HTopic

A HTopic topic is a distributed topic corresponding to a cluster-side List.

The topic behavior can be configured on the server: see the general List documentation for complete details about topics.

Defining Topics

Topics are fully identified by their type and unique name, regardless of the types specified for topic messages. In other words, an HTopic<string> and an HTopic<int> named with the same name are backed by the same cluster structure. Obviously, refering to a topic with types other than the expected types can have unspecified consequences (probably, serialization errors) and is not recommended.

The messages type can be just about any valid .NET type, provided that it can be (de)serialized by the Hazelcast .NET Client (see the Serialization documentation). It does not necessarily need to be (de)serializable by the cluster, as long as the cluster does not need to handle them as objects, and can treat them as plain binary blobs. As soon as the cluster needs to handle the objects themselves, the types must also be (de)serializable by the cluster.

Creating & Destroying Topics

A topic is obtained from the Hazelcast .NET Client, and is created on-demand: if a topic with the specified name already exists on the cluster, it is returned, otherwise it is created on the cluster. For instance:

var optic = await client.GetTopicAsync<string>("my-topic");

Topics should be disposed after usage, in order to release their resources. Note that this only releases client-side resources, but the actual data remain available on the cluster for further usage. In order to wipe the topic and its data entirely from the cluster, it needs to be destroyed:

await topic.DestroyAsync();

Using Topics

The HTopic structure is completely documented in the associated IHTopic<T> reference documentation. It provides a method to publish messages:

• PublishAsync(message) publishes a message

The HTopic structure exposes events (see events general documentation) at topic level. A complete list of events is provided in the CollectionItemEventHandlers<T> documentation. The following example illustrates how to subscribe, and unsubscribe, to topic events:

var id = await topic.SubscribeAsync(events => events
    .Message((sender, args) => {
        logger.LogInformation($"Got message {args.Payload} at {args.PublishTime}.")
    }));

// ...

await topic.UnsubscribeAsync(id);

Note that the handler methods passed to e.g. Message can be asynchronous, too.