Chapter 3 Message Queue Clients: Design and Features

This chapter addresses architectural and configuration issues that depend upon Message Queue’s implementation of the Java Message Specification. It covers the following topics:

• Client Design Considerations

• Managing Client Threads

• Managing Memory and Resources

• Programming Issues for Message Consumers

• Factors Affecting Performance

• Client Connection Failover (Auto-Reconnect)

• Custom Client Acknowledgment

• Communicating with C Clients

Client Design Considerations

The choices you make in designing a JMS client affect portability, allocating work between connections and sessions, reliability and performance, resource use, and ease of administration. This section discusses basic issues that you need to address in client design. It covers the following topics:

• Developing Portable Clients

• Choosing Messaging Domains

• Connections and Sessions

• Producers and Consumers

• Balancing Reliability and Performance

Developing Portable Clients

The Java Messaging Specification was developed to abstract access to message-oriented middleware systems (MOMs). A client that writes JMS code should be portable to any provider that implements this specification. If code portability is important to you, be sure that you do the following in developing clients:

• Make sure your code does not depend on extensions or features that are specific to Message Queue.

• Look up, using JNDI, (rather than instantiate) administered objects for connection factories and destinations.

  Administered objects encapsulate provider-specific implementation and configuration information. Besides allowing for portability, administered objects also make it much easier to share connection factories between applications and to tune a JMS application for performance and resource use. So, even if portability is not important to you, you might still want to leave the work of creating and configuring these objects to an administrator. For more information, see Looking Up a Connection Factory With JNDI and Looking Up a Destination With JNDI.

Choosing Messaging Domains

As described in Chapter 2, Client Programming Model, in Sun Java System Message Queue 3.7 UR1 Technical Overview, JMS supports two distinct message delivery models: point-to-point and publish/subscribe. These two message delivery models can be handled using different API objects—with slightly different semantics—representing different programming domains, as shown in Table 3–1, or they can be handled by base (unified domain) types.

Using the point-to-point or publish/subscribe domains offers the advantage of a clean API that prevents certain types of programming errors; for example, creating a durable subscriber for a queue destination. However, the non-unified domains have the disadvantage that you cannot combine point-to-point and publish/subscribe operations in the same transaction or in the same session. If you need to do that, you should choose the unified domain API.

The JMS 1.1 specification continues to support the separate JMS 1.02 programming domains. (The example applications included with the Message Queue product as well as the code examples provided in this book all use the separate JMS 1.02 programming domains.) You can choose the API that best suits your needs. The only exception are those developers needing to write clients for the Sun Java System Application Server 7 environment, as explained in the following note.

Developers of applications that run in the Sun Java System Application Server 7 environment are limited to using the JMS 1.0.2 API. This is because Sun Java System Application Server 7 complies with the J2EE 1.3 specification, which supports only JMS 1.0.2. Any JMS messaging performed in servlets and EJBs—including message-driven beans must be based on the domain-specific JMS APIs and cannot use the JMS 1.1 unified domain APIs. Developers of J2EE applications that will run in J2EE 1.4-compliant servers can, however, use the simpler JMS 1.1 APIs.

Connections and Sessions

A connection is a relatively heavy-weight object because of the authentication and communication setup that must be done when a connection is created. For this reason, it’s a good idea to use as few connections as possible. The real allocation of work occurs in sessions, which are light-weight, single-threaded contexts for producing and consuming messages. When you are thinking about structuring your client, it is best to think of the work that is done at the session level.

A session

• Is a factory for its message producers and consumers

• Supplies provider-optimized message factories

• Supports a single series of transactions that combine work spanning its producers and consumers into atomic units

• Defines a serial order for the messages it consumes and the messages it produces

• Retains messages until they have been acknowledged

• Serializes execution of message listeners registered with its message consumers

The requirement that sessions be operated on by a single thread at a time places some restrictions on the combination of producers and consumers that can use the same session. In particular, if a session has an asynchronous consumer, it may not have any other synchronous consumers. For a discussion of the connection and session’s use of threads, see Managing Client Threads. With the exception of these restrictions, let the needs of your application determine the number of sessions, producers, and consumers.

Producers and Consumers

Aside from the reliability your client requires, the design decisions that relate to producers and consumers include the following:

• Do you want to use a point-to-point or a publish/subscribe domain?

  There are some interesting permutations here. There are times when you would want to use publish/subscribe even when you have only one subscriber. On the other hand, performance considerations might make the point-to-point model more efficient than the publish/subscribe model, when the work of sorting messages between subscribers is too costly. Sometimes You cannot make these decisions cannot in the abstract, but must actually develop and test different prototypes.

• Are you using an asynchronous message consumer that does not receive messages often or a producer that is seldom used?

  Let the administrator know how to set the ping interval, so that your client gets an exception if the connection should fail. For more information see Using the Client Runtime Ping Feature.

• Are you using a synchronous consumer in a distributed application?

  You might need to allow a small time interval between connecting and calling the receiveNoWait() method in order not to miss a pending message. For more information, see Synchronous Consumption in Distributed Applications.

• Do you need message compression?

  Benefits vary with the size and format of messages, the number of consumers, network bandwidth, and CPU performance; and benefits are not guaranteed. For a more detailed discussion, see Message Compression.

Assigning Client Identifiers

A connection can have a client identifier. This identifier is used to associate a JMS client’s connection to a message service, with state information maintained by the message service for that client. The JMS provider must ensure that a client identifier is unique, and applies to only one connection at a time. Currently, client identifiers are used to maintain state for durable subscribers. In defining a client identifier, you can use a special variable substitution syntax that allows multiple connections to be created from a single ConnectionFactory object using different user name parameters to generate unique client identifiers. These connections can be used by multiple durable subscribers without naming conflicts or lack of security.

Message Queue allows client identifiers to be set in one of two ways:

• Programmatically: You use the setClientID method of the Connection object. If you use this method, you must set the client id before you use the connection. Once the connection is used, the client identifier cannot be set or reset.

• Administratively: The administrator specifies the client ID when creating the connection factory administrative object. See Client Identification in Sun Java System Message Queue 3.7 UR1 Administration Guide.

Message Order and Priority

In general, all messages sent to a destination by a single session are guaranteed to be delivered to a consumer in the order they were sent. However, if they are assigned different priorities, a messaging system will attempt to deliver higher priority messages first.

Beyond this, the ordering of messages consumed by a client can have only a rough relationship to the order in which they were produced. This is because the delivery of messages to a number of destinations and the delivery from those destinations can depend on a number of issues that affect timing, such as the order in which the messages are sent, the sessions from which they are sent, whether the messages are persistent, the lifetime of the messages, the priority of the messages, the message delivery policy of queue destinations (see Chapter 15, Physical Destination Property Reference, in Sun Java System Message Queue 3.7 UR1 Administration Guide), and message service availability.

Using Selectors Efficiently

The use of selectors can have a significant impact on the performance of your application. It’s difficult to put an exact cost on the expense of using selectors since it varies with the complexity of the selector expression, but the more you can do to eliminate or simplify selectors the better.

One way to eliminate (or simplify) selectors is to use multiple destinations to sort messages. This has the additional benefit of spreading the message load over more than one producer, which can improve the scalability of your application. For those cases when it is not possible to do that, here are some techniques that you can use to improve the performance of your application when using selectors:

• Have consumers share selectors. As of version 3.5 of Message Queue, message consumers with identical selectors “share” that selector in imqbrokerd which can significantly improve performance. So if there is a way to structure your application to have some selector sharing, consider doing so.

