_id: The identifier of the transaction in which the DML statement was run.

  • scn: The system change number (SCN) at the time when the change was made.

  • old_values: The old column values related to the change. These are the column values for the row before the DML change. If the type of the DML statement is UPDATE or DELETE, then these old values include some or all of the columns in the changed row before the DML statement. If the type of the DML statement is INSERT, then there are no old values. For UPDATE and DELETE statements, row LCRs created by a capture process can include some or all of the old column values in the row, but row LCRs created by a synchronous capture always contain all of the new column values in the row.

  • new_values: The new column values related to the change. These are the column values for the row after the DML change. If the type of the DML statement is UPDATE or INSERT, then these new values include some or all of the columns in the changed row after the DML statement. If the type of the DML statement is DELETE, then there are no new values. For UPDATE and INSERT statements, row LCRs created by a capture process can include some or all of the new column values in the row, but row LCRs created by a synchronous capture always contain all of the new column values in the row.

A row LCR captured by a capture process or synchronous capture can also contain transaction control statements. These row LCRs contain directives such as COMMIT and ROLLBACK. Such row LCRs are internal and are used by an apply process to maintain transaction consistency between a source database and a destination database.

DDL LCRs

A DDL LCR describes a data definition language (DDL) change. A DDL statement changes the structure of the database. For example, a DDL statement can create, alter, or drop a database object.

Each DDL LCR contains the following information:

  • source_database_name: The name of the source database where the DDL change occurred.

  • command_type: The type of DDL statement that produced the change, for example ALTER TABLE or CREATE INDEX.

  • object_owner: The schema name of the user who owns the database object on which the DDL statement was run.

  • object_name: The name of the database object on which the DDL statement was run.

  • object_type: The type of database object on which the DDL statement was run, for example TABLE or PACKAGE.

  • ddl_text: The text of the DDL statement.

  • logon_user: The logon user, which is the user whose session executed the DDL statement.

  • current_schema: The schema that is used if no schema is specified for an object in the DDL text.

  • base_table_owner: The base table owner. If the DDL statement is dependent on a table, then the base table owner is the owner of the table on which it is dependent.

  • base_table_name: The base table name. If the DDL statement is dependent on a table, then the base table name is the name of the table on which it is dependent.

  • tag: A raw tag that can be used to track the LCR.

  • transaction_id: The identifier of the transaction in which the DDL statement was run.

  • scn: The SCN when the change was made.

Note:

Both row LCRs and DDL LCRs contain the source database name of the database where a change originated. If captured LCRs will be propagated by a propagation or applied by an apply process, then, to avoid propagation and apply problems, Oracle recommends that you do not rename the source database after a capture process has started capturing changes.

See Also:

Extra Information in LCRs

In addition to the information discussed in the previous sections, row LCRs and DDL LCRs optionally can include the following extra information (or LCR attributes):

  • row_id: The rowid of the row changed in a row LCR. This attribute is not included in DDL LCRs or row LCRs for index-organized tables.

  • serial#: The serial number of the session that performed the change captured in the LCR.

  • session#: The identifier of the session that performed the change captured in the LCR.

  • thread#: The thread number of the instance in which the change captured in the LCR was performed. Typically, the thread number is relevant only in an Oracle Real Application Clusters (Oracle RAC) environment.

  • tx_name: The name of the transaction that includes the LCR.

  • username: The name of the current user who performed the change captured in the LCR.

You can use the INCLUDE_EXTRA_ATTRIBUTE procedure in the DBMS_CAPTURE_ADM package to instruct a capture process or synchronous capture to capture one or more extra attributes.

User Messages

Messages that do not contain LCRs are called user messages. User messages can be of any type (except an LCR type). User messages can be created by an application and consumed by an application. For example, a business application might create a user message for each order, and these messages might be processed by another application.

You can capture the following types of user messages with Oracle Streams:

  • A persistent user message is a non-LCR message of a user-defined type that is enqueued into a persistent queue. A persistent user message can be enqueued in one of the following ways:

    • Created explicitly by an application and enqueued

    • Dequeued by an apply process and enqueued by the same apply process using the SET_ENQUEUE_DESTINATION procedure in the DBMS_APPLY_ADM package

    A persistent user message can be enqueued into the persistent queue portion of an ANYDATA queue or a typed queue.

  • A buffered user message is a non-LCR message of a user-defined type that is created explicitly by an application and enqueued into a buffered queue. A buffered user message can be enqueued into the buffered queue portion of an ANYDATA queue or a typed queue.

Note:

Capture processes and synchronous captures never capture user messages.

Summary of Information Capture Options With Oracle Streams

Table 2-1 summarizes the capture options available with Oracle Streams.

Table 2-1 Information Capture Options With Oracle Streams

Capture Type Capture Mechanism Message Types Enqueued Into Use When

Implicit Capture with an Oracle Streams Capture Process

Mining of Redo Log

Captured LCRs

Buffered Queue

You want to capture changes to many tables.

You want to capture changes to schemas or an entire database.

You want to capture DDL changes.

You want to capture changes at a downstream database.

Implicit Capture with Synchronous Capture

Internal Mechanism

Persistent LCRs

Persistent Queue

You want to capture DML changes to a small number of tables.

You want to capture DML changes immediately after they occur.

Explicit Capture by Applications

Manual Message Creation and Enqueue

Buffered LCRs

Persistent LCRs

Buffered User Messages

Persistent User Messages

Buffered Queue or Persistent Queue

You want to capture user messages that will be consumed by applications.

You want to capture LCRs in a heterogeneous replication environment.

You want to construct LCRs by using an application instead of by using a capture process or a synchronous capture.


Note:

A single database can use any combination of the capture options summarized in the table.

Instantiation in an Oracle Streams Environment

An Oracle Streams environment can share a database object within a single database or between multiple databases. In an Oracle Streams environment that shares database objects and uses implicit capture to capture changes to the database object, the source database is the database where the change originated. The source database is one of the following depending on the type of implicit capture used:

After changes are captured, they can be applied locally or propagated to other databases and applied at destination databases.

In an Oracle Streams environment that shares database objects, you must instantiate the shared source database objects before changes to them can be dequeued and processed by an apply process. If a database where changes to the source database objects will be applied is a different database than the source database, then the destination database must have a copy of these database objects.

In Oracle Streams, the following general steps instantiate a database object:

  1. Prepare the object for instantiation at the source database.

  2. If a copy of the object does not exist at the destination database, then create an object physically at the destination database based on an object at the source database. You can use export/import, transportable tablespaces, or RMAN to copy database objects for instantiation. If the database objects already exist at the destination database, then this step is not necessary.

  3. Set the instantiation SCN for the database object at the destination database. An instantiation system change number (SCN) instructs an apply process at the destination database to apply only changes that committed at the source database after the specified SCN.

In some cases, Step 1 and Step 3 are completed automatically. For example, when you add rules for an object to the positive rule set for a capture process by running a procedure in the DBMS_STREAMS_ADM package, the procedure prepares the object for instantiation automatically. Also, when you use export/import or transportable tablespaces to copy database objects from a source database to a destination database, instantiation SCNs can be set for these objects automatically during import. Instantiation is required whenever an apply process dequeues captured LCRs, even if the apply process sends the LCRs to an apply handler that does not execute them.

See Also:

Implicit Capture with an Oracle Streams Capture Process

This section explains the concepts related to the Oracle Streams capture process.

This section contains these topics:

Introduction to Capture Processes

Every Oracle database has a set of two or more redo log files. The redo log files for a database are collectively known as the database redo log. The primary function of the redo log is to record all of the changes made to the database.

Redo logs are used to guarantee recoverability in the event of human error or media failure. A capture process is an optional Oracle background process that scans the database redo log to capture data manipulation language (DML) and data definition language (DDL) changes made to database objects. When a capture process is configured to capture changes from a redo log, the database where the changes were generated is called the source database for the capture process.

When a capture process captures a database change, it converts it into a specific message format called a logical change record (LCR). After capturing an LCR, a capture process enqueues a message containing the LCR into a queue. A capture process is always associated with a single ANYDATA queue, and it enqueues messages into this queue only. For improved performance, captured LCRs always are stored in a buffered queue, which is System Global Area (SGA) memory associated with a queue. You can create multiple queues and associate a different capture process with each queue.

Captured LCRs can be sent to queues in the same database or other databases by propagations. Captured LCRs can also be dequeued by apply processes. In some situations, an optimization enables capture processes to send LCRs to apply processes more efficiently. This optimization is called combined capture and apply.

