For a broker to recover in case of failure, it needs to re-create the state of its message delivery operations. To do this, the broker must save state information to a persistent data store. When the broker restarts, it uses the saved data to re-create destinations and durable subscriptions, recover persistent messages, roll back open transactions, and rebuild its routing table for undelivered messages. It can then resume message delivery.
A persistent data store is thus a key aspect of providing for reliable message delivery. This chapter describes the two different persistence implementations supported by the Message Queue broker and how to set each of them up:
A broker's persistent data store holds information about physical destinations, durable subscriptions, messages, transactions, and acknowledgments.
Message Queue supports both file-based and JDBC-based persistence modules, as shown in the following figure. File-based persistence uses individual files to store persistent data; JDBC-based persistence uses the Java Database Connectivity (JDBC) interface to connect the broker to a JDBC-based data store. While file-based persistence is generally faster than JDBC-based persistence, some users prefer the redundancy and administrative control provided by a JDBC database. The broker configuration property
imq.persist.store (see Table 17-5) specifies which of the two persistence modules (
jdbc) to use.
Message Queue brokers are configured by default to use a file-based persistent store, but you can reconfigure them to plug in any data store accessible through a JDBC-compliant driver. The broker configuration property
imq.persist.store (see Table 17-5) specifies which of the two forms of persistence to use.
By default, Message Queue uses a file-based data store, in which individual files store persistent data (such as messages, destinations, durable subscriptions, transactions, and routing information).
The file-based data store is located in a directory identified by the name of the broker instance (instanceName) to which the data store belongs:
Each destination on the broker has its own subdirectory holding messages delivered to that destination.
Because the data store can contain messages of a sensitive or proprietary nature, you should secure the
/fs370 directory against unauthorized access; see Securing a File-Based Data Store.
Broker configuration properties related to file-based persistence are listed under File-Based Persistence Properties. These properties let you configure various aspects of how the file-based data store behaves.
All persistent data other than messages is stored in separate files: one file each for destinations, durable subscriptions, and transaction state information. Most messages are stored in a single file consisting of variable-size records. You can compact this file to alleviate fragmentation as messages are added and removed (see Managing Physical Destination Disk Utilization). In addition, messages above a certain threshold size are stored in their own individual files rather than in the variable-sized record file. You can configure this threshold size with the broker property
The broker maintains a file pool for these individual message files: instead of being deleted when it is no longer needed, a file is returned to the pool of free files in its destination directory so that it can later be reused for another message. The broker property
imq.persist.file.destination.message.filepool.limit specifies the maximum number of files in the pool. When the number of individual message files for a destination exceeds this limit, files will be deleted when no longer needed instead of being returned to the pool.
When returning a file to the file pool, the broker can save time at the expense of storage space by simply tagging the file as available for reuse without deleting its previous contents. You can use the
imq.persist.file.message.filepool.cleanratio broker property to specify the percentage of files in each destination's file pool that should be maintained in a "clean" (empty) state rather than simply marked for reuse. The higher you set this value, the less space will be required for the file pool, but the more overhead will be needed to empty the contents of files when they are returned to the pool. If the broker's
imq.persist.file.message.cleanup property is
true, all files in the pool will be emptied at broker shutdown, leaving them in a clean state; this conserves storage space but slows down the shutdown process.
In writing data to the data store, the operating system has some leeway in whether to write the data synchronously or "lazily" (asynchronously). Lazy storage can lead to data loss in the event of a system crash, if the broker believes the data to have been written to the data store when it has not. To ensure absolute reliability (at the expense of performance), you can require that all data be written synchronously by setting the broker property
true. In this case, the data is guaranteed to be available when the system comes back up after a crash, and the broker can reliably resume operation.
A file-based data store is automatically created when you create a broker instance. However, you can configure the data store using the properties described in File-Based Persistence Properties.
For example, by default, Message Queue performs asynchronous write operations to disk. However, to attain the highest reliability, you can set the broker property
imq.persist.file.sync to write data synchronously instead. See Table 17-6.
When you start a broker instance, you can use the
-- reset option to clear the file-based data store. For more information about this option and its suboptions, see Broker Utility.
The persistent data store can contain, among other information, message files that are being temporarily stored. Since these messages may contain proprietary information, it is important to secure the data store against unauthorized access. This section describes how to secure data in a file-based data store.