• Use IN instead of multiple string comparisons. For example, the following expression:

  color IN (’red’, ’green’, ’white’)

  is much more efficient than this expression

  color = ’red’ OR color = ’green’ OR color = ’white’

  especially if the above expression usually evaluates to false.

• Use BETWEEN instead of multiple integer comparisons. For example:

  size BETWEEN 6 AND 10

  is generally more efficient than

  size >= 6 AND size  <= 10

  especially if the above expression usually evaluates to true.

• Order the selector expression so that Message Queue can determine its evaluation as soon as possible. (Evaluation proceeds from left to right.) This can easily double or triple performance when using selectors, depending on the complexity of the expression.

  • If you have two expressions joined by an OR, put the expression that is most likely to evaluate to TRUE first.

  • If you have two expressions joined by an AND, put the expression that is most likely to evaluate to FALSE first.

  For example, if size is usually greater than 6, but color is rarely red you’d want the order of an OR expression to be:

  size > 6 OR color = ’red’

  If you are using AND:

  color = ’red’ AND size > 6

Balancing Reliability and Performance

Reliable messaging is implemented in a variety of ways: through the use of persistent messages, acknowledgments or transactions, durable subscriptions, and connection failover.

In general, the more reliable the delivery of messages, the more overhead and bandwidth are required to achieve it. The trade-off between reliability and performance is a significant design consideration. You can maximize performance and throughput by choosing to produce and consume nonpersistent messages. On the other hand, you can maximize reliability by producing and consuming persistent messages in a transaction using a transacted session. For a detailed discussion of design options and their impact on performance, see Factors Affecting Performance.

Managing Client Threads

Using client threads effectively requires that you balance performance, throughput, and resource needs. To do this, you need to understand JMS restrictions on thread usage, what threads Message Queue allocates for itself, and the architecture of your applications. This section addresses these issues and offers some guidelines for managing client threads.

JMS Threading Restrictions

The Java Messaging Specification mandates that a session not be operated on by more than one thread at a time. This leads to the following restrictions:

• A session may not have an asynchronous consumer and a synchronous consumer.

• A session that has an asynchronous consumer can only produce messages from within the onMessage() method (the message listener). The only call that you can make outside the message listener is to close the session.

• A session may include any number of synchronous consumers, any number of producers, and any combination of the two. That is, the single-thread requirement cannot be violated by these combinations. However, performance may suffer.

The system does not enforce the requirement that a session be single threaded. If your client application violates this requirement, you will get a JMSIllegalState exception or unexpected results.

Thread Allocation for Connections

When the Message Queue client runtime creates a connection, it creates two threads: one for consuming messages from the socket, and one to manage the flow of messages for the connection. In addition, the client runtime creates a thread for each client session. Thus, at a minimum, for a connection using one session, three threads are created. For a connection using three sessions, five threads are created, and so on.

Managing threads in a JMS application often involves trade-offs between performance and throughput. Weigh the following considerations when dealing with threading issues.

• When you create several asynchronous message consumers in the same session, messages are delivered serially by the session thread to these consumers. Sharing a session among several message consumers might starve some consumers of messages while inundating other consumers. If the message rate across these consumers is high enough to cause an imbalance, you might want to separate the consumers into different sessions. To determine whether message flow is unbalanced, you can monitor destinations to see the rate of messages coming in. See Chapter 4, Using the Metrics Monitoring API.

• You can reduce the number of threads allocated to the client application by using fewer connections and fewer sessions. However, doing this might slow your application’s throughput.

• You might be able to use certain JVM runtime options to improve thread memory usage and performance. For example, if you are running on the Solaris platform, you may be able to run with the same number (or more) threads by using the following vm options with the client: Refer to the JDK documentation for details.

  • Use the Xss128K option to decrease the memory size of the heap.

  • Use the xconcurrentIO option to improve thread performance in the 1. 3 VM.

Managing Memory and Resources

This section describes memory and performance issues that you can manage by increasing JVM heap space and by managing the size of your messages. It covers the following topics:

• Managing Memory

• Managing Message Size

• Managing the Dead Message Queue

• Managing Physical Destination Limits

You can also improve performance by having the administrator set connection factory attributes to meter the message flow over the client-broker connection and to limit the message flow for a consumer. For a detailed explanation, please see Reliability and Flow Control in Sun Java System Message Queue 3.7 UR1 Administration Guide.

Managing Memory

A client application running in a JVM needs enough memory to accommodate messages that flow in from the network as well as messages the client creates. If your client gets OutOfMemoryError errors, chances are that not enough memory was provided to handle the size or the number of messages being consumed or produced.

Your client might need more than the default JVM heap space. On most systems, the default is 64 MB but you will need to check the default values for your system.

Consider the following guidelines:

• Evaluate the normal and peak system memory footprints when sizing heap space.

• You can start by doubling the heap size using a command like the following:

  java -Xmx128m MyClass

• The best size for the heap space depends on both the operating system and the JDK release. Check the JDK documentation for restrictions.

• The size of the VM’s memory allocation pool must be less than or equal to the amount of virtual memory that is available on the system.

Managing Message Size

In general, for better manageability, you can break large messages into smaller parts, and use sequencing to ensure that the partial messages sent are concatenated properly. You can also use a Message Queue JMS feature to compress the body of a message. This section describes the programming interface that allows you to compress messages and to compare the size of compressed and uncompressed messages.

Message compression and decompression is handled entirely by the client runtime, without involving the broker. Therefore, applications can use this feature with a pervious version of the broker, but they must use version 3.6 or later of the Message Queue client runtime library.

Message Compression

You can use the Message.setBooleanProperty() method to specify that the body of a message be compressed. If the JMS_SUN_COMPRESS property is set to true, the client runtime, will compress the body of the message being sent. This happens after the producer’s send method is called and before the send method returns to the caller. The compressed message is automatically decompressed by the client runtime before the message is delivered to the message consumer.

For example, the following call specifies that a message be compressed:

MyMessage.setBooleanProperty(“JMS_SUN_COMPRESS”,true);

Compression only affects the message body; the message header and properties are not compressed.

Two read-only JMS message properties are set by the client runtime after a message is sent.

Applications can test the properties (JMS_SUN_UNCOMPRESSED_SIZE and JMS_SUN_COMPRESSED_SIZE) after a send returns to determine whether compression is advantageous. That is, applications wanting to use this feature, do not have to explicitly receive a compressed and uncompressed version of the message to determine whether compression is desired.

If the consumer of a compressed message wants to resend the message in an uncompressed form, it should call the Message.clearProperties() to clear the JMS_SUN_COMPRESS property. Otherwise, the message will be compressed before it is sent to its next destination.

Advantages and Limitations of Compression

Although message compression has been added to improve performance, such benefit is not guaranteed. Benefits vary with the size and format of messages, the number of consumers, network bandwidth, and CPU performance. For example, the cost of compression and decompression might be higher than the time saved in sending and receiving a compressed message. This is especially true when sending small messages in a high-speed network. On the other hand, applications that publish large messages to many consumers or who publish in a slow network environment, might improve system performance by compressing messages.

Depending on the message body type, compression may also provide minimal or no benefit. An application client can use the JMS_SUN_UNCOMPRESSED_SIZE and JMS_SUN_COMPRESSED_SIZE properties to determine the benefit of compression for different message types.

Message consumers deployed with client runtime libraries that precede version 3.6 cannot handle compressed messages. Clients wishing to send compressed messages must make sure that consumers are compatible. C clients cannot currently consume compressed messages.