A capture process can run on its source database or on a remote database. When a capture process runs on its source database, the capture process is a local capture process. When a capture process runs on a remote database, the capture process is a downstream capture process, and the remote database is called the downstream database.

Figure 2-1 shows a capture process capturing LCRs.

Figure 2-1 Capture Process

Description of Figure 2-1 follows
Description of "Figure 2-1 Capture Process"

Note:

  • A capture process can be associated only with an ANYDATA queue, not with a typed queue.

  • A capture process and a synchronous capture should not capture changes made to the same table.

Capture Process Rules

A capture process either captures or discards changes based on rules that you define. Each rule specifies the database objects and types of changes for which the rule evaluates to TRUE. You can place these rules in a positive rule set or negative rule set for the capture process.

If a rule evaluates to TRUE for a change, and the rule is in the positive rule set for a capture process, then the capture process captures the change. If a rule evaluates to TRUE for a change, and the rule is in the negative rule set for a capture process, then the capture process discards the change. If a capture process has both a positive and a negative rule set, then the negative rule set is always evaluated first.

You can specify capture process rules at the following levels:

  • A table rule captures or discards either row changes resulting from DML changes or DDL changes to a particular table. Subset rules are table rules that include a subset of the row changes to a particular table.

  • A schema rule captures or discards either row changes resulting from DML changes or DDL changes to the database objects in a particular schema.

  • A global rule captures or discards either all row changes resulting from DML changes or all DDL changes in the database.

Note:

The capture process does not capture certain types of changes and changes to certain data types in table columns. Also, a capture process never captures changes in the SYS, SYSTEM, or CTXSYS schemas.

Data Types Captured by Capture Processes

When capturing the row changes resulting from DML changes made to tables, a capture process can capture changes made to columns of the following data types:

  • VARCHAR2

  • NVARCHAR2

  • FLOAT

  • NUMBER

  • LONG

  • DATE

  • BINARY_FLOAT

  • BINARY_DOUBLE

  • TIMESTAMP

  • TIMESTAMP WITH TIME ZONE

  • TIMESTAMP WITH LOCAL TIME ZONE

  • INTERVAL YEAR TO MONTH

  • INTERVAL DAY TO SECOND

  • RAW

  • LONG RAW

  • CHAR

  • NCHAR

  • UROWID

  • CLOB with BASICFILE storage

  • NCLOB with BASICFILE storage

  • BLOB with BASICFILE storage

  • XMLType stored as CLOB

Note:

Some of these data types might not be supported by Oracle Streams in earlier releases of Oracle Database. If your Oracle Streams environment includes one or more databases from an earlier release, then ensure that row LCRs do not flow into a database that does not support all of the data types in the row LCRs. See the Oracle Streams documentation for the earlier release for information about supported data types.

Types of DML Changes Captured by Capture Processes

When you specify that DML changes made to certain tables should be captured, a capture process captures the following types of DML changes made to these tables:

  • INSERT

  • UPDATE

  • DELETE

  • MERGE

  • Piecewise updates to LOBs

A capture process converts each MERGE change into an INSERT or UPDATE change. MERGE is not a valid command type in a row LCR.

See Also:

Supplemental Logging in an Oracle Streams Environment

Supplemental logging places additional column data into a redo log whenever an operation is performed. A capture process captures this additional information and places it in LCRs. Supplemental logging is always configured at a source database, regardless of location of the capture process that captures changes to the source database.

Typically, supplemental logging is required in Oracle Streams replication environments. In these environments, an apply process needs the additional information in the LCRs to properly apply DML changes and DDL changes that are replicated from a source database to a destination database. However, supplemental logging can also be required in environments where changes are not applied to database objects directly by an apply process. In such environments, an apply handler can process the changes without applying them to the database objects, and the supplemental information might be needed by the apply handlers.

See Also:

Local Capture and Downstream Capture

You can configure a capture process to run locally on a source database or remotely on a downstream database. A single database can have one or more capture processes that capture local changes and other capture processes that capture changes from a remote source database. That is, you can configure a single database to perform both local capture and downstream capture.

Local Capture

Local capture means that a capture process runs on the source database. Figure 2-1 shows a database using local capture.

The Source Database Performs All Change Capture Actions

If you configure local capture, then the following actions are performed at the source database:

  • The DBMS_CAPTURE_ADM.BUILD procedure is run to extract (or build) the data dictionary to the redo log.

  • Supplemental logging at the source database places additional information in the redo log. This information might be needed when captured changes are applied by an apply process.

  • The first time a capture process is started at the database, Oracle Database uses the extracted data dictionary information in the redo log to create a LogMiner data dictionary, which is separate from the primary data dictionary for the source database. Additional capture processes can use this existing LogMiner data dictionary, or they can create new LogMiner data dictionaries.

  • A capture process scans the redo log for changes using LogMiner.

  • The rules engine evaluates changes based on the rules in one or more of the capture process rule sets.

  • The capture process enqueues changes that satisfy the rules in its rule sets into a local ANYDATA queue.

  • If the captured changes are shared with one or more other databases, then one or more propagations propagate these changes from the source database to the other databases.

  • If database objects at the source database must be instantiated at a destination database, then the objects must be prepared for instantiation, and a mechanism such as an Export utility must be used to make a copy of the database objects.

Advantages of Local Capture

The following are the advantages of using local capture:

  • Configuration and administration of the capture process is simpler than when downstream capture is used. When you use local capture, you do not need to configure redo data copying to a downstream database, and you administer the capture process locally at the database where the captured changes originated.

  • A local capture process can scan changes in the online redo log before the database writes these changes to an archived redo log file. When you use an archived-log downstream capture process, archived redo log files are copied to the downstream database after the source database has finished writing changes to them, and some time is required to copy the redo log files to the downstream database. However, a real-time downstream capture process can capture changes in the online redo log sent from the source database.

  • The amount of data being sent over the network is reduced, because the redo data is not copied to the downstream database. Even if captured LCRs are propagated to other databases, the captured LCRs can be a subset of the total changes made to the database, and only the LCRs that satisfy the rules in the rule sets for a propagation are propagated.

  • Security might be improved because only the source (local) database can access the redo data. For example, if you want to capture changes in the hr schema only, then, when you use local capture, only the source database can access the redo data to enqueue changes to the hr schema into the capture process queue. However, when you use downstream capture, the redo data is copied to the downstream database, and the redo data contains all of the changes made to the database, not just the changes made to the hr schema.

  • Some types of custom rule-based transformations are simpler to configure if the capture process is running at the local source database. For example, if you use local capture, then a custom rule-based transformation can use cached information in a PL/SQL session variable which is populated with data stored at the source database.

  • In an Oracle Streams environment where messages are captured and applied in the same database, it might be simpler, and use fewer resources, to configure local queries and computations that require information about captured changes and the local data.

Downstream Capture

Downstream capture means that a capture process runs on a database other than the source database. The following types of downstream capture configurations are possible: real-time downstream capture and archived-log downstream capture. The downstream_real_time_mine capture process parameter controls whether a downstream capture process performs real-time downstream capture or archived-log downstream capture. A real-time downstream capture process and one or more archived-log downstream capture processes can coexist at a downstream database.

Note:

  • References to "downstream capture processes" in this document apply to both real-time downstream capture processes and archived-log downstream capture processes. This document distinguishes between the two types of downstream capture processes when necessary.

  • A downstream capture process only can capture changes from a single source database. However, multiple downstream capture processes at a single downstream database can capture changes from a single source database or multiple source databases.

  • To configure downstream capture, the source database must be an Oracle Database 10g Release 1 or later database.

Real-Time Downstream Capture

A real-time downstream capture configuration works in the following way:

  • Redo transport services use the log writer process (LGWR) at the source database to send redo data to the downstream database either synchronously or asynchronously. At the same time, the LGWR records redo data in the online redo log at the source database.

  • A remote file server process (RFS) at the downstream database receives the redo data over the network and stores the redo data in the standby redo log.

  • A log switch at the source database causes a log switch at the downstream database, and the ARCHn process at the downstream database archives the current standby redo log file.

  • The real-time downstream capture process captures changes from the standby redo log whenever possible and from the archived standby redo log files whenever necessary. A capture process can capture changes in the archived standby redo log files if it falls behind. When it catches up, it resumes capturing changes from the standby redo log.