A broker using file-based persistence writes persistent data to a flat-file data store:
where instanceName is a name identifying the broker instance. This directory is created when the broker instance is started for the first time. The procedure for securing this directory depends on the operating system platform on which the broker is running:
On Solaris and Linux, the directory's permissions are determined by the file mode creation mask (
umask) of the user who started the broker instance. Hence, permission to start a broker instance and to read its persistent files can be restricted by setting the mask appropriately. Alternatively, an administrator (superuser) can secure persistent data by setting the permissions on the
instances directory to
On Windows, the directory's permissions can be set using the mechanisms provided by the Windows operating system. This generally involves opening a Properties dialog for the directory.
Because many activities can occur during a transaction, persisting a transaction's state over the complete life cycle of the transaction can adversely affect overall performance, especially when the
imq.persist.file.sync.enabled property is set to
true to avoid data loss in case of a system crash.
Message Queue provides a transaction logging mechanism that can improve performance of transaction persistence. This transaction log offers tuning parameters that can improve performance of file-based persistence for other objects, such as message payloads.
To enable this transaction logging mechanism, set the
imq.persist.file.newTxnLogenabled broker property to
After enabling the transaction log, essential changes to the state of a JMS transaction are written to the transaction log. When the transaction is committed, all details regarding it are gathered and written to the persistent store. Additionally, the logging mechanism periodically performs a "checkpoint" operation to ensure that the persistent store and the transaction log are synchronized and that the log size remains manageable.
As a further refinement, the operation of the logging mechanism is subject to the value of the
imq.persist.file.sync.enabled broker property:
true, write operations to the transaction log are written synchronously to disk. Non-transacted message and non-transacted message acknowledgements are also written synchronously to the transaction log before being written asynchronously to the persistent store.
false, write operations to the transaction log are written asynchronously to disk. Non-transacted message and non-transacted message acknowledgements are not written to the transaction log.
The tuning parameters supported by the transaction logging mechanism are:
Information about these parameters can be found in Table 17-7.
Instead of using a file-based data store, you can set up a broker to access any data store accessible through a JDBC-compliant driver. This involves setting the appropriate JDBC-related broker configuration properties and using the Database Manager utility (
imqdbmgr) to create the proper database schema. See Configuring a JDBC-Based Data Store for specifics.
The full set of properties for configuring a broker to use a JDBC database are listed in Table 17-8. You can specify these properties either in the instance configuration file (
config.properties) of each broker instance or by using the
-D command line option to the Broker utility (
imqbrokerd) or the Database Manager utility (
In practice, however, JDBC properties are preconfigured by default, depending on the database vendor being used for the data store. The property values are set in the
default.properties file, and only need to be explicitly set if you are overriding the default values. In general, you only need to set the following properties:
This property specifies that a JDBC-based data store (as opposed to the default file-based data store) is used to store persistent data.
This property identifies the database vendor being used for the data store; all of the remaining properties are qualified by this vendor name.
This property specifies the user name to be used by the broker in accessing the database.
This property specifies the password for accessing the database, if required;
.needpassword is a boolean flag specifying whether a password is needed. For security reasons, the database access password should be specified only in a password file referenced with the
-passfile command line option; if no such password file is specified, the
imqdbmgr commands will prompt for the password interactively.
This set of properties represents any additional, vendor-specific properties that are required.
Specifies the vendor-specific options passed to the database when creating the table schema.
imq.persist.store=jdbc imq.persist.jdbc.dbVendor=mysql imq.persist.jdbc.mysql.user=userName imq.persist.jdbc.mysql.password=password imq.persist.jdbc.mysql.property.url=jdbc:mysql://hostName:port/dataBase
If you expect to have messages that are larger than 1 MB, configure MySQL's
max_allowed_packet variable accordingly when starting the database. For more information see Appendix B of the MySQL 5.0 Reference Manual.
In addition, in an enhanced broker cluster, in which a JDBC database is shared by multiple broker instances, each broker must be uniquely identified in the database (unnecessary for an embedded database, which stores data for only one broker instance). The configuration property
imq.brokerid specifies a unique instance identifier to be appended to the names of database tables for each broker. See Enhanced Broker Cluster Properties.
After setting all of the broker's needed JDBC configuration properties, you must also install your JDBC driver's
.jar file in
IMQ_HOME/lib/ext and then create the database schema for the JDBC-based data store (see To Set Up a JDBC-Based Data Store).