Compression Examples

Example 3–1 shows how you set and send a compressed message:

Example 3–1 Sending a Compressed Message

//topicSession and myTopic are assumed to have been created
topicPublisher publisher = topicSession.createPublisher(myTopic);
BytesMessage bytesMessage=topicSession.createBytesMessage();

//byteArray is assumed to have been created
bytesMessage.writeBytes(byteArray);

//instruct the client runtime to compress this message
bytesMessage.setBooleanProperty("JMS_SUN_COMPRESS", true);

//publish message to the myTopic destination
publisher.publish(bytesMessage);

Example 3–2 shows how you examine compressed and uncompressed message body size. The bytesMessage was created as in Example 3–1:

Example 3–2 Comparing Compressed and Uncompressed Message Size

//get uncompressed body size
int uncompressed=bytesMessage.getIntProperty(“JMS_SUN_UNCOMPRESSED_SIZE”);

//get compressed body size
int compressed=bytesMessage.getIntProperty(“JMS_SUN_COMPRESSED_SIZE”);

Managing the Dead Message Queue

When a message is deemed undeliverable, it is automatically placed on a special queue called the dead message queue. A message placed on this queue retains all of its original headers (including its original destination) and information is added to the message’s properties to explain why it became a dead message. An administrator or a developer can access this queue, remove a message, and determine why it was placed on the queue.

• For an introduction to dead messages and the dead message queue, see Destinations and Routing Services in Sun Java System Message Queue 3.7 UR1 Technical Overview

• For a description of the destination properties and of the broker properties that control the system’s use of the dead message queue, see Chapter 15, Physical Destination Property Reference, in Sun Java System Message Queue 3.7 UR1 Administration Guide

This section describes the message properties that you can set or examine programmatically to determine the following:

• Whether a dead message can be sent to the dead message queue.

• Whether the broker should log information when a message is destroyed or moved to the dead message queue.

• Whether the body of the message should also be stored when the message is placed on the dead message queue.

• Why the message was placed on the dead message queue and any ancillary information.

Message Queue 3.6 clients can set properties related to the dead message queue on messages and send those messages to clients compiled against earlier versions. However clients receiving such messages cannot examine these properties without recompiling against 3.6 libraries.

The dead message queue is automatically created by the broker and called mq.sys.dmq. You can use the message monitoring API, described in Chapter 4, Using the Metrics Monitoring API, to determine whether that queue is growing, to examine messages on that queue, and so on.

You can set the properties described in Table 3–2 for any message to control how the broker should handle that message if it deems it to be undeliverable. Note that these message properties are needed only to override destination, or broker-based behavior.

The properties described in Table 3–3 are set by the broker for a message placed in the dead message queue. You can examine the properties for the message to retrieve information about why the message was placed on the queue and to gather other information about the message and about the context within which this action was taken.

Managing Physical Destination Limits

When creating a topic or queue destination, the administrator can specify how the broker should behave when certain memory limits are reached. Specifically, when the number of messages reaching a physical destination exceeds the number specified with the maxNumMsgs property or when the total amount of memory allowed for messages exceeds the number specified with the maxTotalMsgBytes property, the broker takes one of the following actions, depending on the setting of the limitBehavior property:

• Slows message producers (FLOW_CONTROL)

• Throws out the oldest message in memory (REMOVE_OLDEST)

• Throws out the lowest priority message in memory (REMOVE_LOW_PRIORITY)

• Rejects the newest messages (REJECT_NEWEST)

If the default value REJECT_NEWEST is specified for the limitBehavior property, the broker throws out the newest messages received when memory limits are exceeded. If the message discarded is a persistent message, the producing client gets an exception which should be handled by resending the message later.

If any of the other values is selected for the limitBehavior property or if the message is not persistent, the application client is not notified if a message is discarded. Application clients should let the administrator know how they prefer this property to be set for best performance and reliability.

Programming Issues for Message Consumers

This section describes two problems that consumers might need to manage: the undetected loss of a connection, or the loss of a message for distributed synchronous consumers.

Using the Client Runtime Ping Feature

Message Queue defines a connection factory attribute for a ping interval. This attribute specifies the interval at which the client runtime should check the client’s connection to the broker. The ping feature is especially useful to Message Queue clients that exclusively receive messages and might therefore not be aware that the absence of messages is due to a connection failure. This feature could also be useful to producers who don’t send messages frequently and who would want notification that a connection they’re planning to use is not available.

The connection factory attribute used to specify this interval is called imqPingInterval. Its default value is 30 seconds. A value of -1 or 0, specifies that the client runtime should not check the client connection.

Developers should set (or have the administrator set) ping intervals that are slightly more frequent than they need to send or receive messages, to allow time to recover the connection in case the ping discovers a connection failure. Note also that the ping may not occur at the exact time specified by the value you supply for interval; the underlying operating system’s use of i/o buffers may affect the amount of time needed to detect a connection failure and trigger an exception.

A failed ping operation results in a JMSException on the subsequent method call that uses the connection. If an exception listener is registered on the connection, it will be called when a ping operation fails.

Preventing Message Loss for Synchronous Consumers

It is always possible that a message can be lost for synchronous consumers in a session using AUTO_ACKNOWLEDGE mode if the provider fails. To prevent this possibility, you should either use a transacted session or a session in CLIENT_ACKNOWLEDGE mode.

Synchronous Consumption in Distributed Applications

Because distributed applications involve greater processing time, such an application might not behave as expected if it were run locally. For example, calling the receiveNoWait method for a synchronous consumer might return null even when there is a message available to be retrieved.

If a client connects to the broker and immediately calls the receiveNoWait method, it is possible that the message queued for the consuming client is in the process of being transmitted from the broker to the client. The client runtime has no knowledge of what is on the broker, so when it sees that there is no message available on the client’s internal queue, it returns with a null, indicating no message.

You can avoid this problem by having your client do either of the following:

• Use one of the synchronous receive methods that specifies a timeout interval.

• Use a queue browser to check the queue before calling the receiveNoWait method.

Factors Affecting Performance

Application design decisions can have a significant effect on overall messaging performance. The most important factors affecting performance are those that impact the reliability of message delivery; among these are the following:

• Delivery Mode (Persistent/Nonpersistent)

• Use of Transactions

• Acknowledgment Mode

• Durable vs. Nondurable Subscriptions

Other application design factors impacting performance include the following:

• Use of Selectors (Message Filtering)

• Message Size

• Message Body Type

The sections that follow describe the impact of each of these factors on messaging performance. As a general rule, there is a trade-off between performance and reliability: factors that increase reliability tend to decrease performance.

Table 3–4 shows how application design factors affect messaging performance. The table shows two scenarios—a high-reliability, low-performance scenario and a high-performance, low-reliability scenario—and the choice of application design factors that characterizes each. Between these extremes, there are many choices and trade-offs that affect both reliability and performance.

Delivery Mode (Persistent/Nonpersistent)

Persistent messages guarantee message delivery in case of broker failure. The broker stores these message in a persistent store until all intended consumers acknowledge that they have consumed the message.

Broker processing of persistent messages is slower than for nonpersistent messages for the following reasons:

• A broker must reliably store a persistent message so that it will not be lost should the broker fail.

• The broker must confirm receipt of each persistent message it receives. Delivery to the broker is guaranteed once the method producing the message returns without an exception.

• Depending on the client acknowledgment mode, the broker might need to confirm a consuming client’s acknowledgment of a persistent message.

For both queues and topics with durable subscribers, performance was approximately 40% faster for non-persistent messages. We obtained these results using 10K-size messages and AUTO_ACKNOWLEDGE mode.

Use of Transactions

A transaction guarantees that all messages produced in a transacted session and all messages consumed in a transacted session will be either processed or not processed (rolled back) as a unit. Message Queue supports both local and distributed transactions.