Figure 2-2 Real-Time Downstream Capture

Description of Figure 2-2 follows
Description of "Figure 2-2 Real-Time Downstream Capture"

The advantage of real-time downstream capture over archived-log downstream capture is that real-time downstream capture reduces the amount of time required to capture changes made at the source database. The time is reduced because the real-time downstream capture process does not need to wait for the redo log file to be archived before it can capture data from it.

Note:

Only one real-time downstream capture process can exist at a downstream database.
Archived-Log Downstream Capture

An archived-log downstream capture configuration means that archived redo log files from the source database are copied to the downstream database, and the capture process captures changes in these archived redo log files. You can copy the archived redo log files to the downstream database using redo transport services, the DBMS_FILE_TRANSFER package, file transfer protocol (FTP), or some other mechanism.

Figure 2-3 Archived-Log Downstream Capture

Description of Figure 2-3 follows
Description of "Figure 2-3 Archived-Log Downstream Capture"

The advantage of archived-log downstream capture over real-time downstream capture is that archived-log downstream capture allows multiple downstream capture processes at a downstream database. You can copy redo log files from multiple source databases to a single downstream database and configure multiple archived-log downstream capture processes to capture changes in these redo log files.

See Also:

Oracle Data Guard Concepts and Administration for more information about redo transport services
The Downstream Database Performs Most Change Capture Actions

If you configure either real-time or archived-log downstream capture, then the following actions are performed at the downstream database:

  • The first time a downstream capture process is started at the downstream database, Oracle Database uses data dictionary information in the redo data from the source database to create a LogMiner data dictionary at the downstream database. The DBMS_CAPTURE_ADM.BUILD procedure is run at the source database to extract the source data dictionary information to the redo log at the source database. Next, the redo data is copied to the downstream database from the source database. Additional downstream capture processes for the same source database can use this existing LogMiner data dictionary, or they can create new LogMiner data dictionaries. Also, a real-time downstream capture process can share a LogMiner data dictionary with one or more archived-log downstream capture processes.

  • A capture process scans the redo data from the source database for changes using LogMiner.

  • The rules engine evaluates changes based on the rules in one or more of the capture process rule sets.

  • The capture process enqueues changes that satisfy the rules in its rule sets into a local ANYDATA queue. The capture process formats the changes as LCRs.

  • If the captured LCRs are shared with one or more other databases, then one or more propagations propagate these LCRs from the downstream database to the other databases.

In a downstream capture configuration, the following actions are performed at the source database:

  • The DBMS_CAPTURE_ADM.BUILD procedure is run at the source database to extract the data dictionary to the redo log.

  • Supplemental logging at the source database places additional information that might be needed for apply in the redo log.

  • If database objects at the source database must be instantiated at other databases in the environment, then the objects must be prepared for instantiation, and a mechanism such as an Export utility must be used to make a copy of the database objects.

In addition, the redo data must be copied from the computer system running the source database to the computer system running the downstream database. In a real-time downstream capture configuration, redo transport services use LGWR to send redo data to the downstream database. Typically, in an archived-log downstream capture configuration, redo transport services copy the archived redo log files to the downstream database.

See Also:

Chapter 6, "How Rules Are Used in Oracle Streams" for more information about rule sets for Oracle Streams clients and for information about how messages satisfy rule sets
Advantages of Downstream Capture

The following are the advantages of using downstream capture:

  • Capturing changes uses fewer resources at the source database because the downstream database performs most of the required work.

  • If you plan to capture changes originating at multiple source databases, then capture process administration can be simplified by running multiple archived-log downstream capture processes with different source databases at one downstream database. That is, one downstream database can act as the central location for change capture from multiple sources. In such a configuration, one real-time downstream capture process can run at the downstream database in addition to the archived-log downstream capture processes.

  • Copying redo data to one or more downstream databases provides improved protection against data loss. For example, redo log files at the downstream database can be used for recovery of the source database in some situations.

  • The ability to configure at one or more downstream databases multiple capture processes that capture changes from a single source database provides more flexibility and can improve scalability.

Optional Database Link From the Downstream Database to the Source Database

When you create or alter a downstream capture process, you optionally can specify the use of a database link from the downstream database to the source database. This database link must have the same name as the global name of the source database. Such a database link simplifies the creation and administration of a downstream capture process. You specify that a downstream capture process uses a database link by setting the use_database_link parameter to TRUE when you run the CREATE_CAPTURE or ALTER_CAPTURE procedure on the downstream capture process.

When a downstream capture process uses a database link to the source database, the capture process connects to the source database to perform the following administrative actions automatically:

  • In certain situations, runs the DBMS_CAPTURE_ADM.BUILD procedure at the source database to extract the data dictionary at the source database to the redo log when a capture process is created.

  • Prepares source database objects for instantiation.

  • Obtains the first SCN for the downstream capture process if the first system change number (SCN) is not specified during capture process creation. The first SCN is needed to create a capture process.

If a downstream capture process does not use a database link, then you must perform these actions manually.

Note:

During the creation of a downstream capture process, if the first_scn parameter is set to NULL in the CREATE_CAPTURE procedure, then the use_database_link parameter must be set to TRUE. Otherwise, an error is raised.

See Also:

"Configuring a Real-Time Downstream Capture Process" for information about when the DBMS_CAPTURE_ADM.BUILD procedure is run automatically during capture process creation if the downstream capture process uses a database link
Operational Requirements for Downstream Capture

The following are operational requirements for using downstream capture:

  • The source database must be running at least Oracle Database 10g and the downstream capture database must be running the same release of Oracle as the source database or later.

  • The downstream database must be running Oracle Database 10g Release 2 or later to configure real-time downstream capture. In this case, the source database must be running Oracle Database 10g Release 1 or later.

  • The operating system on the source and downstream capture sites must be the same, but the operating system release does not need to be the same. In addition, the downstream sites can use a different directory structure than the source site.

  • The hardware architecture on the source and downstream capture sites must be the same. For example, a downstream capture configuration with a source database on a 32-bit Sun system must have a downstream database that is configured on a 32-bit Sun system. Other hardware elements, such as the number of CPUs, memory size, and storage configuration, can be different between the source and downstream sites.

SCN Values Related to a Capture Process

This section describes system change number (SCN) values that are important for a capture process. You can query the DBA_CAPTURE data dictionary view to display these values for one or more capture processes.

Captured SCN and Applied SCN

The captured SCN is the SCN that corresponds to the most recent change scanned in the redo log by a capture process. The applied SCN for a capture process is the SCN of the most recent message dequeued by the relevant apply processes. All messages lower than this SCN have been dequeued by all apply processes that apply changes captured by the capture process. The applied SCN for a capture process is equivalent to the low-watermark SCN for an apply process that applies changes captured by the capture process.

First SCN and Start SCN

The following sections describe the first SCN and start SCN for a capture process:

First SCN

The first SCN is the lowest SCN in the redo log from which a capture process can capture changes. If you specify a first SCN during capture process creation, then the database must be able to access redo data from the SCN specified and higher.

The DBMS_CAPTURE_ADM.BUILD procedure extracts the source database data dictionary to the redo log. When you create a capture process, you can specify a first SCN that corresponds to this data dictionary build in the redo log. Specifically, the first SCN for the capture process being created can be set to any value returned by the following query:

COLUMN FIRST_CHANGE# HEADING 'First SCN' FORMAT 999999999
COLUMN NAME HEADING 'Log File Name' FORMAT A50

SELECT DISTINCT FIRST_CHANGE#, NAME FROM V$ARCHIVED_LOG
  WHERE DICTIONARY_BEGIN = 'YES';

The value returned for the NAME column is the name of the redo log file that contains the SCN corresponding to the first SCN. This redo log file, and all subsequent redo log files, must be available to the capture process. If this query returns multiple distinct values for FIRST_CHANGE#, then the DBMS_CAPTURE_ADM.BUILD procedure has been run more than once on the source database. In this case, choose the first SCN value that is most appropriate for the capture process you are creating.

In some cases, the DBMS_CAPTURE_ADM.BUILD procedure is run automatically when a capture process is created. When this happens, the first SCN for the capture process corresponds to this data dictionary build.

Start SCN

The start SCN is the SCN from which a capture process begins to capture changes. You can specify a start SCN that is different than the first SCN during capture process creation, or you can alter a capture process to set its start SCN. The start SCN does not need to be modified for normal operation of a capture process. Typically, you reset the start SCN for a capture process if point-in-time recovery must be performed on one of the destination databases that receive changes from the capture process. In these cases, the capture process can be used to capture the changes made at the source database after the point-in-time of the recovery.

