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12c Release 1 (12.1)

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25 Using Transaction Guard

Transaction Guard provides a generic tool for applications to use for at-most-once execution in case of planned and unplanned outages. Applications use the logical transaction ID to determine the outcome of the last transaction open in a database session following an outage. Without Transaction Guard, applications that attempt to replay operations following outages can cause logical corruption by committing duplicate transactions.

Transaction Guard provides these benefits:

This chapter assumes that you are familiar with the major relevant concepts and techniques of the technology or product environment in which you are using Transaction Guard.

Topics:

See Also:

25.1 Problem that Transaction Guard Solves

In applications without Transaction Guard, a fundamental problem for recovering applications after an outage is that the commit message that is sent back to the client is not durable. If there is a break between the client and the server, the client sees an error message indicating that the communication failed. This error does not inform the application if the submission executed any commit operations, if a procedural call completed and executed all expected commits and session state changes, or if a call failed part way through or, yet worse, is still running disconnected from the client.

Without Transaction Guard, it is impossible or extremely difficult to determine the outcome of the last commit operation, in a guaranteed and scalable manner, after a communication failure to the server. If an application must determine whether the submission to the database was committed, the application must add custom exception code to query the outcome for every possible commit point in the application. Given that a system can fail anywhere, this is almost impractical because the query must be specific to each submission. After an application is built and is in production, this is completely impractical. Moreover, a query cannot give the correct answer because the transaction could commit immediately after that query executed. Indeed, after a communication failure the server may still be running the submission not yet aware that the client has disconnected. For PL/SQL or Java in the database, for a procedural submission, there is also no record as to whether that submission ran to completion or was aborted part way through. While such a procedure may have committed, subsequent work may not have been done for the procedure.

Failing to recognize that the last submission has committed, or will commit sometime soon or has not run to completion, can lead applications that attempt to replay, thus causing duplicate transaction submissions and other forms of "logical corruption" because the software might try to reissue already persisted changes.

Without Transaction Guard, if a transaction has been started and commit has been issued, the commit message that is sent back to the client is not durable. The client is left not knowing whether the transaction committed. The transaction cannot be validly resubmitted if the nontransactional state is incorrect or if it already committed. In the absence of guaranteed commit and completion information, resubmission can lead to transactions applied more than once and in a session with the incorrect state.

25.2 Solution that Transaction Guard Provides

Effective with Oracle Database 12c Release 1 (12.1.0.1), Transaction Guard provides new, integrated tools for applications to use to achieve idempotence automatically and transparently, and in a manner that scales. Its key features are the following:

  • Durability of COMMIT outcome by saving a logical transaction identifier (LTXID) at commit for all supported transaction types against the database (Oracle Database 12c Release 1 (12.1.0.1) or later). This includes idempotence for transactions executed using autocommit, from inside PL/SQL, from remote transactions, and from callouts that cannot otherwise be identified using generic means.

  • Use of the LTXID to support at-most-once execution semantics, such that database transactions protected by logical transaction identifiers cannot be duplicated when there are multiple copies of that transaction in flight identified by the LTXID.

  • Blocking of a commit of in-flight work to ensure that regardless of the outage situation, another submission of the same transaction protected by that LTXID cannot commit.

  • Identification of whether work committed at an LTXID was committed as part of a top-level call (client to server), or was embedded in a procedure (such as PL/SQL) at the server. An embedded commit state indicates that while a commit completed, the entire procedure in which the commit executed has not yet run to completion. Any work beyond the commit cannot be guaranteed to have completed until that procedure itself returns to the database engine.

  • Identification of whether the database to which the commit resolution is directed is ahead of, in sync with, or behind the original submission, and rejection when there are gaps in the submission sequence of transactions from a client. It is considered an error to attempt to obtain an outcome if the server or client are not in sync on an LTXID sequence.

  • A callback on the JDBC Thin client driver that fires when the LTXID changes. This can be used by higher layer applications such as WebLogic Server and third parties to maintain the current LTXID ready to use if needed.