A message produced or acknowledged in a transacted session is slower than in a non-transacted session for the following reasons:

• Additional information must be stored with each produced message.

• In some situations, messages in a transaction are stored when normally they would not be. For example, a persistent message delivered to a topic destination with no subscriptions would normally be deleted, however, at the time the transaction is begun, information about subscriptions is not available.

• Information on the consumption and acknowledgment of messages within a transaction must be stored and processed when the transaction is committed.

Acknowledgment Mode

Other than using transactions, you can ensure reliable delivery by having the client acknowledge receiving a message. If a session is closed without the client acknowledging the message or if the message broker fails before the acknowledgment is processed, the broker redelivers that message, setting a JMSRedelivered flag.

For a non-transacted session, the client can choose one of three acknowledgment modes, each of which has its own performance characteristics:

• AUTO_ACKNOWLEDGE. The system automatically acknowledges a message once the consumer has processed it. This mode guarantees at most one redelivered message after a provider failure.

• CLIENT_ACKNOWLEDGE. The application controls the point at which messages are acknowledged. All messages processed in that session since the previous acknowledgment are acknowledged. If the broker fails while processing a set of acknowledgments, one or more messages in that group might be redelivered.

  (Using CLIENT_ACKNOWLEDGE mode is similar to using transactions, except there is no guarantee that all acknowledgments will be processed together if a provider fails during processing.)

• DUPS_OK_ACKNOWLEDGE. This mode instructs the system to acknowledge messages in a lazy manner. Multiple messages can be redelivered after a provider failure.

• NO_ACKNOWLEDGE In this mode, the broker considers a message acknowledged as soon as it has been written to the client. The broker does not wait for an acknowledgment from the receiving client. This mode is best used by typic subscribers who are not worried about reliability.

Performance is impacted by acknowledgment mode for the following reasons:

• Extra control messages between broker and client are required in AUTO_ACKNOWLEDGE and CLIENT_ACKNOWLEDGE modes. The additional control messages add processing overhead and can interfere with JMS payload messages, causing processing delays.

• In AUTO_ACKNOWLEDGE and CLIENT_ACKNOWLEDGE modes, the client must wait until the broker confirms that it has processed the client’s acknowledgment before the client can consume more messages. (This broker confirmation guarantees that the broker will not inadvertently redeliver these messages.)

• The Message Queue persistent store must be updated with the acknowledgment information for all persistent messages received by consumers, thereby decreasing performance.

Durable vs. Nondurable Subscriptions

Subscribers to a topic destination have either durable and nondurable subscriptions. Durable subscriptions provide increased reliability at the cost of slower throughput for the following reasons:

• The Message Queue message broker must persistently store the list of messages assigned to each durable subscription so that should the broker fail, the list is available after recovery.

• Persistent messages for durable subscriptions are stored persistently, so that should a broker fail, the messages can still be delivered after recovery, when the corresponding consumer becomes active. By contrast, persistent messages for nondurable subscriptions are not stored persistently (should a broker fail, the corresponding consumer connection is lost and the message would never be delivered).

We compared performance for durable and non-durable subscribers in two cases: persistent and nonpersistent 10k-sized messages. Both cases use AUTO_ACKNOWLEDGE acknowledgment mode. We found a performance impact only in the case of persistent messages, which slowed messages conveyed to durable subscribers by about 30%.

Use of Selectors (Message Filtering)

Application developers can have the messaging provider sort messages according to criteria specified in the message selector associated with a consumer and deliver to that consumer only those messages whose property value matches the message selector. For example, if an application creates a subscriber to the topic WidgetOrders and specifies the expression NumberOfOrders >1000 for the message selector, messages with a NumberOfOrders property value of 1001 or more are delivered to that subscriber.

Creating consumers with selectors lowers performance (as compared to using multiple destinations) because additional processing is required to handle each message. When a selector is used, it must be parsed so that it can be matched against future messages. Additionally, the message properties of each message must be retrieved and compared against the selector as each message is routed. However, using selectors provides more flexibility in a messaging application and may lower resource requirements at the expense of speed.

Message Size

Message size affects performance because more data must be passed from producing client to broker and from broker to consuming client, and because for persistent messages a larger message must be stored.

However, by batching smaller messages into a single message, the routing and processing of individual messages can be minimized, providing an overall performance gain. In this case, information about the state of individual messages is lost.

In our tests, which compared throughput in kilobytes per second for 1K, 10K, and 100K-sized messages to a queue destination using AUTO_ACKNOWLEDGE mode, we found that non-persistent messaging was about 50% faster for 1K messages, about 20% faster for 10K messages, and about 5% faster for 100K messages. The size of the message affected performance significantly for both persistent and non-persistent messages. 100k messages are about 10 times faster than 10K, and 10K messages are about 5 times faster than 1K.

Message Body Type

JMS supports five message body types, shown below roughly in the order of complexity:

• Bytes: Contains a set of bytes in a format determined by the application

• Text: Is a simple java.lang.String

• Stream: Contains a stream of Java primitive values

• Map: Contains a set of name-and-value pairs

• Object: Contains a Java serialized object

While, in general, the message type is dictated by the needs of an application, the more complicated types (map and object) carry a performance cost — the expense of serializing and deserializing the data. The performance cost depends on how simple or how complicated the data is.

Client Connection Failover (Auto-Reconnect)

Message Queue supports client connection failover. A failed connection can be automatically restored not only to the original broker, but to a different broker in a broker cluster. There are circumstances under which the client-side state cannot be restored on any broker during an automatic reconnection attempt; for example, when the client uses transacted sessions or temporary destinations. At such times the connection exception handler is called and the application code has to catch the exception and restore state.

This section explains how automatic reconnection is enabled, how the broker behaves during a reconnect, how automatic reconnection impacts producers and consumers, and how producers and consumers should handle exceptions that result from connection failover. For additional information about this feature, please see Connection Handling in Sun Java System Message Queue 3.7 UR1 Administration Guide.

Enabling Auto-Reconnect

The developer or the administrator can enable automatic reconnection by setting the connection factory imqReconnectEnabled attribute to true. The connection factory administered object must also be configured to specify the following:

• A list of message-service addresses (using the imqAddressList attribute). When the client runtime needs to establish or reestablish a connection to a message service, it attempts to connect to the brokers in the list until it finds (or fails to find) an available broker. If you specify only a single broker instance on the imqAddressList attribute, the configuration won’t support recovery from hardware failure.

  When you specify more than one broker, you can decide whether to use parallel brokers or a broker cluster. In a parallel configuration, there is no communication between brokers, while in a broker cluster, the brokers interact to distribute message delivery loads. (Refer to Chapter 4, Broker Clusters, in Sun Java System Message Queue 3.7 UR1 Technical Overview for more information on broker clusters.)

  • To enable parallel-broker reconnection, set theimqReconnectListBehavior attribute to PRIORITY . Typically, you would specify no more than a pair of brokers for this type of reconnection. This way, the messages are published to one broker, and all clients fail over together from the first broker to the second.

  • To enable clustered-broker reconnection, set the imqReconnectListBehavior attribute to RANDOM. This way, the client runtime randomizes connection attempts across the list, and client connections are distributed evenly across the broker cluster.

    Each broker in a cluster uses its own separate persistent store (which means that undelivered persistent messages are unavailable until a failed broker is back online). If one broker crashes, its client connections are reestablished on other brokers.

• The number of iterations to be made over the list of brokers (using the imqAddressListIterations attribute) when attempting to create a connection or to reconnect.

• The number of attempts to reconnect to a broker if the first connection fails (using the imqReconnectAttempts attribute).

