why DeferredResult ends on setResult() on trying to use SSE - html

Question

i am trying to implement a Server Sent Events (SSE) webpage which is powered by Spring. My test code does the following:
Browser uses EventSource(url) to connect to server. Spring accepts the request with the following controller code:
#RequestMapping(value="myurl", method = RequestMethod.GET, produces = "text/event-stream")
#ResponseBody
public DeferredResult<String> subscribe() throws Exception {
final DeferredResult<String> deferredResult = new DeferredResult<>();
resultList.add(deferredResult);
deferredResult.onCompletion(() -> {
logTimer.info("deferedResult "+deferredResult+" completion");
resultList.remove(deferredResult);
});
return deferredResult;
}
So mainly it puts the DeferredResult in a List and register a completion callback so that i can remove this thing from the List in case of completion.
Now i have a timer method, that will periodically output current timestamp to all registered "browser" via their DeferredResults.
#Scheduled(fixedRate=10000)
public void processQueues() {
Date d = new Date();
log.info("outputting to "+ LoginController.resultList.size()+ " connections");
LoginController.resultList.forEach(deferredResult -> deferredResult.setResult("data: "+d.getTime()+"\n\n"));
}
The data is sent to the browser and the following client code works:
var source = new EventSource('/myurl');
source.addEventListener('message', function (e) {
console.log(e.data);
$("#content").append(e.data).append("<br>");
});
Now the problem:
The completion callback on the DeferredResult is called on every setResult() call in the timer thread. So for some reason the connection is closed after the setResult() call. SSE in the browser reconnects as per spec and then same thing again. So on client side i have a polling behavior, but i want an kept open request where i can push data on the same DeferredResult over and over again.
Do i miss something here? Is DeferredResult not capable of sending multiple results? i put in a 10 seconds delay in the timer thread to see if the request only terminates after setResult(). So in the browser the request is kept open until the timer pushes the data but then its closed.
Thanks for any hint on that. One more note: I added async-supported to all filters/servlets in tomcat.

Answer 1

Indeed DeferredResult can be set only once (notice that setResult returns a boolean). It completes processing with the full range of Spring MVC processing options, i.e. meaning that all you know about what happens during a Spring MVC request remains more or less the same, except for the asynchronously produced return value.
What you need for SSE is something more focused, i.e. write each value to the response using an HttpMessageConverter. I've created a ticket for that https://jira.spring.io/browse/SPR-12212.
Note that Spring's SockJS support does have an SSE transport which takes care of a few extras such as cross-domain requests with cookies (important for IE). It's also used on top of a WebSocket API and WebSocket-style messaging (even if WebSocket is not available on either the client or the server side) which fully abstracts the details of HTTP long polling.
As a workaround you can also write directly to the Servlet response using an HttpMessageConverter.