Note:

An existing capture process must be stopped before setting its start SCN.
Start SCN Must Be Greater Than or Equal to First SCN

If you specify a start SCN when you create or alter a capture process, then the start SCN specified must be greater than or equal to the first SCN for the capture process. A capture process always scans any unscanned redo log records that have higher SCN values than the first SCN, even if the redo log records have lower SCN values than the start SCN. So, if you specify a start SCN that is greater than the first SCN, then the capture process might scan redo log records for which it cannot capture changes, because these redo log records have a lower SCN than the start SCN.

Scanning redo log records before the start SCN should be avoided if possible because it can take some time. Therefore, Oracle recommends that the difference between the first SCN and start SCN be as small as possible during capture process creation to keep the initial capture process startup time to a minimum.

Caution:

When a capture process is started or restarted, it might need to scan redo log files with a FIRST_CHANGE# value that is lower than start SCN. Removing required redo log files before they are scanned by a capture process causes the capture process to abort. You can query the DBA_CAPTURE data dictionary view to determine the first SCN, start SCN, and required checkpoint SCN. A capture process needs the redo log file that includes the required checkpoint SCN, and all subsequent redo log files.

See Also:

A Start SCN Setting That Is Prior to Preparation for Instantiation

If you want to capture changes to a database object and apply these changes using an apply process, then only changes that occurred after the database object has been prepared for instantiation can be applied. Therefore, if you set the start SCN for a capture process lower than the SCN that corresponds to the time when a database object was prepared for instantiation, then any captured changes to this database object prior to the prepare SCN cannot be applied by an apply process.

This limitation can be important during capture process creation. If a database object was never prepared for instantiation prior to the time of capture process creation, then an apply process cannot apply any captured changes to the object from a time before capture process creation time.

In some cases, database objects might have been prepared for instantiation before a new capture process is created. For example, if you want to create a new capture process for a source database whose changes are already being captured by one or more existing capture processes, then some or all of the database objects might have been prepared for instantiation before the new capture process is created. If you want to capture changes to a certain database object with a new capture process from a time before the new capture process was created, then the following conditions must be met for an apply process to apply these captured changes:

  • The database object must have been prepared for instantiation before the new capture process is created.

  • The start SCN for the new capture process must correspond to a time before the database object was prepared for instantiation.

  • The redo logs for the time corresponding to the specified start SCN must be available. Additional redo logs previous to the start SCN might be required as well.

See Also:

Oracle Streams Capture Processes and RESTRICTED SESSION

When you enable restricted session during system startup by issuing a STARTUP RESTRICT statement, capture processes do not start, even if they were running when the database shut down. When restricted session is disabled with an ALTER SYSTEM statement, each capture process that was running when the database shut down is started.

When restricted session is enabled in a running database by the SQL statement ALTER SYSTEM ENABLE RESTRICTED SESSION clause, it does not affect any running capture processes. These capture processes continue to run and capture changes. If a stopped capture process is started in a restricted session, then the capture process does not actually start until the restricted session is disabled.

Capture Process Components

A capture process is an optional Oracle background process whose process name is CPnn, where nn can include letters and numbers. A capture process captures changes from the redo log by using the infrastructure of LogMiner. Oracle Streams configures LogMiner automatically. The underlying LogMiner process name is MSnn, where nn can include letters and numbers. You can create, alter, start, stop, and drop a capture process, and you can define capture process rules that control which changes a capture process captures.

A capture process consists of the following components:

  • One reader server that reads the redo log and divides the redo log into regions.

  • One or more preparer servers that scan the regions defined by the reader server in parallel and perform prefiltering of changes found in the redo log. Prefiltering involves sending partial information about changes, such as schema and object name for a change, to the rules engine for evaluation, and receiving the results of the evaluation. You can control the number of preparer servers using the parallelism capture process parameter.

  • One builder server that merges redo records from the preparer servers. These redo records either evaluated to TRUE during partial evaluation or partial evaluation was inconclusive for them. The builder server preserves the system change number (SCN) order of these redo records and passes the merged redo records to the capture process.

  • The capture process (CPnn) performs the following actions for each change when it receives merged redo records from the builder server:

    • Formats the change into an LCR

    • If the partial evaluation performed by a preparer server was inconclusive for the change in the LCR, then sends the LCR to the rules engine for full evaluation

    • Receives the results of the full evaluation of the LCR if it was performed

    • Discards the LCR if it satisfies the rules in the negative rule set for the capture process or if it does not satisfy the rules in the positive rule set

    • Enqueues the LCR into the queue associated with the capture process if the LCR satisfies the rules in the positive rule set for the capture process

Each reader server, preparer server, and builder server is a process.

Capture User

Changes are captured in the security domain of the capture user for a capture process. The capture user captures all changes that satisfy the capture process rule sets. In addition, the capture user runs all custom rule-based transformations specified by the rules in these rule sets. The capture user must have the necessary privileges to perform these actions, including EXECUTE privilege on the rule sets used by the capture process, EXECUTE privilege on all custom rule-based transformation functions specified for rules in the positive rule set, and privileges to enqueue messages into the capture process queue. A capture process can be associated with only one user, but one user can be associated with many capture processes.

See Also:

Capture Process States

The state of a capture process describes what the capture process is doing currently. You can view the state of a capture process by querying the STATE column in the V$STREAMS_CAPTURE dynamic performance view. The following capture process states are possible:

  • INITIALIZING - Starting up.

  • WAITING FOR DICTIONARY REDO - Waiting for redo log files containing the dictionary build related to the first SCN to be added to the capture process session. A capture process cannot begin to scan the redo log files until all of the log files containing the dictionary build have been added.

  • DICTIONARY INITIALIZATION - Processing a dictionary build.

  • MINING (PROCESSED SCN = scn_value) - Mining a dictionary build at the SCN scn_value.

  • LOADING (step X of Y) - Processing information from a dictionary build and currently at step X in a process that involves Y steps, where X and Y are numbers.

  • CAPTURING CHANGES - Scanning the redo log for changes that satisfy the capture process rule sets.

  • WAITING FOR REDO - Waiting for new redo log files to be added to the capture process session. The capture process has finished processing all of the redo log files added to its session. This state is possible if there is no activity at a source database. For a downstream capture process, this state is possible if the capture process is waiting for new log files to be added to its session.

  • EVALUATING RULE - Evaluating a change against a capture process rule set.

  • CREATING LCR - Converting a change into a logical change record (LCR).

  • ENQUEUING MESSAGE - Enqueuing an LCR that satisfies the capture process rule sets into the capture process queue.

  • PAUSED FOR FLOW CONTROL - Unable to enqueue LCRs either because of low memory or because propagations and apply processes are consuming messages slower than the capture process is creating them. This state indicates flow control that is used to reduce spilling of captured LCRs when propagation or apply has fallen behind or is unavailable.

  • SHUTTING DOWN - Stopping.

See Also:

Multiple Capture Processes in a Single Database

If you run multiple capture processes in a single database, increase the size of the System Global Area (SGA) for each instance. Use the SGA_MAX_SIZE initialization parameter to increase the SGA size. Also, if the size of the Oracle Streams pool is not managed automatically in the database, then increase the size of the Oracle Streams pool by 10 MB for each capture process parallelism. For example, if you have two capture processes running in a database, and the parallelism parameter is set to 4 for one of them and 1 for the other, then increase the Oracle Streams pool by 50 MB (4 + 1 = 5 parallelism).

Also, Oracle recommends that each ANYDATA queue used by a capture process, propagation, or apply process store captured LCRs from at most one capture process from a particular source database. Therefore, use a separate queue for each capture process that captures changes originating at a particular source database, and make sure each queue has its own queue table. Also, do not propagate messages from two or more capture processes with the same source database to the same queue.

Note:

The size of the Oracle Streams pool is managed automatically if the MEMORY_TARGET, MEMORY_MAX_TARGET, or SGA_TARGET initialization parameter is set to a nonzero value.

See Also:

Capture Process Checkpoints

A checkpoint is information about the current state of a capture process that is stored persistently in the data dictionary of the database running the capture process. A capture process tries to record a checkpoint at regular intervals called checkpoint intervals.