  • Namespace uniqueness across globally disparate databases and across databases that are consolidated into a container (pluggable) infrastructure. This includes Oracle Real Application Clusters (Oracle RAC), Data Guard, and pluggable databases.

  • Service name uniqueness across global databases and across databases that are consolidated into a container (pluggable databases) infrastructure. This ensures that connections are properly directed to the transaction information.

25.3 Transaction Guard Concepts and Scope

This section explains some key concepts for Transaction Guard, and what Transaction Guard covers and does not cover.

Topics:

See Also:

25.3.1 Logical Transaction Identifier (LTXID)

Applications use a concept called the logical transaction identifier (LTXID) to determine the outcome of the last transaction open in a database session following an outage. The logical transaction ID is stored in the OCI session handle and in a connection object for the JDBC Thin and ODP.Net drivers. The logical transaction ID is the foundation of the at-most-once semantics.

The Transaction Guard protocol ensures that:

  • Execution of each logical transaction is unique.

  • Duplication is detected at supported commit time to ensure that for all commit points, the protocol must not be circumvented.

  • When the transaction is committed, the logical transaction ID is persisted for the duration of the retention period for retries (default = 24 hours, maximum = 30 days).

  • When obtaining the outcome, an LTXID is blocked to ensure that an earlier in-flight version of that LTXID cannot commit, by enforcing the uncommitted status. If the earlier version with the same LTXID was already committed or forced, then blocking the LTXID returns the same result.

The logical session number is automatically assigned at session establishment. It is an opaque structure that cannot be read by an application. For scalability, each LTXID carries a running number called the commit number, which is increased when a database transaction is committed for each round trip to the database. This running commit number is zero-based.

25.3.2 At-Most-Once Execution

Transaction Guard uses the logical transaction identifier (LTXID) to avoid duplicate transactions. This ability to ensure at most one execution of a transaction is referred to as transaction idempotence. The LTXID is persisted on commit and is reused following a rollback. During normal runtime, an LTXID is automatically held in the session at both the client and server for each database transaction. At commit, the LTXID is persisted as part of committing the transaction.

The at-most-once protocol requires that the database maintain the LTXID for the retention period agreed for replay. The default retention period is 24 hours, although you might need a shorter or longer period, conceivably even a week or longer. The longer the retention period, the longer the at-most-once check blocks an old transaction using an old LTXID from replay. The setting is available on each service. When multiple databases are involved, as is the case when using Data Guard and Active Data Guard, the LTXID is replicated to each database involved through the use of redo.

The getLTXID API, provided for Oracle JDBC Thin (with similar APIs for OCI, OCCI, and ODP.Net clients), lets an application retrieve the logical transaction identifier that was in use on the dead session. This is needed to determine the status of this last transaction.

The DBMS_APP_CONT.GET_LTXID_OUTCOME PL/SQL subprogram (described in Oracle Database PL/SQL Packages and Types Reference) lets an application find the outcome of an action for a specified logical transaction identifier. Calling DBMS_APP_CONT.GET_LTXID_OUTCOME may involve the server blocking the LTXID from committing so that the outcome is known. This is a requirement if a transaction using that LTXID is in flight or is about to commit. An application using Transaction Guard obtains the LTXID following a recoverable error, and then calls DBMS_APP_CONT.GET_LTXID_OUTCOME before attempting a replay.

25.3.3 Transaction Guard Coverage

Transaction Guard supports commits from these sources:

  • Local transactions

  • Data definition language (DDL) transactions

  • Data control language (DCL) transactions

  • Distributed transactions

  • Remote transactions

  • Parallel transactions

  • Commit on success (auto-commit)

  • PL/SQL with embedded commit-supported client drivers

  • Supported client drivers:

    • 12c JDBC type 4 driver

    • 12c OCI and OCCI client drivers

    • 12c Oracle Data Provider for .NET (ODP.NET) client driver

25.3.4 Transaction Guard Exclusions

Transaction Guard intentionally excludes recursive transactions and autonomous transactions so that they can be replayed.

