如何为 REST API 实现观察者模式？

Question

I'm looking to create a REST API to which clients subscribe to certain data. When the data changes (due to some external event) I want to notify the clients (observers) with the new data.

I want to use Spring for the REST API's, I have no clue how to register and notify the observers though.

Some guidance and or good practises would be very helpful.

Thank you

Answer 1

You can use Spring 5 with WebFlux. It's a combination of an Iterator and the Observer pattern. The client always gets a new Object, whenever there is one on the server. You can start learning more on that on the Spring documentation pages or on eg New in Spring 5: Functional Web Framework

Answer 2

I hesitate to put my answer up because the other is much better, but since I've done some code.

In spring boot you can register call back urls, an example controller is:

@RestController
public class Controller {

    private List<Listener> listeners = new ArrayList<>();

    @RequestMapping(value = "/register/{name}", method = RequestMethod.POST)
    public ResponseEntity<Void> register(@PathVariable("name") String name, @RequestParam("callbackurl") String callBackUrl) throws Exception {
        System.out.println("register, name=" + name + ", callBackUrl=" + callBackUrl);
        Listener listener = new Listener(name, URLDecoder.decode(callBackUrl, "UTF-8"));
        listeners.add(listener);
        System.out.println(listener);
        return new ResponseEntity<>(HttpStatus.OK);
    }

    @RequestMapping(value = "/callback/*", method = RequestMethod.POST)
    public ResponseEntity callBack(@RequestBody String message) {
        System.out.println("call back with message=" + message);
        return new ResponseEntity(HttpStatus.OK);
    }

    @Scheduled(fixedRate = 10000)
    public void notifyListeners() {
        System.out.println("notifying listeners");
        for (Listener listener : listeners) {
            System.out.println("listener " + listener);
            CloseableHttpClient client = HttpClients.createDefault();
            HttpPost httpPost = new HttpPost(listener.getCallBackUrl());
            try {
                httpPost.setEntity(new StringEntity("hello listener " + listener));
                CloseableHttpResponse response = client.execute(httpPost);
                client.close();
            } catch (Exception e) {
            }
        }
    }
}

Can be tested like so, register 2 call backs, the URL http://127.0.0.1:8080/callback/app1 is encoded so it can be a paramter.

curl -X POST http://127.0.0.1:8080/register/listener1?callbackurl=http%3A%2F%2F127.0.0.1%3A8080%2Fcallback%2Fapp1
curl -X POST http://127.0.0.1:8080/register/listener1?callbackurl=http%3A%2F%2F127.0.0.1%3A8080%2Fcallback%2Fapp2

In my case for simplicity the client and server are the same application, but they could be different.