To configure a broker to use a JDBC database, you set JDBC-related properties in the broker's instance configuration file and create the appropriate database schema. The Message Queue Database Manager utility (
imqdbmgr) uses your JDBC driver and the broker configuration properties to create the schema and manage the database. You can also use the Database Manager to delete corrupted tables from the database or if you want to use a different database as a data store. See Database Manager Utility for more information.
If you use an embedded database, it is best to create it under the following directory:
If an embedded database is not protected by a user name and password, it is probably protected by file system permissions. To ensure that the database is readable and writable by the broker, the user who runs the broker should be the same user who created the embedded database using the
Set JDBC-related properties in the broker's instance configuration file.
The relevant properties are discussed, with examples, in JDBC-Based Persistence Properties and listed in full in Table 17-8. In particular, you must specify a JDBC-based data store by setting the broker's
imq.persist.store property to
Place a copy of, or a symbolic link to, your JDBC driver's
.jar file in
IMQ_HOME/lib/ext, the Message Queue external resource files directory.
Check the message table schema for your database in
default.properties file, locate the message table schema for your database by searching for "imq.persist.jdbc.vendor.table.MQMSG". Read any commentary notes about the schema and check that the maximum message size is sufficient; if it is not, adjust the size of the MESSAGE column as needed.
Create the database schema needed for Message Queue persistence.
imqdbmgr create all command (for an embedded database) or the
imqdbmgr create tbl command (for an external database); see Database Manager Utility.
Change to the directory where the Database Manager utility resides:
imqdbmgr create all
You can display information about a JDBC-based data store using the Database Manager utility (
imqdbmgr) as follows:
Change to the directory where the Database Manager utility resides:
The output should resemble the following
dbmgr query [04/Oct/2005:15:30:20 PDT] Using plugged-in persistent store: version=400 brokerid=Mozart1756 database connection url=jdbc:oracle:thin:@Xhome:1521:mqdb database user=scott Running in standalone mode. Database tables have already been created.
The persistent data store can contain, among other information, message files that are being temporarily stored. Since these messages may contain proprietary information, it is important to secure the data store against unauthorized access. This section describes how to secure data in a JDBC-based data store.
A broker using JDBC-based persistence writes persistent data to a JDBC-compliant database. For a database managed by a database server (such as Oracle), it is recommended that you create a user name and password to access the Message Queue database tables (tables whose names start with
MQ). If the database does not allow individual tables to be protected, create a dedicated database to be used only by Message Queue brokers. See the documentation provided by your database vendor for information on how to create user name/password access.
The user name and password required to open a database connection by a broker can be provided as broker configuration properties. However it is more secure to provide them as command line options when starting up the broker, using the
-dbpassword options (see Broker Utility).
For an embedded database that is accessed directly by the broker by means of the database's JDBC driver, security is usually provided by setting file permissions on the directory where the persistent data will be stored, as described above under Securing a File-Based Data Store To ensure that the database is readable and writable by both the broker and the Database Manager utility, however, both should be run by the same user.
Changes in the file formats for both file-based and JDBC-based persistent data stores were introduced in Message Queue 3.7, with further JDBC changes in version 4.0 and 4.1. As a result of these changes, the persistent data store version numbers have been updated to 370 for file-based data stores and 410 for JDBC-based stores. You can use the
query command to determine the version number of your existing data store.
On first startup, the Message Queue Broker utility (
imqbrokerd) will check for the presence of an older persistent data store and automatically migrate it to the latest format:
File-based data store versions 200 and 350 are migrated to the version 370 format.
JDBC-based data store versions 350, 370, and 400 are migrated to the version 410 format. (If you need to upgrade a version 200 data store, you will need to step through an intermediate Message Queue 3.5 or 3.6 release.)
The upgrade leaves the older copy of the persistent data store intact, allowing you to roll back the upgrade if necessary. To do so, you can uninstall the current version of Message Queue and reinstall the earlier version you were previously running. The older version's message brokers will locate and use the older copy of the data store.
Beginning in Message Queue 4.5, the
imq.persist.file.newTxnLog property is
true by default. This setting can generate an error when starting a broker that is using an older persistent data store. To resolve the error, set the property value to
false and start the broker, thus migrating the data store to the latest format. Then, you can stop the broker, set the property value back to
true, and start the broker without encountering an error.