WAS v8.5 > Administer applications and their environment > Welcome to administering Messaging resources > Manage messaging with the default messaging provider > Configure resources for the default messaging provider

Protecting an MDB or SCA application from system resource problems

We can configure the system so that if there is a problem with a dependent external system resource, the enterprise application is stopped before messages are moved unnecessarily to an exception destination. This configuration also handles occasional problems with messages without blocking the enterprise application.

This task assumes that we have deployed an enterprise application containing a MDB, or a business-level application containing a SCA composite, that interacts with external system resources.

The destination to which the MDB or SCA composite listens must use an exception destination. This exception destination can be the system default, or one configured specifically for the destination.

To complete this task, you need the following information:

• The enterprise application containing the MDB, or the business-level application containing the SCA composite.
• The dependent external system resources.
• An acceptable value for the Sequential failed message threshold, that is, the maximum number of sequential failures of delivery of messages, after which the MDB or SCA composite is stopped. This property applies to sets of messages.
• An acceptable value for the Delay between failing message retries, that is, the time in milliseconds before a failing message is available to be delivered to the MDB or SCA composite. Other messages might be delivered during this period, unless theSequential failed message threshold and the maximum concurrency is set to 1.
• An acceptable value for the Maximum failed deliveries per message, that is, the maximum number of failed attempts to process a message, after which the message is forwarded from its intended destination to the exception destination. This property applies to individual messages.

When an MDB or SCA composite fails to process a message, the message is rolled back and made available to the MDB or SCA composite again.

Typically, the messaging system is configured in one of the following ways:

1. Failed messages are retried a finite number of times, and if they continue to fail, they are moved to an exception destination allowing subsequent messages to be processed.
2. Failed messages are retried indefinitely until the problem is rectified.

Configuration (1) protects the MDB or SCA composite from an occasional problem message that prevents subsequent messages from being processed. However, if there is a prolonged problem with a resource the enterprise application or business-level application depends on, for example a database, all messages sent to the destination might be moved to the exception destination.

Configuration (2) blocks the delivery of messages until the original failing message problem is resolved. This configuration prevents messages being moved unnecessarily to an exception destination, but it also blocks subsequent messages as soon as a single problem message fails to be processed.

We can configure the activation specification for an MDB or SCA composite so the MDB or SCA composite endpoint is stopped automatically when a number of failures with sequential messages are detected. These failures indicate a problem with a dependent resource. When the problem is resolved, the MDB or SCA composite endpoint is restarted manually. This configuration tolerates occasional message failures, allowing individual problem messages to be moved to the exception destination without blocking the entire MDB or SCA composite.

Use the following steps to protect an enterprise application from dependent external system resource failures.

1. cd deployed enterprise application containing the MDB, or the business-level application containing the SCA composite.

2. From the MDB or SCA composite, navigate to its JMS activation specification. Click Resources -> JMS -> Activation specifications -> activation_specification_name.

3. Enter a value for the Sequential failed message threshold and the Delay between failing message retries.

4. Save the configuration.
5. cd destination to which the MDB or SCA composite is listening. Click one of the following paths, as appropriate:
   • Service integration -> Buses -> bus_name -> [Destination resources] Destinations -> queue_name
   • Service integration -> Buses -> bus_name -> [Destination resources] Destinations -> topic_space_name

6. Enter a value for the Maximum failed deliveries per message.

7. Save your changes to the master configuration.
   

Results

You have configured the enterprise application, or business-level application, to protect itself from the sort of external resource problem that can occur at any time. This means that, in the event of a system resource problem, the MDB or SCA composite is stopped automatically when the Sequential failed message threshold is reached for any message.

When the system resource that failed becomes available, we can restart the system resource and resume the MDB or SCA composite. The messages that failed during the system resource downtime are retried instead of being left on an exception destination.

Subtopics

• Example 1: Handling a planned outage of an MDB or SCA application external resource
  We can configure the system so that, if there is a problem with a dependent external system resource, the enterprise or business-level application can continue.
• Example 2: Automatically stopping an MDB or SCA composite when a system resource becomes unavailable
  To prepare for a system resource becoming unavailable, configure the system to stop the MDB or Service Component Architecture (SCA) composite automatically after a small number of message failures, and to alert you to the problem.
• Example 3: The system experiences problems with a problem message
  To prepare for a problem message, configure the system to move that message to an exception destination and allow other messages to be processed successfully.
• Example 4: Automatically stopping an MDB or SCA composite when no exception destination is specified
  To prepare for a system resource becoming unavailable or a problem message, configure the system to stop the message-driven bean (MDB) or SCA composite automatically. To maintain message ordering, do not use an exception destination.

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