WebSocket/REST: Client connections?

Question

I understand the main principles behind both. I have however a thought which I can\'t answer.

Benchmarks show that WebSockets can serve more messages as this website

Answer 1

This is my experience so far, I am happy to discuss my conclusions about using WebSockets in big applications approached with CQRS:

Real Time Apps

Are you creating a financial application, game, chat or whatever kind of application that needs low latency, frequent, bidirectional communication? Go with WebSockets:

• Well supported.
• Standard.
• You can use either publisher/subscriber model or request/response model (by creating a correlationId with each request and subscribing once to it).

Small size apps

Do you need push communication and/or pub/sub in your client and your application is not too big? Go with WebSockets. Probably there is no point in complicating things further.

Regular Apps with some degree of high load expected

If you do not need to send commands very fast, and you expect to do far more reads than writes, you should expose a REST API to perform CRUD (create, read, update, delete), specially C_UD.

• Not all devices prefer WebSockets. For example, mobile devices may prefer to use REST, since maintaining a WebSocket connection may prevent the device from saving battery.
• You expect an outcome, even if it is a time out. Even when you can do request/response in WebSockets using a correlationId, still the response is not guaranteed. When you send a command to the system, you need to know if the system has accepted it. Yes you can implement your own logic and achieve the same effect, but what I mean, is that an HTTP request has the semantics you need to send a command.
• Does your application send commands very often? You should strive for chunky communication rather than chatty, so you should probably batch those change request.

You should then expose a WebSocket endpoint to subscribe to specific topics, and to perform low latency query-response, like filling autocomplete boxes, checking for unique items (eg: usernames) or any kind of search in your read model. Also to get notification on when a change request (write) was actually processed and completed.

What I am doing in a pet project, is to place the WebSocket endpoint in the read model, then on connection the server gives a connectionID to the client via WebSocket. When the client performs an operation via REST, includes an optional parameter that indicates "when done, notify me through this connectionID". The REST server returns saying if the command was sent correctly to a service bus. A queue consumer processes the command, and when done (well or wrong), if the command had notification request, another message is placed in a "web notification queue" indicating the outcome of the command and the connectionID to be notified. The read model is subscribed to this queue, gets messessages and forward them to the appropriate WebSocket connection.

However, if your REST API is going to be consumed by non-browser clients, you may want to offer a way to check of the completion of a command using the async REST approach: https://www.adayinthelifeof.nl/2011/06/02/asynchronous-operations-in-rest/

I know, that is quite appealing to have an low latency UP channel available to send commands, but if you do, your overall architecture gets messed up. For example, if you are using a CQRS architecture, where is your WebSocket endpoint? in the read model or in the write model?

• If you place it on the read model, then you can easy access to your read DB to answer fast search queries, but then you have to couple somehow the logic to process commands, being the read model the responsible of send the commands to the write model and notify if it is unable to do so.

• If you place it on the write model, then you have it easy to place commands, but then you need access to your read model and read DB if you want to answer search queries through the WebSocket.

By considering WebSockets part of your read model and leaving command processing to the REST interface, you keep your loose coupling between your read model and your write model.