• The interval, in milliseconds, between reconnect attempts, using the imqReconnectInterval attribute.

Single-Broker Auto-Reconnect

Configure your connection-factory object as follows:

Example 3–3 Example of Command to Configure a Single Broker

imqobjmgr add -t cf -l "cn=myConnectionFactory" \
    -o "imqAddressList=mq://jpgserv/jms" \
    -o "imqReconnect=true" \
    -o "imqReconnectAttempts=10"
               

This command creates a connection-factory object with a single address in the broker address list. If connection fails, the client runtime will try to reconnect with the broker 10 times. If an attempt to reconnect fails, the client runtime will sleep for three seconds (the default value for the imqReconnectInterval attribute) before trying again. After 10 unsuccessful attempts, the application will receive a JMSException .

You can ensure that the broker starts automatically with at system start-up time. See Sun Java System Message Queue 3.7 UR1 Installation Guide for information on how to configure automatic broker start-up. For example, on the Solaris platform, you can use /etc/rc.d scripts.

Parallel Broker Auto-Reconnect

Configure your connection-factory objects as follows:

Example 3–4 Example of Command to Configure Parallel Brokers

imqobjmgr add -t cf -l "cn=myCF" \
    -o "imqAddressList=myhost1, mqtcp://myhost2:12345/jms" \
    -o "imqReconnect=true" \
    -o "imqReconnectRetries=5"
               

This command creates a connection factory object with two addresses in the broker list. The first address describes a broker instance running on the host myhost1 with a standard port number (7676). The second address describes a jms connection service running at a statically configured port number (12345).

Clustered-Broker Auto-Reconnect

Configure your connection-factory objects as follows:

Example 3–5 Example of Command to Configure a Broker Cluster

imqobjmgr add -t cf -l "cn=myConnectionFactory" \
    -o "imqAddressList=mq://myhost1/ssljms, \
            mq://myhost2/ssljms, \
            mq://myhost3/ssljms, \
            mq://myhost4/ssljms” \
    -o "imqReconnect=true" \
    -o "imqReconnectRetries=5" \
    -o "imqAddressListBehavior=RANDOM"
               

This command creates a connection factory object with four addresses in the imqAddressList. All the addresses point to jms services running on SSL transport on different hosts. Since the imqAddressListBehavior attribute is set to RANDOM, the client connections that are established using this connection factory object will be distributed randomly among the four brokers in the address list.

This is a clustered broker configuration, so you must configure one of the brokers in the cluster as the master broker. In the connection-factory address list, you can also specify a subset of all the brokers in the cluster.

Auto-Reconnect Behaviors

A broker treats an automatic reconnection as it would a new connection. When the original connection is lost, all resources associated with that connection are released. For example, in a broker cluster, as soon as one broker fails, the other brokers assume that the client connections associated with the failed broker are gone. After auto-reconnect takes place, the client connections are recreated from scratch.

Sometimes the client-side state cannot be fully restored by auto-reconnect. Perhaps a resource that the client needs cannot be recreated. In this case, the client runtime calls the client’s connection exception handler and the client must take appropriate action to restore state. For additional information, see Handling Exceptions When Failover Occurs.

If the client is automatically-reconnected to a different broker instance, persistent messages can only be sent after the original broker recovers. Other state information held by the failed or disconnected broker can be lost. The messages held by the original broker, once it is restored, might be delivered out of order. This is because broker instances in a cluster do not use a shared, highly available persistent store.

A transacted session is the most reliable method of ensuring that a message isn’t lost if you are careful in coding the transaction. If auto-reconnect happens in the middle of a transaction, the client runtime throws an exception when the transaction is committed, and the transaction is rolled back. At that point, you must make sure that the client restarts the whole transaction. (This is especially important when you use a broker cluster.) For additional information, see Handling Exceptions When Failover Occurs.

Automatic reconnection affects producers and consumers differently:

• During reconnection, producers cannot send messages. The production of messages (or any operation that involves communication with the message broker) is blocked until the connection is reestablished.

• For consumers, automatic reconnection is supported for all client acknowledgment modes. After a connection is reestablished, the broker will redeliver all unacknowledged messages it had previously delivered, marking them with a Redeliver flag. The client can examine this flag to determine whether any message has already been consumed (but not yet acknowledged). In the case of nondurable subscribers, some messages might be lost because the broker does not hold their messages once their connections have been closed. Any messages produced for nondurable subscribers while the connection is down cannot be delivered when the connections is reestablished. For additional information, see Handling Exceptions When Failover Occurs.

Auto-Reconnect Limitations

Notice the following points when using the auto-reconnect feature:

• Messages might be redelivered to a consumer after auto-reconnect takes place. In auto-acknowledge mode, you will get no more than one redelivered message. In other session types, all unacknowledged persistent messages are redelivered.

• While the client runtime is trying to reconnect, any messages sent by the broker to nondurable topic consumers are lost.

• Any messages that are in queue destinations and that are unacknowledged when a connection fails are redelivered after auto-reconnect. However, in the case of queues delivering to multiple consumers, these messages cannot be guaranteed to be redelivered to the original consumers. That is, as soon as a connection fails, an unacknowledged queue message might be rerouted to other connected consumers.

• In the case of a broker cluster, the failure of the master broker prevents the following operations from succeeding on any other broker in the cluster:

  • Creating or destroying a new durable subscription.

  • Creating or destroying a new physical destination using the imqcmd create dst command.

  • Starting a new broker process. (However, the brokers that are already running continue to function normally even if the master broker goes down.)

    You can configure the master broker to restart automatically using Message Queue broker support for rc scripts or the Windows service manager.

• Auto-reconnect doesn’t work if the client uses a ConnectionConsumer to consume messages. In that case, the client runtime throws an exception.

Handling Exceptions When Failover Occurs

Several kinds of exceptions can occur as a result of the client being reconnected after a failover. How the client application should handle these exceptions depends on whether a session is transacted, on the kind of exception thrown, and on the client's role--as producer or consumer. The following sections discuss the implications of these factors.

Independently of how the exception is raised, the client application must never call System.exit()to exit the application because this would prevent the Message Queue client runtime from reconnecting to an alternate or restarted broker.

When a failover occurs, exception messages may be shown on the application's console and recorded in the broker's log. These messages are for information only. They may be useful in troubleshooting, but minimizing or eliminating the impact of a failover is best handled preemptively by the application client in the ways described in the following sections.

Handling Exceptions in a Transacted Session

A transacted session might fail to commit and (throw an exception) either because a failover occurs while statements within the transaction are being executed or because the failover occurs during the call to Session.commit(). In the first case, the failover is said to occur during an open transaction; in the second case, the failover occurs during the commit itself.

In the case of a failover during an open transaction, when the client application calls Session.commit(), the client runtime will throw a TransactionRolledBackException and roll back the transaction causing the following to happen.

• Messages that have been produced (but not committed) in the transacted session are discarded and not delivered to the consumer.

• All messages that have been consumed (but not committed) in the transacted session are redelivered to the consumer with the Redeliver flag set.

• A new transaction is automatically started.

If the client application itself had called Session.rollback after a failover (before the Session.commit is executed) the same things would happen as if the application had received a TransactionRollbackException. After receiving a TransactionRollbackException or calling Session.rollback(), the client application must retry the failed transaction. That is, it must re-send and re-consume the messages that were involved in the failed-over transaction.

In the second case, when the failover occurs during a call to Session.commit, there may be three outcomes:

• The transaction is committed successfully and the call to Session.commit does not return an exception. In this case, the application client does not have to do anything.

