Performance considerations

Any queries about subscriptions are expensive on resources and time, because these queries are synchronous and blocking. For example, querying whether a client is subscribed to a topic, what clients are subscribed to a specific topic, or what topic a specific client subscribes to.

If your publishers respond to add topic notifications or subscription notifications, ensure that these responses are efficient. These publisher actions are now serialized in a single thread and as a result the publisher can become a bottleneck and hold up processing.

Using topic data

Where a topic is inherently stateful and has associated data, the use of topic data is recommended. Topic data automatically handles topic loading.

Topic loading

Typically, on subscription, the publisher provides the client with the current state of the data for the topic. It can do this by creating a new topic load message and populating it with a representation of the state. Rather than doing this every time a client subscribes it is generally more efficient for the publisher to create a topic load message only when the state changes and send this same message out to every client that subscribes.

This provision of the current state is known as the topic load. This can be done in one of the following ways:

Topic load in subscription method

If the topic has not already been loaded by a topic loader (see below), the loaded parameter of the subscription method is false. In this case, the normal action is for the publisher to send a topic load message to the client (passed as a parameter to subscription) through its send method.

Topic loaders

A topic loader is an object that implements the TopicLoader interface and can be used to perform any topic load processing that is required for one or more topics. Topic loaders can be declared for a Publisher using the Publisher.addTopicLoader method. This is typically done in the initialLoad processing and must be done before any topics that are loaded by the topic loader are added.

Hierarchic subscription

When a client subscribes to a topic the publisher can choose to subscribe the client to other topics or to subordinate topics. This can be done using the Client.subscribe methods.

A client itself can request subscription to a hierarchy of topics using topic selectors but this is an alternative method of handling hierarchies.