Required Checkpoint SCN

The system change number (SCN) that corresponds to the lowest checkpoint for which a capture process requires redo data is the required checkpoint SCN. The redo log file that contains the required checkpoint SCN, and all subsequent redo log files, must be available to the capture process. If a capture process is stopped and restarted, then it starts scanning the redo log from the SCN that corresponds to its required checkpoint SCN. The required checkpoint SCN is important for recovery if a database stops unexpectedly. Also, if the first SCN is reset for a capture process, then it must be set to a value that is less than or equal to the required checkpoint SCN for the captured process. You can determine the required checkpoint SCN for a capture process by querying the REQUIRED_CHECKPOINT_SCN column in the DBA_CAPTURE data dictionary view.

Maximum Checkpoint SCN

The SCN that corresponds to the last checkpoint recorded by a capture process is the maximum checkpoint SCN. If you create a capture process that captures changes from a source database, and other capture processes already exist which capture changes from the same source database, then the maximum checkpoint SCNs of the existing capture processes can help you to decide whether the new capture process should create a new LogMiner data dictionary or share one of the existing LogMiner data dictionaries. You can determine the maximum checkpoint SCN for a capture process by querying the MAX_CHECKPOINT_SCN column in the DBA_CAPTURE data dictionary view.

Checkpoint Retention Time

The checkpoint retention time is the amount of time, in number of days, that a capture process retains checkpoints before purging them automatically. A capture process periodically computes the age of a checkpoint by subtracting the NEXT_TIME of the archived redo log file that corresponds to the checkpoint from FIRST_TIME of the archived redo log file containing the required checkpoint SCN for the capture process. If the resulting value is greater than the checkpoint retention time, then the capture process automatically purges the checkpoint by advancing its first SCN value. Otherwise, the checkpoint is retained. The DBA_REGISTERED_ARCHIVED_LOG view displays the FIRST_TIME and NEXT_TIME for archived redo log files, and the REQUIRED_CHECKPOINT_SCN column in the DBA_CAPTURE view displays the required checkpoint SCN for a capture process.

Figure 2-4 shows an example of a checkpoint being purged when the checkpoint retention time is set to 20 days.

Figure 2-4 Checkpoint Retention Time Set to 20 Days

Description of Figure 2-4 follows
Description of "Figure 2-4 Checkpoint Retention Time Set to 20 Days"

In Figure 2-4, with the checkpoint retention time set to 20 days, the checkpoint at SCN 435250 is purged because it is 21 days old, while the checkpoint at SCN 479315 is retained because it is 8 days old.

Whenever the first SCN is reset for a capture process, the capture process purges information about archived redo log files prior to the new first SCN from its LogMiner data dictionary. After this information is purged, the archived redo log files remain on the hard disk, but the files are not needed by the capture process. The PURGEABLE column in the DBA_REGISTERED_ARCHIVED_LOG view displays YES for the archived redo log files that are no longer needed. These files can be removed from disk or moved to another location without affecting the capture process.

If you create a capture process using the CREATE_CAPTURE procedure in the DBMS_CAPTURE_ADM package, then you can specify the checkpoint retention time, in days, using the checkpoint_retention_time parameter. The default checkpoint retention time is 60 days if the checkpoint_retention_time parameter is not specified in the CREATE_CAPTURE procedure, or if you use the DBMS_STREAMS_ADM package to create the capture process. The CHECKPOINT_RETENTION_TIME column in the DBA_CAPTURE view displays the current checkpoint retention time for a capture process.

You can change the checkpoint retention time for a capture process by specifying a new time period in the ALTER_CAPTURE procedure in the DBMS_CAPTURE_ADM package. If you do not want checkpoints for a capture process to be purged automatically, then specify DBMS_CAPTURE_ADM.INFINITE for the checkpoint_retention_time parameter in CREATE_CAPTURE or ALTER_CAPTURE.

Note:

To specify a checkpoint retention time for a capture process, the compatibility level of the database running the capture process must be 10.2.0 or higher. If the compatibility level is lower than 10.2.0 for a database, then the checkpoint retention time for all capture processes running on the database is infinite.

A New First SCN Value and Purged LogMiner Data Dictionary Information

If you reset the first SCN value for an existing capture process, or if the first SCN is reset automatically when checkpoints are purged, then Oracle automatically purges LogMiner data dictionary information prior to the new first SCN setting. If the start SCN for a capture process corresponds to redo information that has been purged, then Oracle Database automatically resets the start SCN to the same value as the first SCN. However, if the start SCN is higher than the new first SCN setting, then the start SCN remains unchanged.

Figure 2-5 shows how Oracle automatically purges LogMiner data dictionary information prior to a new first SCN setting, and how the start SCN is not changed if it is higher than the new first SCN setting.

Figure 2-5 Start SCN Higher than Reset First SCN

Description of Figure 2-5 follows
Description of "Figure 2-5 Start SCN Higher than Reset First SCN"

Given this example, if the first SCN is reset again to a value higher than the start SCN value for a capture process, then the start SCN no longer corresponds to existing information in the LogMiner data dictionary. Figure 2-6 shows how Oracle Database resets the start SCN automatically if it is lower than a new first SCN setting.

Figure 2-6 Start SCN Lower than Reset First SCN

Description of Figure 2-6 follows
Description of "Figure 2-6 Start SCN Lower than Reset First SCN"

As you can see, the first SCN and start SCN for a capture process can continually increase over time, and, as the first SCN moves forward, it might no longer correspond to an SCN established by the DBMS_CAPTURE_ADM.BUILD procedure.

See Also:

ARCHIVELOG Mode and a Capture Process

The following list describes how different types of capture processes read the redo data:

  • A local capture process reads from the redo log buffer whenever possible. If it cannot read from the log buffer, then it reads from the online redo logs. If it cannot read from the log buffer or the online redo logs, then it reads from the archived redo log files. Therefore, the source database must be running in ARCHIVELOG mode when a local capture process is configured to capture changes.

  • A real-time downstream capture process reads online redo data from its source database whenever possible and archived redo log files that contain redo data from the source database otherwise. In this case, the redo data from the source database is stored in the standby redo log at the downstream database, and the archiver at the downstream database archives the redo data in the standby redo log. Therefore, both the source database and the downstream database must be running in ARCHIVELOG mode when a real-time downstream capture process is configured to capture changes.

  • An archived-log downstream capture process always reads archived redo log files from its source database. Therefore, the source database must be running in ARCHIVELOG mode when an archived-log downstream capture process is configured to capture changes.

You can query the REQUIRED_CHECKPOINT_SCN column in the DBA_CAPTURE data dictionary view to determine the required checkpoint SCN for a capture process. When the capture process is restarted, it scans the redo log from the required checkpoint SCN forward. Therefore, the redo log file that includes the required checkpoint SCN, and all subsequent redo log files, must be available to the capture process.

You must keep an archived redo log file available until you are certain that no capture process will need that file. The first SCN for a capture process can be reset to a higher value, but it cannot be reset to a lower value. Therefore, a capture process will never need the redo log files that contain information prior to its first SCN. Query the DBA_LOGMNR_PURGED_LOG data dictionary view to determine which archived redo log files will never be needed by any capture process.

When a local capture process falls behind, there is a seamless transition from reading an online redo log to reading an archived redo log, and, when a local capture process catches up, there is a seamless transition from reading an archived redo log to reading an online redo log. Similarly, when a real-time downstream capture process falls behind, there is a seamless transition from reading the standby redo log to reading an archived redo log, and, when a real-time downstream capture process catches up, there is a seamless transition from reading an archived redo log to reading the standby redo log.

Note:

At a downstream database in a downstream capture configuration, log files from a remote source database should be kept separate from local database log files. In addition, if the downstream database contains log files from multiple source databases, then the log files from each source database should be kept separate from each other.

See Also:

Capture Process Creation

You can create a capture process using a procedure in the DBMS_STREAMS_ADM package or the DBMS_CAPTURE_ADM package. Using a procedure the DBMS_STREAMS_ADM package to create a capture process is simpler because the procedure automatically uses defaults for some configuration options. In addition, when you use a procedure in the DBMS_STREAMS_ADM package, a rule set is created for the capture process, and rules can be added to the rule set automatically. The rule set is a positive rule set if the inclusion_rule parameter is set to TRUE (the default) in the procedure, or it is a negative rule set if the inclusion_rule parameter is set to FALSE in the procedure.

Alternatively, using the CREATE_CAPTURE procedure in the DBMS_CAPTURE_ADM package to create a capture process is more flexible, and you create one or more rule sets and rules for the capture process either before or after it is created. You can use the procedures in the DBMS_STREAMS_ADM package or the DBMS_RULE_ADM package to add rules to a rule set for the capture process. To create a capture process at a downstream database, you must use the DBMS_CAPTURE_ADM package.