As of Oracle Database 12c Release 1 (12.1.0.1), Transaction Guard also excludes:

  • XA transactions

  • Active Data Guard with read/write database links for forwarding transactions

  • Golden Gate and Logical Standby

25.4 Database Configuration for Transaction Guard

This section contains information relevant to configuring the database for using Transaction Guard.

Topics:

25.4.1 Configuration Checklist

To use Transaction Guard with an application, you must do the following:

  • Use Oracle Database 12c Release 1 (12.1.0.1) or later.

  • Use an application service for all database work. Create the service using the srvctl command if you are using Oracle RAC, or using the DBMS_SERVICE.CREATE_SERVICE PL/SQL subprogram if you are not using Oracle RAC.

    Do not use the default database services, because these services are for administration purposes and cannot be manipulated. That is, do not use a service name that is set to db_name or db_unique_name.

  • Grant permission on the DBMS_APP_CONT package to the database users who will call GET_LTXID_OUTCOME:

    GRANT EXECUTE ON DBMS_APP_CONT TO <user-name>;
    

To use Transaction Guard with an application, Oracle recommends that you do the following:

  • Locate and define the transaction history table for optimal performance. (The transaction history table is described in Section 25.4.2.)

  • If you are using Oracle RAC or Oracle Data Guard, ensure that FAN is configured to communicate to interrupt clients fast on error.

  • Set the following parameter: AQ_HA_NOTIFICATIONS = TRUE (if using OCI FAN).

25.4.2 Transaction History Table

The transaction history table maintains the mapping of logical transaction identifiers (LTXIDs) to database transaction. This table can be accessed only by databases users with DBA privileges. It is maintained automatically by Oracle Database, and users must not issue DDL or DML statements directly against the transaction history table.

The transaction history table (LTXID_TRANS) is created by default in the SYSAUX tablespace at database creation and upgrade. New partitions are added when instances are added, using the storage of the last partition. However, if the location of this tablespace is not optimal for performance, the DBA can move partitions to another tablespace. For example, the following statement alters the transaction history table to move it to a tablespace named FastPace:

ALTER TABLE LTXID_TRANS move partition LTXID_TRANS_4
 tablespace FastPace
 storage ( initial 10G next 10G
 minextents 1 maxextents 121 );

See Also:

Oracle Database SQL Language Reference for information about the ALTER TABLE statement

25.4.3 Service Parameters

Configure the services for commit outcome and retention. For example:

COMMIT_OUTCOME = TRUE
RETENTION_TIMEOUT = <retention-value>

COMMIT_OUTCOME determines whether transaction commit outcome is accessible after the commit has executed. This feature makes the outcome of the commit durable, and it is used by applications to enforce the status of the last transaction executed before an outage. The feature is used internally by the Oracle replay driver and by WebLogic Server, and it is available for use by other applications to determine an outcome. The COMMIT_OUTCOME possible values are FALSE (the default) and TRUE, and the value must be TRUE for Transaction Guard to be in effect.

The following considerations apply to COMMIT_OUTCOME:

  • Using the DBMS_APP_CONT.GET_LTXID_OUTCOME PL/SQL procedure requires that COMMIT_OUTCOME be TRUE.

  • COMMIT_OUTCOME has no effect on Active Data Guard and read-only databases. Using Transaction Guard with read/write Active Data Guard combined with DML forwarding is not supported.

  • COMMIT_OUTCOME is allowed on user-defined database services. Use on the database service is excluded because this service does not switch across Data Guard and cannot be started, stopped, or disabled for planned outages at the primary database.

RETENTION_TIMEOUT is used in conjunction with COMMIT_OUTCOME to set the amount of time that the commit outcome is retained. The retention timeout value is specified in seconds; the default is 86400 (24 hours), and the maximum is 2592000 (30 days). You can use the srvctl command or the DBMS_SERVICE PL/SQL package to specify the retention timeout value.

