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Else, commit (that's what just happened): else session.commit( ); } return; Otherwise, just continue on and process the message. Java Message Service Point-to-point queues have the same transactional capabilities as publish-and-subscribe, with one caveat: when a rollback( ) occurs on a QueueSession, the messages are placed back on the queue. Because a queue may have multiple recipients, the messages could now go to another consuming client. This may result in messages being processed out of order, since the other client could have processed newer messages while the older messages were being rolled back. This is an area where behavior may vary, depending on how your vendor interprets the standard. Finally, you may have noticed that we used Wholesaler and Retailer for this example instead of their "Q" counterparts. This is because a publish-and-subscribe session and point-to-point session cannot jointly particpate in a JMS transaction. JMS transactions cannot span multiple sessions without the use of a JTA-compliant transaction manager (see the next section, Section 6.4.3). We wanted to show the use of JMS as first-class middleware in its own right, without using a supporting application server just to provide a shared transaction between a p2p and a pub/sub session. The inability of p2p and pub/sub sessions to share a transaction is a weakness in JMS that may be addressed in a future version of the specification. 6.4.3 Distributed Transactions Distributed systems sometimes use a two-phase commit (2PC) process that allows multiple distributed resources to participate in one transaction. This typically involves an underlying transaction manager that takes care of coordinating the prepare, commit, or rollback of each resource participating in the transaction. In most cases, the resources involved in the transaction are databases, but they can be other things, like JMS providers. Transactions can be either local transactions or global transactions. Local transactions involve work performed on a single resource: one database or JMS provider. Global transactions involve work performed across several different resources, i.e., some combination of databases and JMS providers. JMS provides transaction facilities for both local and global transactions. The transacted sessions discussed previously are local transactions in JMS; they involve a single JMS provider. The 2PC protocol is designed to facilitate global transactions, transactions that span multiple resources. As an example, an enterprise application may need to process (consume and produce) messages as well as make changes to a database. In some cases, the processing of messages and database updates needs to be treated as a single unit of work, so that a failure to update the database or consume a message will cause the entire unit of work to fail. This is the basic premise behind a transaction: all the tasks must complete or fail together. To create a unit of work that spans different resources, the resources must be able to cooperate with a transaction manager in a 2PC. 102
Java Message Service The 2PC protocol is used by a transaction manager to coordinate the interactions of resources in a global transaction. A resource can only participate in a global transaction if it supports the 2PC protocol, which is usually implemented using the XA interface developed by The Open Group. In the Java enterprise technologies, the XA interface is implemented by the Java Transaction API and XA interfaces (javax.transaction and javax.transaction.xa). Any resource that implements these interfaces can be enrolled in a global transaction by a transaction manager that supports these interfaces. JMS providers that implement the JTA XA APIs can therefore participate as a resource in two-phase commit. The JMS specification provides XA versions of the following JMS objects: XAConnectionFactory, XAQueueConnection, XAQueueConnectionFactory, XAQueueSession, XASession, XATopicConnection, XATopicConnectionFactory, and XATopicSession. Each of these objects works like its corresponding non-XA-compliant object. The XATopicSession, for example, provides the same methods as the TopicSession. An application server's transaction manager uses these XA interfaces directly, but a JMS client only sees the nontransactional versions. The following code shows a JMS client that uses JDBC and JMS together in one transaction managed by an external transaction manager. JNDI is used to obtain all the resources including the JTA TransactionManager, the JDBC XADataSource, and the JMS XATopicConnectionFactory: InitialContext jndiContext = new InitialContext(env); // Obtain the JTA TransactionManager a JNDI namespace TransactionManager txMngr =jndiContext.lookup("../../tx/txmngr"); // Start the Global Transaction txMngr.begin( ); // Get the transaction object that represents the new global transaction Transaction transaction = txMnger.getTransaction( ); // Obtain the JDBC DataSource from a JNDI namespace XADataSource dataSrc = jndiContext.lookup("../../jdbc/data"); // Obtain an XA-compliant Connection XAConnection jdbcCon = dataSrc.getConnection( ); // Obtain the XAResource from the connection XAResource jdbcXA = jdbcCon.getXAResource( ); /* * Enlist the XAResource in the transaction. This adds the JDBC * resource to the global transaction. */ transaction.enlist(jdbcXA); //... do some JDBC work // Obtain an XA-compliant TopicConnectionFactory from a JNDI namespace XATopicConnectionFactory factory = jndiContext.lookup("../.../jms/factory"); // Obtain an XA-compliant TopicConnection XATopicConnection jmsCon = factory.createXATopicConnection( ); // Obtain an XA-compliant Session XATopicSession session = jmsCon.createXATopicSession( ); // Obtain the XAResource from the session 103
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Java Message Service Richard Monson
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Chapter 7. Deployment Consideration
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Java Message Service Although this
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Java Message Service Examples devel
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Java Message Service Chapter 1. Und
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Java Message Service Before we disc
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Java Message Service to naming and
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1.3.1 Enterprise Application Integr
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Java Message Service Figure 1.5. Dy
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Java Message Service With J2EE, JSP
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Java Message Service recognition of
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TopicConnectionFactory conFactory =
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Java Message Service Before examini
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Understanding JNDI Java Message Ser
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Java Message Service The TopicConne
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protected void writeMessage(String
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Java Message Service When the Topic
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Java Message Service publishing a m
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Java Message Service Messages come
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3.1.1.3 JMSMessageID Java Message S
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3.2 Properties Java Message Service
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Java Message Service The following
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Notification of certain bids on inv
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Java Message Service When a consume
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UTF-8 Java Message Service UTF-8 en
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Java Message Service Table 3.1 show
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Java Message Service Essentially, M
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A.2.6 QueueSender Java Message Serv
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} throws JMSException; public Conne
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Appendix B. Message Headers Java Me
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Java Message Service are delivered
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Java Message Service JMSTimestamp P
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UTC Java Message Service UTC (Unive
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Java Message Service JMSReplyTo Pur
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Java Message Service JMSType Purpos
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} public void setByteProperty(Strin
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Java Message Service Each of the ac
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JMSXAppID Java Message Service This
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Appendix D. Message Selectors Java
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The BETWEEN operator can be used to
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D.5 Declaring a Message Selector Ja
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Colophon Java Message Service Our l
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