When you create a capture process using a procedure in the DBMS_STREAMS_ADM package and generate one or more rules in the positive rule set for the capture process, the objects for which changes are captured are prepared for instantiation automatically, unless it is a downstream capture process and there is no database link from the downstream database to the source database.

When you create a capture process using the CREATE_CAPTURE procedure in the DBMS_CAPTURE_ADM package, you should prepare for instantiation any objects for which you plan to capture changes. Prepare these objects for instantiations as soon as possible after capture process creation. You can prepare objects for instantiation using one of the following procedures in the DBMS_CAPTURE_ADM package:

  • PREPARE_TABLE_INSTANTIATION prepares a single table for instantiation.

  • PREPARE_SCHEMA_INSTANTIATION prepares for instantiation all of the objects in a schema and all objects added to the schema in the future.

  • PREPARE_GLOBAL_INSTANTIATION prepares for instantiation all of the objects in a database and all objects added to the database in the future.

These procedures can also enable supplemental logging for the key columns or for all columns in the table or tables prepared for instantiation.

Note:

After creating a capture process, avoid changing the DBID or global name of the source database for the capture process. If you change either the DBID or global name of the source database, then the capture process must be dropped and re-created.

See Also:

The LogMiner Data Dictionary for a Capture Process

A capture process requires a data dictionary that is separate from the primary data dictionary for the source database. This separate data dictionary is called a LogMiner data dictionary. There can be more than one LogMiner data dictionary for a particular source database. If there are multiple capture processes capturing changes from the source database, then two or more capture processes can share a LogMiner data dictionary, or each capture process can have its own LogMiner data dictionary. If the LogMiner data dictionary that is needed by a capture process does not exist, then the capture process populates it using information in the redo log when the capture process is started for the first time.

The DBMS_CAPTURE_ADM.BUILD procedure extracts data dictionary information to the redo log, and this procedure must be run at least once on the source database before any capture process configured to capture changes originating at the source database is started. The extracted data dictionary information in the redo log is consistent with the primary data dictionary at the time when the DBMS_CAPTURE_ADM.BUILD procedure is run. This procedure also identifies a valid first SCN value that can be used to create a capture process.

You can perform a build of data dictionary information in the redo log multiple times, and a particular build might or might not be used by a capture process to create a LogMiner data dictionary. The amount of information extracted to a redo log when you run the BUILD procedure depends on the number of database objects in the database. Typically, the BUILD procedure generates a large amount of redo data that a capture process must scan subsequently. Therefore, you should run the BUILD procedure only when necessary.

In most cases, if a build is required when a capture process is created using a procedure in the DBMS_STREAMS_ADM or DBMS_CAPTURE_ADM package, then the procedure runs the BUILD procedure automatically. However, the BUILD procedure is not run automatically during capture process creation in the following cases:

  • You use CREATE_CAPTURE and specify a non-NULL value for the first_scn parameter. In this case, the specified first SCN must correspond to a previous build.

  • You create a downstream capture process that does not use a database link. In this case, the command at the downstream database cannot communicate with the source database to run the BUILD procedure automatically. Therefore, you must run it manually on the source database and specify the first SCN that corresponds to the build during capture process creation.

A capture process requires a LogMiner data dictionary because the information in the primary data dictionary might not apply to the changes being captured from the redo log. These changes might have occurred minutes, hours, or even days before they are captured by a capture process. For example, consider the following scenario:

  1. A capture process is configured to capture changes to tables.

  2. A database administrator stops the capture process. When the capture process is stopped, it records the SCN of the change it was currently capturing.

  3. User applications continue to make changes to the tables while the capture process is stopped.

  4. The capture process is restarted three hours after it was stopped.

In this case, to ensure data consistency, the capture process must begin capturing changes in the redo log at the time when it was stopped. The capture process starts capturing changes at the SCN that it recorded when it was stopped.

The redo log contains raw data. It does not contain database object names and column names in tables. Instead, it uses object numbers and internal column numbers for database objects and columns, respectively. Therefore, when a change is captured, a capture process must reference a data dictionary to determine the details of the change.

Because a LogMiner data dictionary might be populated when a capture process is started for the first time, it might take some time to start capturing changes. The amount of time required depends on the number of database objects in the database. You can query the STATE column in the V$STREAMS_CAPTURE dynamic performance view to monitor the progress while a capture process is processing a data dictionary build.

See Also:

Scenario Illustrating Why a Capture Process Needs a LogMiner Data Dictionary

Consider a scenario in which a capture process has been configured to capture changes to table t1, which has columns a and b, and the following changes are made to this table at three different points in time:

Time 1: Insert values a=7 and b=15.

Time 2: Add column c.

Time 3: Drop column b.

If for some reason the capture process is capturing changes from an earlier time, then the primary data dictionary and the relevant version in the LogMiner data dictionary contain different information. Table 2-2 illustrates how the information in the LogMiner data dictionary is used when the current time is different than the change capturing time.

Table 2-2 Information About Table t1 in the Primary and LogMiner Data Dictionaries

Current Time Change Capturing Time Primary Data Dictionary LogMiner Data Dictionary

1

1

Table t1 has columns a and b.

Table t1 has columns a and b at time 1.

2

1

Table t1 has columns a, b, and c at time 2.

Table t1 has columns a and b at time 1.

3

1

Table t1 has columns a and c at time 3.

Table t1 has columns a and b at time 1.


Assume that the capture process captures the change resulting from the insert at time 1 when the actual time is time 3. If the capture process used the primary data dictionary, then it might assume that a value of 7 was inserted into column a and a value of 15 was inserted into column c, because those are the two columns for table t1 at time 3 in the primary data dictionary. However, a value of 15 actually was inserted into column b, not column c.

Because the capture process uses the LogMiner data dictionary, the error is avoided. The LogMiner data dictionary is synchronized with the capture process and continues to record that table t1 has columns a and b at time 1. So, the captured change specifies that a value of 15 was inserted into column b.

Multiple Capture Processes for the Same Source Database

If one or more capture processes are capturing changes made to a source database, and you want to create a new capture process that captures changes to the same source database, then the new capture process can either create a new LogMiner data dictionary or share one of the existing LogMiner data dictionaries with one or more other capture processes.

Whether a new LogMiner data dictionary is created for a new capture process depends on the setting for the first_scn parameter when you run CREATE_CAPTURE to create a capture process.

If multiple LogMiner data dictionaries exist, and you specify NULL for the first_scn parameter during capture process creation, then the new capture process automatically attempts to share the LogMiner data dictionary of one of the existing capture processes that has taken at least one checkpoint. You can view the maximum checkpoint SCN for all existing capture processes by querying the MAX_CHECKPOINT_SCN column in the DBA_CAPTURE data dictionary view. During capture process creation, if the first_scn parameter is NULL and the start_scn parameter is non-NULL, then an error is raised if the start_scn parameter setting is lower than all of the first SCN values for all existing capture processes.

If multiple LogMiner data dictionaries exist, and you specify a non-NULL value for the first_scn parameter during capture process creation, then the new capture process creates a new LogMiner data dictionary the first time it is started. In this case, before you create the new capture process, you must run the BUILD procedure in the DBMS_CAPTURE_ADM package on the source database. The BUILD procedure generates a corresponding valid first SCN value that you can specify when you create the new capture process.

You can find a first SCN generated by the BUILD procedure by running the following query:

COLUMN FIRST_CHANGE# HEADING 'First SCN' FORMAT 999999999
COLUMN NAME HEADING 'Log File Name' FORMAT A50

SELECT DISTINCT FIRST_CHANGE#, NAME FROM V$ARCHIVED_LOG
  WHERE DICTIONARY_BEGIN = 'YES';

This query can return more than one row if the BUILD procedure was run more than once.

The most important factor to consider when deciding whether a new capture process should share an existing LogMiner data dictionary or create a new one is the difference between the maximum checkpoint SCN values of the existing capture processes and the start SCN of the new capture process. If the new capture process shares a LogMiner data dictionary, then it must scan the redo log from the point of the maximum checkpoint SCN of the shared LogMiner data dictionary onward, even though the new capture process cannot capture changes prior to its first SCN. If the start SCN of the new capture process is much higher than the maximum checkpoint SCN of the existing capture process, then the new capture process must scan a large amount of redo data before it reaches its start SCN.