If you are using Oracle RAC or Oracle RAC One, use the srvctl command to create and modify services. Example 25-1 and Example 25-2 show the use of srvctl. You can also use Global Data Services (GDSCTL).

Example 25-1 Adding and Modifying a Service for a Server Pool

srvctl add service -database orcl -service GOLD -poolname ora.Srvpool  -commit_outcome TRUE  -retention 604800
srvctl modify service -database orcl -service GOLD -commit_outcome TRUE  -retention 604800

Example 25-2 Adding an Administrator-Managed Service

srvctl add service -database codedb -service GOLD -prefer serv1 -available serv2 -commit_outcome TRUE  -retention 604800

If you are using a single-instance database, use the DBMS_SERVICE.MODIFY_SERVICE PL/SQL procedure to modify services and use FAN. Example 25-3 modifies a service (but substitute the actual service name for <service-name>).

Example 25-3 Modifying a Service (PL/SQL)

DECLARE
  params dbms_service.svc_parameter_array;
BEGIN
  params('COMMIT_OUTCOME'):='true';
  params('RETENTION_TIMEOUT'):=604800;
  dbms_service.modify_service('<service-name>',params);
END;
/

See Also:

25.5 Developing Applications that Use Transaction Guard

To use Transaction Guard, review the requirements and recommendations in Section 25.4.1, "Configuration Checklist," and follow these steps in the error handling when a recoverable error occurs:

  1. Check that the error is a recoverable error that has made the database session unavailable.

  2. Acquire the LTXID from the previous failed session using the client driver provided APIs (getLTXID for JDBC, OCI_ATTR_GET with LTXID for OCI, and LogicalTransactionId for ODP.net).

  3. Acquire a new session with that sessions' own LTXID.

  4. Invoke the DBMS_APP_CONT.GET_LTXID_OUTCOME PL/SQL procedure with the LTXID obtained from the API. The return state tells the driver if the last transaction was COMMITTED (TRUE/FALSE) and USER_CALL_COMPLETED (TRUE/FALSE). This PL/SQL function returns an error if the client and database are out of sync (for example, not the same database or restored database).

  5. The application can return the result to the user to decide. An application can replay itself. If the replay itself incurs an outage, then the LTXID for the replaying session is used for the DBMS_APP_CONT.GET_LTXID_OUTCOME procedure.

Topics:

25.5.1 Typical Transaction Guard Usage

The following pseudocode shows a typical usage of Transaction Guard:

Receive a FAN down event (or recoverable error)
 
FAN aborts the dead session
 
If recoverable error  (new OCI_ATTRIBUTE for OCI, isRecoverable for JDBC)
     Get last LTXID from dead session using getLTXID or from your callback
     Obtain a new session 
     Call GET_LTXID_OUTCOME with last LTXID to obtain COMMITTED and USER_CALL_COMPLETED status
 
If COMMITTED and USER_CALL_COMPLETED  
     Then return result
 
ELSEIF COMMITTED and NOT USER_CALL_COMPLETED   
     Then return result with a warning (that details such as out binds or row count were not returned)
 
ELSEIF NOT COMMITTED 
     Cleanup and resubmit request, or return uncommitted result to the client

25.5.2 Details for Using the LTXID

For replay and returning results, the application or third party container needs access to the next LTXID to be committed at the server for each session. The LTXID can be obtained using APIs (getLTXID for JDBC and OCI_ATTR_GET with LTXID for OCI) from a failed session after a recoverable outage.

The JDBC Thin driver also provides a callback that executes on each commit number change received from the database. A third party container can use this callback to save the current LTXID in preparation to use if failover is needed. Within each session, the current LTXID is in use, so the callback can override earlier ones.

If failovers cascade without completing (that is, if during recovery from one failure, another failure occurs), the application must obtain and then pass the LTXID in effect on the current session into GET_LTXID_OUTCOME.