• The runtime throws a TransactionRolledbackException and does not commit the transaction. The transaction is automatically rolled back by the Message Queue runtime. In this case, the client application must retry the transaction as described for the case in which an open transaction is failed-over.

• A JMXException is thrown. This signals the fact that the transaction state is unknown: It might have either succeeded or failed. A client application should handle this case by assuming failure, pausing for three seconds, calling Session.rollback, and then retrying the operations. However, since the commit might have succeeded, when retrying the transacted operations, a producer should set application-specific properties on the messages it re-sends to signal that these might be duplicate messages. Likewise, consumers that retry receive operations should not assume that a message that is redelivered is necessarily a duplicate. In other words, to ensure once and only once delivery, both producers and consumers need to do a little extra work to handle this edge case. The code samples presented next illustrate good coding practices for handling this situation.

Handling Exceptions in a Non-Transacted Session

If a connection is failed-over for a producer, a client application may receive a JMSException. The application thread that receives the exception should pause for a few seconds and then resend the messages. The client application may want to set a flag on the resent messages to indicate that they could be duplicates.

If a connection is failed over for a message consumer, the consequences vary with the sessions acknowledge mode:

• In client-acknowledge mode, calling Message.acknowledge or MessageConsumer.receive during a failover will raise a JMSException. The consumer should call Session.recover to recover or re-deliver the unacknowledged messages and then call Message.acknowledge or MessageConsumer.receive.

• In auto-acknowledge mode, after getting a JMSException, the synchronous consumer should pause a few seconds and then call MessageConsumer.receive to continue receiving messages. Any message that failed to be acknowledged (due to the failover) will be redelivered with the redelivered flags set to true.

• In dups-OK-acknowledge mode, the synchronous consumer should pause a few seconds after getting an exception and then call MessageConsumer.receive to continue receiving messages. In this case, it's possible that messages delivered and acknowledged (before the failover) could be redelivered.

Failover Producer Example

The following code sample illustrates good coding practices for handling exceptions during a failover. It is designed to send non-transacted, persistent messages forever and to handle JMSExceptions when a failover occurs. The program is able to handle either a true or false setting for the imqReconnectEnabled property. To run the program enter one of the following commands.

java dura.example.FailoverProducer

java -DimqReconnectEnabled=true dura.example.FailoverProducer

/*
 * @(#)FailoverProducer.java	1.1 06/06/09
 * Copyright 2006 Sun Microsystems, Inc. All Rights Reserved
 * SUN PROPRIETARY/CONFIDENTIAL
 * Use is subject to license terms. */

package dura.example;

import javax.jms.*;
import com.sun.messaging.ConnectionConfiguration;
import java.util.*;

public class FailoverProducer implements ExceptionListener {

	//connection factory
    private com.sun.messaging.TopicConnectionFactory factory;
    //connection
    private TopicConnection pconn = null;
    //session
    private TopicSession psession = null;
    //publisher
    private TopicPublisher publisher = null;
    //topic
    private Topic topic = null;
    //This flag indicates whether this test client is closed.
    private boolean isClosed = false;
    //auto reconnection flag
    private boolean autoReconnect = false;
    //destination name for this example.
    private static final String DURA_TEST_TOPIC = "DuraTestTopic";
    //the message counter property name 
    public static final String MESSAGE_COUNTER = "MESSAGE_COUNTER";
    //the message in-doubt-bit property name
    public static final String MESSAGE_IN_DOUBT = "MESSAGE_IN_DOUBT";

    /**
     * Constructor.  Get imqReconnectEnabled property value from 
     * System property.
     */
    public FailoverProducer () {
    	
    	try {
    		autoReconnect = 
    		Boolean.getBoolean(ConnectionConfiguration.imqReconnectEnabled);
    	} catch (Exception e) {
    		this.printException(e);
    	}
    	
    }

    /**
     * Connection is broken if this handler is called.  
     * If autoReconnect flag is true, this is called only 
     * if no more retries from MQ.
     */
    public void onException (JMSException jmse) {
        this.printException (jmse);
    }

    /**
     * create MQ connection factory.
     * @throws JMSException
     */
    private void initFactory() throws JMSException {
        //get connection factory
        factory = new com.sun.messaging.TopicConnectionFactory();
    }

    /**
     * JMS setup.  Create a Connection,Session, and Producer.
     * 
     * If any of the JMS object creation fails (due to system failure),
     * it retries until it succeeds.
     *
     */
    private void initProducer() {
    	
        boolean isConnected = false;

        while ( isClosed == false && isConnected == false ) {
            
        	try {
                println("producer client creating connection ...");

                //create connection
                pconn = factory.createTopicConnection();
                
                //set connection exception listener
                pconn.setExceptionListener(this);

                //create topic session
                psession = pconn.createTopicSession(false,
                    Session.CLIENT_ACKNOWLEDGE);
                
                //get destination
                topic = psession.createTopic(DURA_TEST_TOPIC);

                //publisher
                publisher = psession.createPublisher(topic);

                //set flag to true
                isConnected = true;

                println("producer ready.");
            }
            catch (Exception e) {

                println("*** connect failed ... sleep for 5 secs.");
                
                try {
                	//close resources.
                	if ( pconn != null ) {
                		pconn.close();
                	}
                	//pause 5 secs.
                    Thread.sleep(5000);
                
                } catch (Exception e1) {
                    ;
                }
            }
        }
    }

    /**
     * Start test.  This sends JMS messages in a loop (forever).
     */
    public void run () {

        try {
        	//create MQ connection factory.
            initFactory();
            
            //create JMS connection,session, and producer
            initProducer();
            
            //send messages forever.
            sendMessages();
        } catch (Exception e) {
            this.printException(e);
        }
    }

    /**
     * Send persistent messages to a topic forever.  This shows how
     * to handle failover for a message producer.
     */
    private void sendMessages() {
    	
    	//this is set to true if send failed.
        boolean messageInDoubt = false;
        
        //message to be sent
        TextMessage m = null;
        
        //msg counter
        long msgcount = 0;

        while (isClosed == false) {
        	
            try {
            	
            	/**
            	 * create a text message 
            	 */
                m = psession.createTextMessage();
               
                /**
                 * the MESSAGE_IN_DOUBT bit is set to true if 
                 * you get an exception for the last message.
                 */
                if ( messageInDoubt == true ) {
                    m.setBooleanProperty (MESSAGE_IN_DOUBT, true);
                    messageInDoubt = false;
                    
                    println("MESSAGE_IN_DOUBT bit is set to true 
                             for msg: " + msgcount);
                } else {
                    m.setBooleanProperty (MESSAGE_IN_DOUBT, false);
                }
                
                //set message counter
                m.setLongProperty(MESSAGE_COUNTER, msgcount);
                
                //set message body
                m.setText("msg: " + msgcount);
                
                //send the msg
                publisher.send(m, DeliveryMode.PERSISTENT, 4, 0);

                println("sent msg: " + msgcount);
                
                /**
                 * reset counetr if reached max long value.
                 */
                if (msgcount == Long.MAX_VALUE) {
                	msgcount = 0;
                	
                	println ("Reset message counter to 0.");
                }
                
                //increase counter
                msgcount ++;
                
                Thread.sleep(1000);

            } catch (Exception e) {

                if ( isClosed == false ) {
                	
                    //set in doubt bit to true.
                    messageInDoubt = true;

                    this.printException(e);
                   
                    //init producer only if auto reconnect is false.
                    if ( autoReconnect == false ) {
                        this.initProducer();
                    }
                }
            }
        }
    }

    /**
     * Close this example program.
     */
    public synchronized void close() {

        try {
            isClosed = true;
            pconn.close();

            notifyAll();
        } catch (Exception e) {
            this.printException(e);
        }
    }
    
    /**
     * print the specified exception.
     * @param e the exception to be printed.
     */
    private void printException (Exception e) {
    	System.out.println(new Date().toString());
    	e.printStackTrace();
    }
    
    /**
     * print the specified message.
     * @param msg the message to be printed.
     */
    private void println (String msg) {
    	System.out.println(new Date() + ": " + msg);
    }
    
    /**
     * Main program to start this example.
     */
    public static void main (String args[]) {
    	FailoverProducer fp = new FailoverProducer();
    	fp.run();
    }

}

Failover Consumer Example

The following code sample, FailoverConsumer, illustrates good coding practices for handling exceptions during a failover. The transacted session is able to receive messages forever. The program sets the auto reconnect property to true, requiring the Message Queue runtime to automatically perform a reconnect when the connected broker fails or is killed. It is designed to work with the dura.example.FailoverProducer, shown in the previous section.

