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 or IHandleMessage so that they would handle all incoming messages.
It is now recommended to plug into the message handling pipeline to implement these cross-cutting concerns.
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.
Consult the data access guidelines when multiple handlers for the same message need to participate in the same transaction.
Overview of the implementation
- Find the list of possible handlers for a message.
- If an order has been specified for any of those handlers, move them to the start of the list.
- 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
Specifying one handler to run first
endpointConfiguration.AddHandler<HandlerB>();
Specifying multiple handlers to run in order
endpointConfiguration.AddHandler<HandlerB>();
endpointConfiguration.AddHandler<HandlerA>();
endpointConfiguration.AddHandler<HandlerC>();