Using messaging to do writes as well as reads

Question

Hi,

I come from a web background where I only have to deal with HTTP so please excuse my ignorance.

I have an app which where clients listen for changes in a message queue which uses stomp. Previously the client only needed to listen to the relevant channels for messages telling them about changes on the server and update themselves accordingly. Simple stuff.

There is now a requirement for the client to be able to edit data and push those changes back to the server. The data on the server is already exposed via restful resources so my first thought was just to make REST put requests to change the data on the server, but then I started to wonder whether I could find a solution using messaging. I could just open up another channel which the clients could publish changes to and the server could subscribe to that channel and update itself accordingly. Implementing this would obviously be simple but I would love to have some of the potential pitfalls pointed out to me ahead of time.

I am familiar with REST so I want to ask some questions in the context of REST:

• Would I map a group of queues to REST/CRUD verbs for each resource i.e. itemPostQueue, itemPutQueue, itemDeleteQueue?
• What about GET's how can I request data to read using a queue?
• What do I use to replace my status code mechanism to catch problems or do I just fire and forget (gulp) or use error/receipt headers in Stomp somehow?

Any answers and advise will be much appreciated.

Regards,

Chris

Answer 1

While I am not clear on why you must use messaging here, a few thoughts:

You could map to REST on the wire like itemPostQueue, but this would likely feel unnatural to a message-oriented person. If you are using some kind of queue with a guaranteed semantic and deliver-once built in, then go ahead and use that mechanism. For a shopping-cart example, then you could put an AddItem message on the wire, and you trust the infrastructure to deliver it once to the server.

There is no direct GET like concept here in message queuing. You can simulate it with a pair of messages, I send you a request and you send me back a response. This is much like RPC, but even further decoupled. So I send you a PublishCart request and later on, the server sends a CartContents message on a channel that the client is listening to.

Status codes are more complex, and generally fall into two camps. First are the actual queue-library messages - deal with them just as you would any normal system message. Second you may have your own messages you want to put on the wire that signal failure at some place in the chain.

One thing that messaging does do is significantly decouple your app. Unlike HTTP, where you know that something happened, with a queue, you send a letter to somebody. It may get there. The postman might drop it in the snow. The dog might eat it. If you don't get a response in some period of time, you try other means to contact your relatives, or to pull back the analogy, to contact the server. Monitoring of the health of the queue infrastructure and depth of queues and the like take on added importance, as they are the plumbing that you are now depending upon.

Good Luck