Publish-Subscribe

NServiceBus has a built-in implementation of the Publish-subscribe pattern.

Publish-subscribe is a messaging pattern where senders of messages, called publishers, do not program the messages to be sent directly to specific receivers, called subscribers. Instead, published messages are characterized into classes, without knowledge of what, if any, subscribers there may be. Similarly, subscribers express interest in one or more classes, and only receive messages that are of interest, without knowledge of what, if any, publishers there are.

Or, in simpler terms

Subscribers let the publisher know they're interested, and the publisher stores their addresses to know where to send which message.

Mechanics

Depending on the features provided by a given transport, there are two possible implementations of Publish-Subscribe mechanics: message-driven (persistence-based) and native.

All subscribers get their copy of the event

To ensure that each subscriber can process and potentially retry the event independently of other subscribers, NServiceBus ensures that each subscriber receives a copy of the published event delivered to their input queue.

Message-driven (persistence-based)

Message-driven publish-subscribe is controlled by subscribe and unsubscribe system messages the subscriber sends to the publisher. The message-driven publish-subscribe implementation is used by the unicast transports. These transports are limited to unicast (point-to-point) communication and have to simulate multicast delivery via a series of point-to-point communications.

Subscribe

The subscribe workflow for unicast transports is as follows:

1. Subscribers request to a publisher the intent to subscribe to specific message types.
2. Publisher stores the subscriber names and the message types in the persistence.

The publisher's address is provided via routing configuration.

Publish

Message-driven publish-subscribe relies on the publisher's access to a persistent store to maintain the mapping between message types and their subscribers.

Available subscription persisters include

• MSMQ
• RavenDB
• NHibernate
• Non-Durable
• Azure Storage
• SQL Server

The publish workflow for unicast transports is as follows:

1. Some code (e.g., a saga or a handler) requests that a message be published.
2. The publisher queries the storage for a list of subscribers.
3. The publisher loops through the list and sends each subscriber a copy of that message.

Disabling publishing

By default, NServiceBus requires a subscription persister to be configured. If an endpoint does not publish any events, the publish functionality can be disabled to avoid providing a subscription persister:

var transportConfiguration = endpointConfiguration.UseTransport(new TransportDefinition());
transportConfiguration.DisablePublishing();

Native

For multicast transports that support publish–subscribe natively neither persistence nor control message exchange is required to complete the publish-subscribe workflow.

Subscribe

The subscribe workflow for multicast transports is as follows:

1. Subscribers send a request to the broker with the intent to subscribe to specific message types.
2. The broker stores the subscription information.

Note that the publisher does not interact with the subscribe workflow in this case.

Publish

The publish workflow for multicast transports is as follows:

1. Some code (e.g., a saga or a handler) requests that a message be published.
2. The publisher sends the message to the Broker.
3. The broker sends a copy of that message to each subscriber.