How to implement the observer pattern for REST API's? - spring

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

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.

Answer 2

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 e.g.
New in Spring 5: Functional Web Framework