A capture process creates a new LogMiner data dictionary when the first_scn parameter is non-NULL during capture process creation. Follow these guidelines when you decide whether a new capture process should share an existing LogMiner data dictionary or create a new one:

  • If one or more maximum checkpoint SCN values is greater than the start SCN you want to specify, and if this start SCN is greater than the first SCN of one or more existing capture processes, then it might be better to share the LogMiner data dictionary of an existing capture process. In this case, you can assume there is a checkpoint SCN that is less than the start SCN and that the difference between this checkpoint SCN and the start SCN is small. The new capture process will begin scanning the redo log from this checkpoint SCN and will catch up to the start SCN quickly.

  • If no maximum checkpoint SCN is greater than the start SCN, and if the difference between the maximum checkpoint SCN and the start SCN is small, then it might be better to share the LogMiner data dictionary of an existing capture process. The new capture process will begin scanning the redo log from the maximum checkpoint SCN, but it will catch up to the start SCN quickly.

  • If no maximum checkpoint SCN is greater than the start SCN, and if the difference between the highest maximum checkpoint SCN and the start SCN is large, then it might take a long time for the capture process to catch up to the start SCN. In this case, it might be better for the new capture process to create a new LogMiner data dictionary. It will take some time to create the new LogMiner data dictionary when the new capture process is first started, but the capture process can specify the same value for its first SCN and start SCN, and thereby avoid scanning a large amount of redo data unnecessarily.

Figure 2-7 illustrates these guidelines.

Figure 2-7 Deciding Whether to Share a LogMiner Data Dictionary

Description of Figure 2-7 follows
Description of "Figure 2-7 Deciding Whether to Share a LogMiner Data Dictionary"

Note:

  • If you create a capture process using one of the procedures in the DBMS_STREAMS_ADM package, then it is the same as specifying NULL for the first_scn and start_scn parameters in the CREATE_CAPTURE procedure.

  • You must prepare database objects for instantiation if a new capture process will capture changes made to these database objects. This requirement holds even if the new capture process shares a LogMiner data dictionary with one or more other capture processes for which these database objects have been prepared for instantiation.

See Also:

The Oracle Streams Data Dictionary

Propagations and apply processes use an Oracle Streams data dictionary to keep track of the database objects from a particular source database. An Oracle Streams data dictionary is populated whenever one or more database objects are prepared for instantiation at a source database. Specifically, when a database object is prepared for instantiation, it is recorded in the redo log. When a capture process scans the redo log, it uses this information to populate the local Oracle Streams data dictionary for the source database. In the case of local capture, this Oracle Streams data dictionary is at the source database. In the case of downstream capture, this Oracle Streams data dictionary is at the downstream database.

When you prepare a database object for instantiation, you are informing Oracle Streams that information about the database object is needed by propagations that propagate changes to the database object and apply processes that apply changes to the database object. Any database that propagates or applies these changes requires an Oracle Streams data dictionary for the source database where the changes originated.

After an object has been prepared for instantiation, the local Oracle Streams data dictionary is updated when a DDL statement on the object is processed by a capture process. In addition, an internal message containing information about this DDL statement is captured and placed in the queue for the capture process. Propagations can then propagate these internal messages to destination queues at databases.

An Oracle Streams data dictionary is multiversioned. If a database has multiple propagations and apply processes, then all of them use the same Oracle Streams data dictionary for a particular source database. A database can contain only one Oracle Streams data dictionary for a particular source database, but it can contain multiple Oracle Streams data dictionaries if it propagates or applies changes from multiple source databases.

Capture Process Parameters

After creation, a capture process is disabled so that you can set the capture process parameters for your environment before starting it for the first time. Capture process parameters control the way a capture process operates. For example, the parallelism capture process parameter controls the number of preparer servers used by a capture process, and the time_limit capture process parameter specifies the amount of time a capture process runs before it is shut down automatically. You set capture process parameters using the DBMS_CAPTURE_ADM.SET_PARAMETER procedure.

See Also:

Capture Process Rule Evaluation

A capture process evaluates changes it finds in the redo log against its positive and negative rule sets. The capture process evaluates a change against the negative rule set first. If one or more rules in the negative rule set evaluate to TRUE for the change, then the change is discarded, but if no rule in the negative rule set evaluates to TRUE for the change, then the change satisfies the negative rule set. When a change satisfies the negative rule set for a capture process, the capture process evaluates the change against its positive rule set. If one or more rules in the positive rule set evaluate to TRUE for the change, then the change satisfies the positive rule set, but if no rule in the positive rule set evaluates to TRUE for the change, then the change is discarded. If a capture process only has one rule set, then it evaluates changes against this one rule set only.

A running capture process completes the following series of actions to capture changes:

  1. Finds changes in the redo log.

  2. Performs prefiltering of the changes in the redo log. During this step, a capture process evaluates rules in its rule sets at a basic level to place changes found in the redo log into two categories: changes that should be converted into LCRs and changes that should not be converted into LCRs. Prefiltering is done in two phases. In the first phase, information that can be evaluated during prefiltering includes schema name, object name, and command type. If more information is needed to determine whether a change should be converted into an LCR, then information that can be evaluated during the second phase of prefiltering includes tag values and column values when appropriate.

    Prefiltering is a safe optimization done with incomplete information. This step identifies relevant changes to be processed subsequently, such that:

    • A capture process converts a change into an LCR if the change satisfies the capture process rule sets. In this case, proceed to Step 3.

    • A capture process does not convert a change into an LCR if the change does not satisfy the capture process rule sets.

    • Regarding MAYBE evaluations, the rule evaluation proceeds as follows:

      • If a change evaluates to MAYBE against both the positive and negative rule set for a capture process, then the capture process might not have enough information to determine whether the change will definitely satisfy both of its rule sets. In this case, further evaluation is necessary. Proceed to Step 3.

      • If the change evaluates to FALSE against the negative rule set and MAYBE against the positive rule set for the capture process, then the capture process might not have enough information to determine whether the change will definitely satisfy both of its rule sets. In this case, further evaluation is necessary. Proceed to Step 3.

      • If the change evaluates to MAYBE against the negative rule set and TRUE against the positive rule set for the capture process, then the capture process might not have enough information to determine whether the change will definitely satisfy both of its rule sets. In this case, further evaluation is necessary. Proceed to Step 3.

      • If the change evaluates to TRUE against the negative rule set and MAYBE against the positive rule set for the capture process, then the capture process discards the change.

      • If the change evaluates to MAYBE against the negative rule set and FALSE against the positive rule set for the capture process, then the capture process discards the change.

  3. Converts changes that satisfy, or might satisfy, the capture process rule sets into LCRs based on prefiltering.

  4. Performs LCR filtering. During this step, a capture process evaluates rules regarding information in each LCR to separate the LCRs into two categories: LCRs that should be enqueued and LCRs that should be discarded.

  5. Discards the LCRs that should not be enqueued because they did not satisfy the capture process rule sets.

  6. Enqueues the remaining captured LCRs into the queue associated with the capture process.

For example, suppose the following rule is defined in the positive rule set for a capture process: Capture changes to the hr.employees table where the department_id is 50. No other rules are defined for the capture process, and the parallelism parameter for the capture process is set to 1.

Given this rule, suppose an UPDATE statement on the hr.employees table changes 50 rows in the table. The capture process performs the following series of actions for each row change:

  1. Finds the next change resulting from the UPDATE statement in the redo log.

  2. Determines that the change resulted from an UPDATE statement to the hr.employees table and must be captured. If the change was made to a different table, then the capture process ignores the change.

  3. Captures the change and converts it into an LCR.

  4. Filters the LCR to determine whether it involves a row where the department_id is 50.

  5. Either enqueues the LCR into the queue associated with the capture process if it involves a row where the department_id is 50, or discards the LCR if it involves a row where the department_id is not 50 or is missing.

    See Also:

Figure 2-8 illustrates capture process rule evaluation in a flowchart.

Figure 2-8 Flowchart Showing Capture Process Rule Evaluation

Description of Figure 2-8 follows
Description of "Figure 2-8 Flowchart Showing Capture Process Rule Evaluation"

Persistent Capture Process Status Upon Database Restart

A capture process maintains a persistent status when the database running the capture process is shut down and restarted. For example, if a capture process is enabled when the database is shut down, then the capture process automatically starts when the database is restarted. Similarly, if a capture process is disabled or aborted when a database is shut down, then the capture process is not started and retains the disabled or aborted status when the database is restarted.

Implicit Capture with Synchronous Capture

This section explains the concepts related to synchronous capture.

