What should be returned from the API for CQRS comm

问题:

As far as I understand, in a CQRS-oriented API exposed through a RESTful HTTP API the commands and queries are expressed through the HTTP verbs, the commands being asynchronous and usually returning 202 Accepted, while the queries get the information you need. Someone asked me the following: supposing they want to change some information, they would have to send a command and then a query to get the resulting state, why to force the client to make two HTTP requests when you can simply return what they want in the HTTP response of the command in a single HTTP request?

回答1:

We had a long conversation in DDD/CRQS mailing list a couple of months ago (link). One part of the discussion was "one way command" and this is what I think you are assuming. You can find out that Greg Young is opposed to this pattern. A command changes the state and therefore prone to failure, meaning it can fail and you should support this. REST API with POST/PUT requests provide perfect support for this but you should not just return 202 Accepted but really give some meaningful result back. Some people return 200 success and also some object that contains a URL to retrieve the newly created or updated object. If the command handler fails, it should return 500 and an error message.

Having fire-and-forget commands is dangerous since it can give a consumer wrong ideas about the system state.

回答2:

My team also recently had a very heated discussion about this very thing. Thanks for posting the question. I have usually been the defender of the "fire and forget" style commands. My position has always been that, if you want to be able to move to an async command dispatcher some day, then you cannot allow commands to return anything. Doing so would kill your chances since an async command doesn't have much of a way to return a value to the original http call. Some of my team mates really challenged this thinking so I had to start thinking if my position was really worth defending.

Then I realized that async or not async is JUST an implementation detail. This led me to realize that, using our frameworks, we can build in middleware to accomplish the same thing our async dispatchers are doing. So, we can build our command handlers the way we want to, returning what ever makes sense, and then let the framework around the handlers deal with the "when".

Example: My team is building an http API in node.js currently. Instead of requiring a POST command to only return a blank 202, we are returning details of the newly created resource. This helps the front-end move on. The front-end POSTS a widget and opens a channel to the server's web socket using the same command as the channel name. the request comes to the server and is intercepted by middleware which passes it to the service bus. When the command is eventually processed synchronously by the handler, it "returns" via the web socket and the front-end is happy. The middleware can be disabled easily, making the API synchronous again.

回答3:

There is nothing stopping you from doing that. If you execute your commands synchronously and create your projections synchronously, then it will be easy for you to just make a query directly after executing the command and returning that result. If you do this asynchronously via the rest-api, then you have no query result to send back. If you do it asynchronously within your system, then you can wait for the projection to be created and then send the response to the client.

The important thing is that you separate your write and read models in classic CQRS style. That does not mean that you cannot do a read in the same request as you do the command. Sure, you can send a command to the server and then with SignalR (or something) wait for a notification that your projection have been created/updated. I do not see a problem with waiting for the projection to be created on the server side instead for on the client.

How you do this will affect you infrastructure and error handling. Also, you will hold the HTTP request open for a longer time if you return the result at once.