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How The Command Processor Works
You don't need to understand how Brighter works under the hood to use it, but if you want to debug, or contribute to the project, it can help to know what is going on.
Ignoring attributes, which create a pipeline, for now, all of CommandProcessor's dispatch methods:
- Accept a IAmARequest derived class as an argument
- Find the registered handler(s) for the type of the command
- Ask the factory you provide to create an instance of that handler
- Call Handle() or HandleAsync() as appropriate on that handler, passing in the command
Let's look at that sequence in more detail, for those of you who want to understand the detail, so that you can walk the code if required. We'll show the code looking for a SendAsync() but the Send() works the same way. Publish() and PublishAsync() are a minor variation in that they may dispatch to multiple handler chains, not just the one.
Let's model what happens, if you use Brighter as the Ports layer behind the an ASP.NET Core web controller.
1: The client makes an HTTP POST request to ASP.NET Core which marshalls the parameters and calls your web controller's POST method.
2: The web controller takes those parameters, along with any other relevant information, such as identity from the bearer token, and creates a Command. Let's call it MyCommand
3: The web controller calls the SendAsync() method on CommandProcessor because it is a Command and we expect one handler. If this was an Event, perhaps raised by the command handler itself, we would call PublishAsync() instead.
4: The SendAsync() method creates a RequestContext, using the supplied RequestContextFactory. Here we use the supplied InMemoryRequestContextFactory, which does not try to persist the state of the request, and is adequate for most purposes.
5: The CommandProcessor then creates a new PipelineBuilder. The PipelineBuilder is a generic type specialized to the type of the Request. (Both Command and Event inherit from Request.) The PipelineBuilder orchestrates building the chain of responsibility that will handle our request.
6: The PipelineBuilder creates an Interpreter. Again this is a generic type, specialized to the type of the Request. The interpreter is going to find the 'target handlers'. A 'target handler' is your code that implements IAmARequestHandlerAsync (usually via RequestHandlerAsync<>) that you register via the SubscriberRegistry. it is where the code which exercises the entities of your domain should live. The PipelineBuilder also creates a LifeTimeScope It will track the handlers we have created as part of this request.
7: The CommandProcessor calls Build() on the PipelineBuilder to create the pipeline.
8: The PipelineBuilder asks the Interpreter for an instance of the registered RequestHandlerAsync<>.
9: The Interpreter uses the SubscriberRegistry to lookup the RequestHandlerAsync<> that we have associated with this Command. We don't show it here, but we passed the SubscriberRegistry to the CommandProcessor when we built it, so it's not created here. The SubscriberRegistry returns the type of RequestHandlerAsync<>. In our example MyCommandHandlerAsync.
10: The Interpreter now tries to build an instance of the handler by calling the HandlerFactory. The HandlerFactory is supplied to the CommandProcessor when it is built. It is user implemented because we don't know how to build your handler, which has its dependencies in your code. However, we do integrate with ServiceCollection if desired. In our case we build a MyHandlerAsync.
11: Having constructed a handler, we now need to build the pipeline based on the attributes that you have decorated the handler with (and any global attributes). We call the BuildPipeline method to create the pipeline.
12: We start with the part of the chain called before the registered handler - attributes flagged HandlerTiming.Before.
13: Because your pipeline's configuration can only change at design time (when you write code) and not at runtime (when you execute) we only want to figure out the attributes of the chain once. So we store the attribute list, once it's been determined, in a memento collection. This collection is a static. The first thing we do when building is see if we have already determined the configuration. If we have, we will use that.
14: If we have not got a pipeline configuration, then we need to build one. Our first step is to call FindHandlerMethod() to get the handler method from our target handler i.e. MyCommandHandler in this case. RequestHandlerAsync does the heavy lifting for you here.
15: We then use RTTI to find the attributes you have tagged your handler method with. In this case, let's pretend we just have RequestLoggingAsync<>. We order them via the Step value on the attribute (note that step collision order behavior is undefined).
16: Once we have the per-handler list of attributes, we check if the CommandProcessor has been configured to use a global inbox. If it already has a UseInboxAsync attribute, or has a NoGlobalInbox attribute in the preAttributes list, we are done. Otherwise, if we have set up a global inbox, we add UseInboxAsync into the list of handlers.
17: We then add the preAttributes into the collection of preAttributes, so that we will not need to use RTTI to build them again. Let's assume that we have configured out CommandProcessor with a global inbox.
18: Next we have to construct this preAttribute chain. We iterate over all of the attributes and create the handlers by calling the supplied HandlerFactory. Again, this is because only your code knows how to create your handlers. If you use our support for ServiceCollection, then we will search the assemblies in the project for classes that implement IAmARequestHandlerAsync (including those we supply) and register them for you. The hander we create is determined from the type information supplied by the Attribute. In our example we will construct a RequestLoggingHandlerAsync and a UseInboxHandlerAsync.
19: We construct the pre-Attributes. once we have constructed a Handler, we set it's successor property to be the next handler in the chain.
20: We then repeat this process for any post handler attributes i.e. those tagged as HandlerTiming.After. We don't have any of those here, so we don't show that again this time. This decision was mainly for simplicity.
21: We then add the handlers to the LifeTimeScope of the pipeline.
22: We then return the handler chain that we just constructed to the CommandProcessor.
23: The CommandProcessor checks that we have a valid pipeline; for a SendAsync we expect exactly one pipeline will handle the Command. For PublishAsync we allow zero or more.
24: Now we call the pipeline by passing our MyCommand to the first handler in the chain, in our case the UseInboxHandlerAsync.
25: The UseInboxHanlderAsync has an inbox as a private member, that was passed in via the constructor via the HandlerFactoryAsync. This is a data store specific implementation of IAmAnInbox. if the OnceOnly parameter is set on the attribute then we call the Inbox's ExistsAsync method to determine if we have already processed the command. If the command has not already been processed, we call the base class's HandleAsync method.
26: The base class's HandleAsync() method we use the successor field (see 19 above) to determine the next handler in the chain and we call it's HandleAsync() method. In this case we call RequestLoggingAsync<>'s HandleAsync method.
27: The RequestLoggingAsync<>'s HandleAsync method logs the call, and again calls the base class's HandleAsync() method to pass the call down the pipeline.
28: Finally, we call MyCommandHandlerAsync whose HandleAsync() command runs our business logic. Again we call the base class's HandleAsync() method, but as there is no successor we return.
29: We return from RequestLoggingAsync<> which has no work left to do.
30: UseInboxHandlerAsync calls IAmAnIbox's AddAsync method to write the command to the Inbox. Then it returns.
31: SendAsync returns, and we are done.