Table 25-1 shows several conditions or situations that require some LTXID-related action, and for each the application action and next LTXID to use.

Table 25-1 LTXID Condition or Situation, Application Actions, and Next LTXID to Use

Condition or Situation Application Action Next LTXID to Use (Callback on LTXID Change for Containers - JDBC Thin Only)

Application receives a recoverable error and calls GET_LTXID_OUTCOME to determine the transaction status.

Application takes a new connection (with its own LTXID-B 0) and calls GET_LTXID_OUTCOME with the LTXID of the last failed session (LTXID-A ).

New LTXID-B 0

Also set using the JDBC callback when registered

Application finds that the last session transaction status is COMMITTED and USER_CALL_COMPLETED.

Returns committed status to client; the application may be able to continue.

(Not applicable)

Application finds that the last session transaction status is COMMITTED and NOT USER_CALL_COMPLETED.

Returns committed status to client and exits - some applications cannot progress as the work in the call is not complete. (for example, an out bind or row count was not returned). Whether the application can continue is application dependent.

(Not applicable)

Application finds that the last session transaction status is NOT COMMITTED.

Application returns the result to the user, or cleans up if needed, and resubmits with the LTXID on the new session in effect, LTXID-B 0.

If the new request executes any commits, server returns commit messages with LTXID-B 2 and increasing.

New LTXID-B 2 .. N

Also set using the JDBC callback when registered

Application receives a recoverable error if it has decided to replay.

Application takes a new connection (with LTXID-C 0) and calls GET_LTXID_OUTCOME with the LTXID of LAST session (LTXID-B N).

LTXID-C 0 on the new session.

Also set using the JDBC callback when registered

Application receives another recoverable error if it has decided to replay.

Application takes a new connection (with LTXID-D 0) and calls GET_LTXID_OUTCOME again with the LTXID of LAST session (LTXID-C N).

LTXID-D 0 on the new session.

Also set using the JDBC callback when registered


25.5.3 Connection-Pool LTXID Usage

Connection pools create a different use case for managing LTXIDs because connections and sessions are preestablished and shared. In the simplest model for connection pools and middle tiers, an LTXID exists on each session handle. It is associated with an application request at check-out from the connection pool, and is disassociated from the application request at check-in back to the pool. Between check-out and check-in, the LTXID on the session is exclusively held by that application request. After check-in, the LTXID belongs to an idle, pooled session. It is associated with the next application request that checks-out that connection.

Using Transaction Guard in this way:

  • Can support duplicate detection and failover for the present HTTP request

  • Might support basic replay by a third party container – replaying the last request following a nonrecoverable outage

25.5.4 Additional Requirements for Transaction Guard Development

Transaction Guard is a tool for developers to use after recoverable errors to provide a known outcome. It must be used when an error is returned indicating that the last session is dead.

The Transaction Guard APIs must not be used in the following cases:

  • Do not use GET_LTXID_OUTCOME on the current session. It will return an error.

  • Do not use GET_LTXID_OUTCOME against a session that did not receive a recoverable error—that is, a live session. It will block that session from committing.

  • Do not use GET_LTXID_OUTCOME from a different user or to a different database. It will return an error.

  • Do not obtain the LTXID and save it for use later, as opposed to using it immediately. The result of GET_LTXID_OUTCOME is valid only for the last open or completed transaction. If it is used with an earlier transaction on the same session, it will return an error.

25.6 Transaction Guard and Its Relationship to Application Continuity

Transaction Guard provides a unique identifier (LTXID) for each database transaction. This identifier can be used to query the commit outcome of the transaction, and can also be used to ensure that the transaction is applied only once. Transaction Guard is used by Application Continuity and automatically enabled by it, but it can also be enabled independently. Transaction Guard prevents the transaction being replayed by Application Continuity from being applied more than once. If the application has implemented an application-level replay, then it requires the application to be integrated with transaction guard to provide idempotence.

For automatic replay, your application must use Application Continuity.

See Also: