Topology Conventions

Conventions are used to apply topology to messages without requiring explicit configuration of every message type.

A basic example of a convention is the default CorrelationId convention, which is automatically applied to all sent messages. As message types are sent, the convention is used to determine if the message contains a property that could be considered a CorrelationId, and uses that property to set the CorrelationId header on the message envelope.

For example, the following message contains a property named CorrelationId, which is an obvious choice. Note that the CorrelatedBy<Guid> interface is not part of the message contract.

public interface OrderCreated
{
    Guid CorrelationId { get; }
}

If there isn't a property named CorrelationId, the convention also checks for CommandId and EventId and uses that property to set the header value (the type must be a Guid, or a Guid?, no magic type conversion happening here).

If the message implements the CorrelatedBy<Guid> interface, that would be used before referencing any properties by name.

During bus creation, it is possible to explictly configure a message type (or any of the message type's inherited interfaces) to use a specific property for the CorrelationId. In the example below, the OrderId property is specified as the CorrelationId.

public interface OrderSubmitted
{
    Guid OrderId { get; }
    Guid CustomerId { get; }
}

Bus.Factory.CreateUsingRabbitMQ(..., cfg =>
{
    cfg.Send<OrderSubmitted>(x =>
    {
        x.UseCorrelationId(context => context.Message.OrderId);
    });
});

The CorrelationId topology convention is implemented here, which can be used as an example of how to create your own conventions, or add additional CorrelationId detectors to the existing convention.

Send topologies are applied to all outbound messages, regardless of whether they are sent or published.