Creating Durable Topic Subscriptions

Durable topics persist messages, which allows the messages to be received later if a subscriber is not online. A durable topic subscription is useful when a subscriber client needs to be able to receive messages that are published even when the client is inactive.

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. 

The following points will elaborate on the different elements that you need to be mindful of when creating a durable topic subscription:

• Creating a durable topic subscription is similar to creating a nondurable subscription, but you must additionally provide a name that identifies the durable subscription as shown below.

  // Create a durable subscriber, supplying a uniquely-identifying name
  TopicSubscriber sub = session.createDurableSubscriber( topic, "mySub1_0001" );

• Shown below is the connection URL used for subscribing to the broker during an AMQP session. See the topic on setting the connection URL for more information.

  connectionfactory.Connectionfactory = amqp://admin:admin@clientID/carbon?brokerlist='tcp://192.168.10.5:5682'

• Reusing a durable topic subscription: To reuse a durable subscription that was created previously, the subscriber client must create a durable topic subscriber using a session with the same client identifier as that associated with the durable subscription.  

Unsubscribing non-durable topic subscriptions

Nondurable message consumers in the publish/subscribe domain automatically deregister themselves when the close() method is called or when they fall out of scope. However, if you want to terminate a durable subscription, you must explicitly notify the broker. To do this, use the unsubscribe() method of the session and pass in the name that identifies the durable subscription:

// Unsubscribe the durable subscriber created above
session.unsubscribe( "mySub1_0001" );

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 let's look at what is meant by 'sharing' a durable subscription and how this possibility can be enabled for the broker.

Understanding shared durable topic subscriptions

Consider the following scenario:

In the above example, there are two subscribers (A and B) with durable subscriptions to the topic and each subscriber performs a specific task with the messages received through the subscription. Therefore, when messages are published to the topic, copies of the messages are dispatched to both subscribers and each subscriber will perform a unique task. This is how the default configuration of EI Message Broker works. You will note here that it is not possible to have two subscriber clients performing the same task. For example, consider that the task that should be performed by subscriber A is time-consuming and requires a lot of CPU memory. In such a situation, we need to be able to scale the workload of subscriber A among multiple subscriber clients. The following diagram depicts this requirement:
 

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 setup 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 broker nodes. Consider the following scenario:
 

In this example, we have a cluster of 3 broker nodes and all 3 nodes have the 'SportsNews' topic created. The following points will elaborate how this clustered setup works:

• There are multiple durable topic subscribers (Sub-1, Sub-2, Sub-3 and Sub-4) connecting to the broker cluster using a shared subscription ID (Sub1). Note that the subscribers Sub-2 and Sub-3 are connecting to the same broker node in the cluster.

• There are 3 other clients (Pub-1, Pub-2 and Pub-3) publishing messages to the SportsNews topic.

  When a client publishes messages to a topic, the client must connect to a particular broker node in the cluster. In the above example, publishers Pub-1, Pub-2 and Pub-3 connects to broker nodes A, B and C respectively. However, since all three nodes belong to a clustered setup, the messages published to each node are received by the slot manager of the cluster and not by the specific node to which the publisher sends the message. The slot manager then allocates all the received messages among the 3 nodes. This means that the messages published to the broker cluster can be dispatched to the subscriber clients from any one of the 3 nodes in the cluster.

• All the messages published to the cluster will be evenly distributed among the subscribers because the subscription is shared. Therefore, the total number of messages that are published to the broker cluster will equal the total number of messages that are received by all the subscribers (x1+x2+x3 = y1+y2+y3+y4).

Enabling shared durable topic subscription in EI Message Broker

You can enable shared topic subscriptions in EI Message Broker by following the steps given below.

1. Open the broker.xml file stored in the <EI_HOME>/wso2/broker/conf folder.

2. Enable the allowSharedTopicSubscriptions element as shown below. Note that this property is only applicable to AMQP.

   <broker>
    <transports>
     <amqp enabled="true">
      <allowSharedTopicSubscriptions>true</allowSharedTopicSubscriptions>
     </amqp>
    </transports>
   </broker>

3. 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.