Subscription durability
Subscriptions can be configured to be durable or non-durable. Subscription durability determines what happens to subscriptions when subscribing applications disconnect from a queue manager.
Durable subscriptions
Durable subscriptions continue to exist when a subscribing application's connection to the queue manager is closed. If a subscription is durable, when the subscribing application disconnects, the subscription remains in place and can be used by the subscribing application when it reconnects requesting the subscription again using the SubName that was returned when the subscription was created.
When subscribing durably, a subscription name ( SubName ) is required. Subscription names must be unique within a queue manager so that it can be used to identify a subscription. This means of identification is necessary when specifying a subscription you want to resume, if you have either deliberately closed the connection to the subscription (using the MQCO_KEEP_SUB option) or have been disconnected from the queue manager. You can resume an existing subscription by using the MQSUB call with the MQSO_RESUME option. Subscription names are also displayed if you use the DISPLAY SBSTATUS command with SUBTYPE ALL or ADMIN.
When an application no longer requires a durable subscription it can be removed using the MQCLOSE function call with the MQCO_REMOVE_SUB option or it can be deleted manually use the MQSC command DELETE SUB.
You can use the DURSUB topic attribute to specify whether or not durable subscriptions can be made to a topic.
On return from an MQSUB call using the MQSO_RESUME option, subscription expiry is set to the original expiry of the subscription and not the remaining expiry time.
A queue manager continues to send publications to satisfy a durable subscription even if that subscriber application is not connected. This leads to a build up of messages on the subscriber queue. The easiest way to avoid this problem is to use a non-durable subscription wherever appropriate. However, where it is necessary to use durable subscriptions, a build up of messages can be avoided if the subscriber subscribes using the Retained publications option. A subscriber can then control when it receives publications by using the MQSUBRQ call.
Non-durable subscriptions
Non-durable subscriptions exist only as long as the subscribing application's connection to the queue manager remains open. The subscription is removed when the subscribing application disconnects from the queue manager either deliberately or by loss of connection. When the connection is closed, the information about the subscription is removed from the queue manager, and is no longer shown if you display subscriptions using the DISPLAY SBSTATUS command. No more messages are put to the subscriber queue.
- If a subscribing application is using a managed destination, any publications that have not been consumed are automatically removed.
- If the subscribing application provides a handle to its own subscriber queue when it subscribes, unconsumed messages are not removed automatically. It is the responsibility of the application to clear the queue if that is appropriate. If the queue is shared by more than one subscriber, or other point-to-point applications, it might not be appropriate to clear the queue completely.
Although not required for non-durable subscriptions, a subscription name if provided, is used by the queue manager. Subscription names must be unique within the queue manager so that it can be used to identify a subscription.