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Creating durable topic subscriptions
If a client subscriber creates a durable subscription to the topic, the subscriber will be able to recover the messages that were published to the topic while the client was inactive. For example, a durable topic subscriber will receive all the messages that are published to the topic while the subscriber is active. However, if the subscriber becomes inactive for a time period and later returns to active state, the messages that were published during the inactive period will be fetched from the database and dispatched to the subscriber.
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It is not possible to unsubscribe a durable consumer from inside the onMessage() method in the listener. If you need to unsubscribe, write a separate class/thread etc. with a different session, give the subscription ID and unsubscribe.
Sharing a durable topic subscription
As explained in the previous section, durable topic subscriptions are unique. That is, the subscription ID and client ID, which are used for identifying the durable subscription should be unique. This means that it is not possible to have multiple subscriber clients sharing a single subscription. Now lets look at what is meant by 'sharing' a durable subscription and how this possibility can be enabled for WSO2 MB.
Understanding shared durable topic subscriptions
Consider the following scenario:
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As shown above, what we need is to establish a single subscription where the messages received are shared by multiple subscriber clients using the round-robin method. In the default set up of WSO2 MB, this is not possible, because when there are multiple subscriber clients connecting to the topic, copies of the same messages are dispatched to all the subscriber clients. Therefore, sharing a subscription is only possible if the subscription ID can be shared by all the subscriber clients.
Shared durable subscriptions in a clustered setup
Now, lets look at how this functionality of sharing a durable topic subscription works when the brokering facility is distributed among multiple MB nodes. Consider the following scenario:
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Note that if you unsubscribe one shared topic subscriber, it will affect all other durable topic subscriptions with the same subscription ID. |
Enabling shared durable topic subscription in WSO2 MB
You can enable shared topic subscriptions in WSO2 MB by following the steps given below.
- Open the
broker.xmlfile stored in the<MB_HOME>/repository/conffolder. Enable the
allowSharedTopicSubscriptionselement as shown below. Note that this property is only applicable to AMQP.Code Block <broker> <transports> <amqp enabled="true"> <allowSharedTopicSubscriptions>true</allowSharedTopicSubscriptions> </amqp> </transports> </broker>
- The JMS clients that are subscribing to the topic should create durable subscriptions using the same subscription ID. See the previous section on creating durable topics subscriptions.