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Returning results from a Handler
We use Command-Query separation so a Command does not have return value and CommandDispatcher.Send() does not return anything. Our project Darker provides a Query Processor that can be used to return results in response to queries. You can use both together to provide CQRS.
This in turn leads to a set of questions that we need to answer about common scenarios:
- How do I handle failure? With no return value, what do I do if my handler fails?
- How do I communicate the outcome of a command?
If we don't allow return values, what do you do on failure?
- The basic failure strategy is to throw an exception. This will terminate the request handling pipeline.
- If you want to Requeue (with Delay) to an External Bus, you should throw a DeferMessageAction exception.
Sometimes you need to provide information to the caller about the outcome of a Command, instead of listening for an Event an.
How do you communicate the outcome of handling a Command? There are two options, which depend on circumstance:
- Raise an Event
- Update a field on the Command
This approach let's you take action in response to a Command by raising an Event within your handler using CommandProcessor.Publish or via an External Bus using CommandProcessor.Post/CommandProcessor.DepositPost.
If you use an Internal Bus these handlers will run immediately, in their own pipeline, before your handler exits. If you use an External Bus you offload the work to another process.
If you are using an Internal Bus and need a return value from a Command you will note that CommandProcessor.Send has a void return value, so you cannot return a value from the handler.
What happens if the caller needs to know the outcome, and can't be signalled via an Event?
In that case add a property to the Command that you can initialize from the Handler. As an example, what happens if you need to return the identity of a newly created entity, so that you can use Darker to retrieve its details? In this case you can create a NewEntityIdentity property in your command that you write a newly created entity's identity to in the Handler, and then inspect the property in your Command in the calling code after the call to commandProcessor.Send(command) completes.
You can think of these as out parameters.
var createTaskCommand = new CreateTaskCommand();
var newTaskId = createTaskCommand.TaskId;