To run this program enter the following command.

java dura.example.FailoverConsumer

/*
 * @(#)FailoverConsumer.java	1.1 06/06/09
 * Copyright 2006 Sun Microsystems, Inc. All Rights Reserved
 * SUN PROPRIETARY/CONFIDENTIAL
 * Use is subject to license terms.
 *
 */
package dura.example;

import java.util.Date;
import javax.jms.Destination;
import javax.jms.ExceptionListener;
import javax.jms.JMSException;
import javax.jms.Message;
import javax.jms.Connection;
import javax.jms.MessageConsumer;
import javax.jms.Session;
import javax.jms.Topic;
import javax.jms.TransactionRolledBackException;
import com.sun.messaging.ConnectionConfiguration;

public class FailoverConsumer implements ExceptionListener, Runnable {

	//JMS connection
    private Connection conn = null;
    //JMS session
    private Session session = null;
    //JMS Message consumer
    private MessageConsumer messageConsumer = null;
    //JMS destination.
    private Destination destination = null;
    //flag indicates whether this program should continue running.
    private boolean isConnected = false;
    //destination name.
    private static final String DURA_TEST_TOPIC = "DuraTestTopic";
    //the commit counter, for information only.
    private long commitCounter = 0;
    
    /**
     * message counter property set by the producer.
     */
    public static final String MESSAGE_COUNTER = "MESSAGE_COUNTER";
    
    /**
     * Message in doubt bit set by the producer
     */
    public static final String MESSAGE_IN_DOUBT = "MESSAGE_IN_DOUBT";
    
    /**
     * receive time out
     */
    public static final long RECEIVE_TIMEOUT = 0;
    
    /**
     * Default constructor -   
     * Set up JMS Environment.
     */
    public FailoverConsumer() {
       setup();
    }

    /*  Connection Exception listener.  This is called when connection
     *  breaks and no reconnect attempts are performed by MQ client runtime.
     */
    public void onException (JMSException e) {

    	print ("Reconnect failed.  Shutting down the connection ...");
    	
    	/**
    	 * Set this flag to false so that the run() method will exit.
    	 */
        this.isConnected = false;
        e.printStackTrace();
    }

    /**
     * Best effort to roll back a jms session.  When a broker crashes, an
     * open-transaction should be rolled back.  But the re-started broker 
     * may not have the uncommitted tranaction information due to system
     * failure.  In a situation like this, an application can just quit
     * calling rollback after retrying a few times  The uncommitted 
     * transaction (resources) will eventually be removed by the broker.
     */
    private void rollBackJMS() {
    	
    	//rollback fail count
    	int failCount = 0;
    	
        boolean keepTrying = true;
        
        while ( keepTrying ) {

            try {

                print ("<<< rolling back JMS ...., consumer commit counter:
                          " +  this.commitCounter);

                session.rollback();
                
                print("<<< JMS rolled back ...., consumer commit counter:
                          " + this.commitCounter);
                keepTrying = false;
            } catch (JMSException jmse) {
               
            	failCount ++;
                jmse.printStackTrace();

                sleep (3000); //3 secs
                
                if ( failCount == 1 ) {

                    print ("<<< rollback failed : total count" + failCount);
                    keepTrying = false;
                }
            }
        }
    }

    
    /**
     * Close the JMS connection and exit the program.
     *
     */
    private void close() {
        try {
       	
            if ( conn != null ) {
                conn.close();	
            }
        
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            this.isConnected = false;
        }
    }

    /*Receive messages in a loop until closed.*/
     
    public void run () {
       
		while (isConnected) {

			try {
				
				/*receive message with specified timeout.*/
				
                Message m = messageConsumer.receive(RECEIVE_TIMEOUT);
                
                /* process the message. */
                processMessage(m);

                /* commit JMS transaction. */
                this.commit();
                
                /*increase the commit counter.*/
                this.commitCounter ++;
               
			} catch (TransactionRolledBackException trbe) {
				
				/**
				 * the transaction is rolled back
				 * a new transaction is automatically started. 
				 */
				trbe.printStackTrace();
			} catch (JMSException jmse) {
				
				/* The transaction is in unknown state.
        * We need to roll back the transaction.*/

                jmse.printStackTrace();
                
                /* roll back if not closed.
                 */
                if ( this.isConnected == true ) {
					this.rollBackJMS();
				}

			} catch (Exception e) {
                
                e.printStackTrace();
                
                /* Exit if this is an unexpected Exception.
                 */
                this.close();
                
            } finally {
            	;//do nothing
            }
		}
		
		print(" <<< consumer exit ...");
	}

    /*Set up connection, destination, etc*/
       /
    protected void setup() {
        
    	try {
        	
			//create connection factory
			com.sun.messaging.ConnectionFactory factory =
			new com.sun.messaging.ConnectionFactory();
			
			//set auto reconnect to true.
			factory.setProperty(ConnectionConfiguration.imqReconnectEnabled, "true");
        	//A value of -1 will retry forever if connection is broken.
			factory.setProperty(ConnectionConfiguration.imqReconnectAttempts, "-1");
			//retry interval - every 10 seconds
			factory.setProperty(ConnectionConfiguration.imqReconnectInterval, "10000");
			//create connection
			conn = factory.createConnection();
			//set client ID
			conn.setClientID(DURA_TEST_TOPIC);
			
			//set exception listener
			conn.setExceptionListener(this);

			//create a transacted session
			session = conn.createSession(true, -1);
            
			//get destination
			destination = session.createTopic(DURA_TEST_TOPIC);
			
			//message consumer
			messageConsumer = session.createDurableSubscriber((Topic)destination,
                                                       DURA_TEST_TOPIC);
			//set flag to true
            this.isConnected = true;
            //we are ready, start the connection
            conn.start();
            
            print("<<< Ready to receive on destination: " + DURA_TEST_TOPIC);

		} catch (JMSException jmse) {
            this.isConnected = false;
            jmse.printStackTrace();

            this.close();
        }
	}

    /**
     * Process the received message message. 
     *  This prints received message counter.
     * @param m the message to be processed.
     */
    private synchronized void processMessage(Message m) {
        
    	try {
    		//in this example, we do not expect a timeout, etc.
    		if ( m == null ) {
    			throw new RuntimeException ("<<< Received null message. 
                                     Maybe reached max time out. ");
    		}
    		
    		//get message counter property
        	long msgCtr = m.getLongProperty (MESSAGE_COUNTER);
        	
