The RabbitMQ transport has the concept of a routing topology, which controls how it creates exchanges, queues, and the bindings between them in the RabbitMQ broker. The routing topology also controls how the transport uses the exchanges it creates to send and publish messages. All endpoints in a system must use the same topology to be able to communicate with each other. For new systems, the conventional routing topology should be used. The direct routing topology is recommended only when adding an endpoint to an existing system that already uses that topology. A custom topology can be useful when integrating with a legacy system.
The conventional routing topology relies on fanout exchanges to route messages.
Each endpoint creates a pair of a fanout exchange and a queue named after the endpoint's name. It also creates a binding between them. Messages are sent to the endpoint by publishing them to the endpoint's exchange. The binding then routes the message to the endpoint's queue.
For each type being published, a series of fanout exchanges are created to model the inheritance hierarchy of the type. For each type involved, an exchange is created, named in the following format:
Namespace:TypeName. Bindings are created between the types, going from child to parent, until the entire hierarchy has been modeled. Exchanges are also created for each interface the type implements.
When an endpoint subscribes to an event, it first ensures that the above infrastructure exists. It then adds a binding from the exchange corresponding to the subscribed type to its own exchange.
When an endpoint publishes an event, it first ensures that the above infrastructure exists. It then publishes the message to the exchange corresponding to the message type being published.
The direct routing topology routes all events through a single exchange,
amq. by default. Events are published using a routing key based on the event type, and subscribers will use that key to filter their subscriptions.
Every endpoint creates a queue named after the endpoint's name. When an endpoint sends a message it publishes it to a default exchange with a routing key equal to the destination endpoint name. This makes use of RabbitMQ default exchanges to move the message to a queue with the same name.
Every endpoint publishes an event using the
amq. exchange with a routing key of the form
Namespace., 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.
To enable the direct routing topology, use the following configuration:
var transport = busConfiguration.UseTransport<RabbitMQTransport>(); transport.UseDirectRoutingTopology();
The default conventions for exchange names and routing keys can be overridden by using the following overload:
var transport = busConfiguration.UseTransport<RabbitMQTransport>(); transport.UseDirectRoutingTopology( routingKeyConvention: MyRoutingKeyConvention, exchangeNameConvention: (address, eventType) => "MyTopic");
The routing topologies provided by the transport always create durable exchanges. Durable queues are created unless durable messages have been disabled globally for the endpoint. Transient queues will be created when
DisableDurableMessages is called.
If the built-in routing topologies do not satisfy the requirements of the system, a custom routing topology may be used. To do this:
- Define the routing topology by creating a class implementing
- Register it with the transport calling
UseRoutingTopologyas shown below.
var transport = busConfiguration.UseTransport<RabbitMQTransport>(); transport.UseRoutingTopology<MyRoutingTopology>();
For each queue required by the endpoint, the transport will first declare that queue and will then call the
Initialize method of the routing topology. The routing topology should then perform all initialization related to that specific queue such as the creation of appropriate exchanges and bindings.