Handler Ordering

In the past, message handlers used to be the only way to implement cross-cutting concerns like authentication, authorization, and certain kinds of validation. As those concerns needed to be applied before any other logic, it was necessary to run those handlers before all other message handlers. These message handler classes implemented IHandleMessage<IMessage> or IHandleMessage<Object> so that they would handle all incoming messages.

If it is not possible to migrate this kind of functionality out of message handlers, there are a number of ways to specify the order in which they will be executed.

Overview of the implementation

1. Find the list of possible handlers for a message.
2. If an order has been specified for any of those handlers, move them to the start of the list.
3. Execute the handlers.

The remaining handlers (i.e. ones not specified in the ordering) are executed in a non-deterministic order.

With the configuration API

endpointConfiguration.ExecuteTheseHandlersFirst(
    typeof(HandlerB),
    typeof(HandlerA),
    typeof(HandlerC));

Specifying one handler to run first

endpointConfiguration.ExecuteTheseHandlersFirst(typeof(HandlerB));

Specifying multiple handlers to run in order

endpointConfiguration.ExecuteTheseHandlersFirst(
    typeof(HandlerB),
    typeof(HandlerA),
    typeof(HandlerC));