        	//get message in-doubt bit
        	boolean indoubt = m.getBooleanProperty(MESSAGE_IN_DOUBT);
        	
        	if ( indoubt) {
        		print("<<< received message: " + msgCtr + ", indoubt bit is true");
        	} else {
        		print("<<< received message: " + msgCtr);
        	}
        	
        } catch (JMSException jmse) {
            jmse.printStackTrace();
        }
    }

    /**
     * Commit a JMS transaction.
     * @throws JMSException
     */
    private void commit() throws JMSException {
        session.commit();
    }
    
    /**
     * Sleep for the specified time.
     * @param millis sleep time in milli-seconds.
     */
    private void sleep (long millis) {
        try {
            Thread.sleep(millis);
        } catch (java.lang.InterruptedException inte) {
            print (inte);
        }
    }
    
    /**
     * Print the specified message.
     * @param msg the message to be printed.
     */
    private static void print (String msg) {
    	System.out.println(new Date() + ": " + msg);
    }
    
    /**
     * Print Exception stack trace.
     * @param e the exception to be printed.
     */
    private static void print (Exception e) {
    	System.out.print(e.getMessage());
    	e.printStackTrace();
    }
    
    /**
     * Start this example program.
     */
    public static void main (String args[]) {
        FailoverConsumer fc = new FailoverConsumer();
        fc.run();
    }

}

Custom Client Acknowledgment

Message Queue supports the standard JMS acknowledgment modes (auto-acknowledge, client-acknowledge, and dups-OK-acknowledge). When you create a session for a consumer, you can specify one of these modes. Your choice will affect whether acknowledgment is done explicitly (by the client application) or implicitly (by the session) and will also affect performance and reliability. This section describes additional options you can use to customize acknowledgment behavior:

• You can customize the JMS client-acknowledge mode to acknowledge one message at a time.

• If performance is key and reliability is not a concern, you can use the proprietary no-acknowledge mode to have the broker consider a message acknowledged as soon as it has been sent to the consuming client.

The following sections explain how you program these options.

Using Client Acknowledge Mode

For more flexibility, Message Queue lets you customize the JMS client-acknowledge mode. In client-acknowledge mode, the client explicitly acknowledges message consumption by invoking the acknowledge() method of a message object. The standard behavior of this method is to cause the session to acknowledge all messages that have been consumed by any consumer in the session since the last time the method was invoked. (That is, the session acknowledges the current message and all previously unacknowledged messages, regardless of who consumed them.)

In addition to the standard behavior specified by JMS, Message Queue lets you use client-acknowledge mode to acknowledge one message at a time.

Observe the following rules when implementing custom client acknowledgment:

• To acknowledge an individual message, call the acknowledgeThisMessage() method. To acknowledge all messages consumed so far, call the acknowledgeUpThroughThisMessage() method. Both are shown in the following code example.

  public interface com.sun.messaging.jms.Message {
            void acknowledgeThisMessage() throws JMSException;
            void acknowledgeUpThroughThisMessage() throws JMSException;
  }

• When you compile the resulting code, include both imq.jar and jms.jar in the class path.

• Don’t call acknowledge(), acknowledgeThisMessage() , or acknowledgeUpThroughThisMessage() in any session except one that uses client-acknowledge mode. Otherwise, the method call is ignored.

• Don’t use custom acknowledgment in transacted sessions. A transacted session defines a specific way to have messages acknowledged.

If a broker fails, any message that was not acknowledged successfully (that is, any message whose acknowledgment ended in a JMSException) is held by the broker for delivery to subsequent clients.

Example 3–6 demonstrates both types of custom client acknowledgment.

Example 3–6 Example of Custom Client Acknowledgment Code

...
import javax.jms.*;
...[Look up a connection factory and create a connection.]

    Session session = connection.createSession(false,
                       Session.CLIENT_ACKNOWLEDGE);

...[Create a consumer and receive messages.]

    Message message1 = consumer.receive();
    Message message2 = consumer.receive();
    Message message3 = consumer.receive();

...[Process messages.]

...[Acknowledge one individual message.
   Notice that the following acknowledges only message 2.]

    ((com.sun.messaging.jms.Message)message2).acknowledgeThisMessage();

...[Continue. Receive and process more messages.]

    Message message4 = consumer.receive();
    Message message5 = consumer.receive();
    Message message6 = consumer.receive();

...[Acknowledge all messages up through message 4. Notice that this
    acknowledges messages 1, 3, and 4, because message 2 was acknowledged 
    earlier.]

    ((com.sun.messaging.jms.Message)message4).acknowledgeUpThroughThisMessage();
...[Continue. Finally, acknowledge all messages consumed in the session.    
    Notice that this acknowledges all remaining consumed messages, that is,
    messages 5 and 6, because this is the standard behavior of the JMS API.]

    message5.acknowledge();

            

Using No Acknowledge Mode

No-acknowledge mode is a nonstandard extension to the JMS API. Normally, the broker waits for a client acknowledgment before considering that a message has been acknowledged and discarding it. That acknowledgment must be made programmatically if the client has specified client-acknowledge mode or it can be made automatically, by the session, if the client has specified auto-acknowledge or dups-OK-acknowledge. If a consuming client specifies no-acknowledge mode, the broker discards the message as soon as it has sent it to the consuming client. This feature is intended for use by nondurable subscribers consuming nonpersistent messages, but it can be used by any consumer.

Using this feature improves performance by reducing protocol traffic and broker work involved in acknowledging a message. This feature can also improve performance for brokers dealing with misbehaving clients who do not acknowledge messages and therefore tie down broker memory resources unnecessarily. Using this mode has no effect on producers.

You use this feature by specifying NO_ACKNOWLEDGE for the acknowledgeMode parameter to the createSession, createQueueSession, or createTopicSession method. No-acknowledge mode must be used only with the connection methods defined in the com.sun.messaging.jms package. Note however that the connection itself must be created using the javax.jms package.

The following are sample variable declarations for connection, queueConnection and topicConnection:

javax.jms.connection Connection;
javax.jms.queueConnection queueConnection
javax.jms.topicConnection topicConnection

The following are sample statements to create different kinds of no-acknowledge sessions:

//to create a no ack session
Session noAckSession  =
     ((com.sun.messaging.jms.Connection)connection)
    .createSession(com.sun.messaging.jms.Session.NO_ACKNOWLEDGE);

// to create a no ack topic session
TopicSession noAckTopicSession  =
     ((com.sun.messaging.jms.TopicConnection) topicConnection)
    .createTopicSession(com.sun.messaging.jms.Session.NO_ACKNOWLEDGE);

//to create a no ack queue session
QueueSession noAckQueueSession  =
     ((com.sun.messaging.jms.QueueConnection) queueConnection)
    .createQueueSession(com.sun.messaging.jms.Session.NO_ACKNOWLEDGE);

Specifying no-acknowledge mode for a session results in the following behavior:

• The client runtime will throw a JMSException if Session.recover() is called.

• The client runtime will ignore a call to the Message.acknowledge() method from a consumer.

• Messages can be lost. As opposed to dups-OK-acknowledge, which can result in duplicate messages being sent, no-acknowledge mode bypasses checks and balances built into the system and may result in message loss.

Communicating with C Clients

Message Queue supports C clients as message producers and consumers.

A Java client consuming messages sent by a C client faces only one restriction: a C client cannot be part of a distributed transaction, and therefore a Java client receiving a message from a C client cannot participate in a distributed transaction either.

A Java client producing messages for a consuming C client must be aware of the following differences in the Java and C interfaces because these differences will affect the C client’s ability to consume messages: C clients

• Can only consume messages of type text and bytes

• Cannot consume messages whose body has been compressed

• Cannot participate in distributed transactions

• Cannot receive SOAP messages