Routing topology

The RabbitMQ transport uses a routing topology to control how exchanges, queues, and bindings are created in the broker. The routing topology also defines how these exchanges are used for sending and publishing messages.

All endpoints in a system must use the same routing topology to communicate. For new systems, use the conventional routing topology. The direct routing topology should only be used when adding endpoints to an existing system that already relies on it. A custom routing topology may be useful when integrating with legacy systems.

Conventional routing topology

The conventional routing topology uses fanout exchanges to route messages.

Sending

Each endpoint creates a fanout exchange and a queue with the same name as the endpoint. A binding connects them.
Messages are sent by publishing them to the endpoint’s exchange, which then routes them to the queue.

Publishing

For each event type, a set of fanout exchanges is created to represent its inheritance hierarchy.

• Each type has an exchange named Namespace:TypeName.
• Bindings connect child types to their parents, modeling the full hierarchy.
• Exchanges are also created for each implemented interface.

When subscribing, an endpoint ensures the infrastructure exists and then adds a binding from the subscribed type’s exchange to its own exchange.

When publishing, the endpoint ensures the infrastructure exists and then publishes to the exchange of the event type.

Enabling

var transport = endpointConfiguration.UseTransport<RabbitMQTransport>();
transport.UseConventionalRoutingTopology();

Direct routing topology

The direct routing topology routes all events through a single exchange (default: amq.topic). Events are published with a routing key based on the event type, and subscribers use that key to filter messages.

Sending

Each endpoint creates a queue named after the endpoint.

When sending, the message is published to the default exchange with a routing key equal to the destination endpoint name. This leverages RabbitMQ default exchanges to deliver messages directly to the queue.

Publishing

Every endpoint publishes an event using the amq.topic exchange with a routing key of the form Namespace.TypeName, corresponding to the type of the event. The event is moved to all queues that have a binding for that event type.

An endpoint that subscribes to a given event creates a binding to the default exchange with the appropriate routing key.

Enabling

To enable the direct routing topology, use the following configuration:

var transport = endpointConfiguration.UseTransport<RabbitMQTransport>();
transport.UseDirectRoutingTopology();

Overriding conventions

The default conventions for exchange names and routing keys can be overridden by using the following overload:

var transport = endpointConfiguration.UseTransport<RabbitMQTransport>();
transport.UseDirectRoutingTopology(
    routingKeyConvention: MyRoutingKeyConvention,
    exchangeNameConvention: () => "MyTopic");

Controlling exchange and queue durability

The routing topologies provided by the transport create durable exchanges and queues by default. To create transient exchanges and queues, use the following:

var transport = endpointConfiguration.UseTransport<RabbitMQTransport>();
transport.DisableDurableExchangesAndQueues();

Custom routing topology

If the built-in routing topologies do not satisfy the requirements of the system, a custom routing topology may be used. To do this:

1. Define the routing topology by creating a class implementing IRoutingTopology.
2. Register it with the transport as shown below:

var transport = endpointConfiguration.UseTransport<RabbitMQTransport>();
transport.UseCustomRoutingTopology(
    topologyFactory: createDurableExchangesAndQueues =>
    {
        return new MyRoutingTopology(createDurableExchangesAndQueues);
    });

The boolean argument supplied to the factory delegate indicates whether the custom routing topology should create durable exchanges and queues on the broker. Read more about durable exchanges and queues in the AMQP Concepts Guide.