This section discusses the following topics:

Introduction to Synchronous Capture

Synchronous capture is an optional Oracle Streams client that captures data manipulation language (DML) changes made to tables. Synchronous capture uses an internal mechanism to capture DML changes to specified tables. When synchronous capture is configured to capture changes to tables, the database that contains these tables is called the source database.

When a DML change it made to a table, it can result in changes to one or more rows in the table. Synchronous capture captures each row change and converts it into a specific message format called a row logical change record (row LCR). After capturing a row LCR, synchronous capture enqueues a message containing the row LCR into a queue. Row LCRs created by synchronous capture always contain values for all the columns in a row, even if some of the columns where not modified by the change.

Figure 2-9 shows a synchronous capture capturing LCRs.

Figure 2-9 Synchronous Capture

Description of Figure 2-9 follows
Description of "Figure 2-9 Synchronous Capture"

Note:

A synchronous capture and a capture process should not capture changes made to the same table.

Synchronous Capture and Queues

Synchronous capture is always associated with a single ANYDATA queue, and it enqueues messages into this queue only. The queue used by synchronous capture must be a commit-time queue. Commit-time queues ensure that messages are grouped into transactions, and that transactions groups are in commit system change number (CSCN) order. Synchronous capture always enqueues row LCRs into the persistent queue. The persistent queue is the portion of a queue that only stores messages on hard disk in a queue table, not in memory. You can create multiple queues and associate a different synchronous capture with each queue.

Although synchronous capture must enqueue messages into a commit-time queue, messages captured by synchronous capture can be propagated to queues that are not commit-time queues. Therefore, any intermediate queues that store messages captured by synchronous capture do not need to be commit-time queue. Also, apply processes that apply messages captured by synchronous capture can use queues that are not commit-time queues.

Note:

  • Synchronous capture can be associated only with an ANYDATA queue, not with a typed queue.

  • Synchronous capture should not enqueue messages that is used by a capture process.

Synchronous Capture Rules

Synchronous capture either captures or discards changes based on rules that you define. Each rule specifies the database objects and types of changes for which the rule evaluates to TRUE. You can place these rules in a positive rule set. If a rule evaluates to TRUE for a change, and the rule is in the positive rule set for synchronous capture, then synchronous capture captures the change. Synchronous capture does not use negative rule sets.

You can specify synchronous capture rules at the table level. A table rule captures or discards row changes resulting from DML changes to a particular table. Subset rules are table rules that include a subset of the row changes to a particular table. Synchronous capture does not use schema or global rules.

All synchronous capture rules must be created with one of the following procedures in the DBMS_STREAMS_ADM package:

  • ADD_TABLE_RULES

  • ADD_SUBSET_RULES

Synchronous capture does not capture changes based on the following types of rules:

  • Rules added to the synchronous capture rules set by any procedure other than ADD_TABLE_RULES or ADD_SUBSET_RULES in the DBMS_STREAMS_ADM package.

  • Rules created by the DBMS_RULE_ADM package.

If these types of rules are in a synchronous capture rule set, then synchronous capture ignores these rules.

A synchronous capture can use a rule set created by the CREATE_RULE_SET procedure in the DBMS_RULE_ADM package, but you must add rules to the rule set with the ADD_TABLE_RULES or ADD_SUBSET_RULES procedure.

If the specified synchronous capture does not exist when you run the ADD_TABLE_RULES or ADD_SUBSET_RULES procedure, then the procedure creates it automatically. You can also use the CREATE_SYNC_CAPTURE procedure in the DBMS_CAPTURE_ADM package to create a synchronous capture.

Note:

  • Synchronous capture does not capture certain types of changes and changes to certain data types in table columns. Also, synchronous capture never captures changes in the SYS, SYSTEM, or CTXSYS schemas.

  • When a rule is in the rule set for a synchronous capture, do not change the following rule conditions: :dml.get_object_name and :dml.get_object_owner. Changing these conditions can cause the synchronous capture not to capture changes to the database object. You can change other conditions in synchronous capture rules.

Data Types Captured by Synchronous Capture

When capturing the row changes resulting from DML changes made to tables, synchronous capture can capture changes made to columns of the following data types:

  • VARCHAR2

  • NVARCHAR2

  • NUMBER

  • FLOAT

  • DATE

  • BINARY_FLOAT

  • BINARY_DOUBLE

  • TIMESTAMP

  • TIMESTAMP WITH TIME ZONE

  • TIMESTAMP WITH LOCAL TIME ZONE

  • INTERVAL YEAR TO MONTH

  • INTERVAL DAY TO SECOND

  • RAW

  • CHAR

  • NCHAR

  • UROWID

See Also:

Types of DML Changes Captured by Synchronous Capture

When you specify that DML changes made to specific tables should be captured, synchronous capture captures the following types of DML changes made to these tables:

  • INSERT

  • UPDATE

  • DELETE

  • MERGE

Synchronous capture converts each MERGE change into an INSERT or UPDATE change. MERGE is not a valid command type in a row LCR.

See Also:

Capture User for Synchronous Capture

Changes are captured in the security domain of the capture user for a synchronous capture. The capture user captures all changes that satisfy the synchronous capture rule set. In addition, the capture user runs all custom rule-based transformations specified by the rules in these rule sets. The capture user must have the necessary privileges to perform these actions, including EXECUTE privilege on the rule set used by synchronous capture, EXECUTE privilege on all custom rule-based transformation functions specified for rules in the rule set, and privileges to enqueue messages into the synchronous capture queue. A synchronous capture can be associated with only one user, but one user can be associated with many synchronous captures.

See Also:

"Configuring an Oracle Streams Administrator" for information about the required privileges

Multiple Synchronous Captures in a Single Database

Oracle recommends that each ANYDATA queue used by a synchronous capture, propagation, or apply process have messages from at most one synchronous capture from a particular source database. Therefore, a separate queue should be used for each synchronous capture that captures changes originating at a particular source database, and each queue should have its own queue table. Also, messages from two or more synchronous captures in the same source database should not be propagated to the same destination queue.

Explicit Capture by Applications

When applications enqueue messages manually, it is called explicit capture. After enqueue, these messages can be propagated by Oracle Streams propagations within the same database or to a different database. These messages can also be consumed by applications, apply processes, and messaging clients. You can use either the DBMS_STREAMS_MESSAGING package or the DBMS_AQADM package to enqueue messages.

The following sections describe conceptual information about enqueuing messages:

See Also:

Types of Messages That Can Be Enqueued Explicitly

Applications can create and enqueue different types of messages for various purposes in an Oracle Streams environment. These messages can be messages of a user-defined type called user messages, or they can be LCRs.

This section contains these topics:

User Messages

An application can construct a message of a user-defined type and enqueue it. The queue can be a queue of the same type as the message, or it can be an ANYDATA queue. Typically, these user messages are consumed by applications or apply processes.

User messages enqueued into a buffered queue are called buffered user messages. Buffered user messages can be dequeued by an application only. An application processes the messages after it dequeues them.

User messages enqueued into a persistent queue are called persistent user messages. Persistent user messages can be dequeued by:

  • Messaging clients: A messaging client passes the messages to the application that invoked the messaging client for processing.

  • Applications: An application processes the messages after it dequeues them.

  • Apply processes: An apply process passes the messages to a message handler for processing. The queue must be an ANYDATA queue for an apply process to dequeue messages from it.

Logical Change Records (LCRs)

An application can construct and enqueue LCRs into an ANYDATA queue. Row LCRs describe the results of DML changes, and DDL LCRs describe DDL changes. Typically, LCRs are consumed by apply processes, but they can also be consumed by messaging clients and applications. Heterogeneous replication environment can use explicit enqueue of LCRs to replicate database changes from a non-Oracle database to an Oracle database.

LCRs enqueued explicitly into a buffered queue are called buffered LCRs. Buffered LCRs can be dequeued only by applications. An application processes the buffered LCRs after it dequeues them.

LCRs enqueued explicitly into a persistent queue are called persistent LCRs. Persistent LCRs can be dequeued by:

  • Messaging clients: A messaging client passes the messages to the application that invoked the messaging client for processing.

  • Applications: An application processes the messages after it dequeues them.

  • Apply processes: An apply process can apply the LCRs directly or pass them to an apply handler for processing.

See Also:

Enqueue Features

The enqueue features available with Oracle Streams Advanced Queuing include the following:

  • Enqueue into a buffered queue or a persistent queue

  • Ordering of messages by priority enqueue time, or commit time

  • Array enqueue of messages

  • Correlation identifiers

  • Message grouping

  • Sender identification

  • Time specification and scheduling

See Also: