I have a Java server and a JavaScript client that have successfully been using Server Sent-Events to send updates. Recently we have been forced to switch between using NGINX built into kubernetes as a proxy between the server and the client to using a kong based proxy that extends NGINX with a different configuration. The new proxy buffers the SSE going through it breaking the protocol and because other applications are using the new proxy I am not allowed to turn off all buffering which was how the previous proxy got around the issue. As an alternative, I have been attempting to add 3 http headers to the SSE response to trigger NGINX to turn off buffering for this particular endpoint:
"Content-Type" : "text/event-stream"
"Cache-Control", "no-cache"
"X-Accel-Buffering", "no"
I have been able to add the "Cache-Control" and "X-Accel-Buffering" headers relatively easily, but I have tried several different approaches to add the "Content-Type" header and nothing is working.
Below is the first attempt, notice the "Content-Type" header is set identically to the other two headers in RestServiceImpl, but from my logs the other two headers are added while "Content-Type" is not.
@Api
@Path("/service")
public interface RestService {
@GET
@Path("/sseconnect")
@Produces(SseFeature.SERVER_SENT_EVENTS)
EventOutput listenToBroadcast(@Context HttpServletResponse response);
}
@Component
public class RestServiceImpl implements RestService {
@Autowired
private Broadcaster broadcaster;
public RestServiceImpl() {
}
@Override
public EventOutput listenToBroadcast(HttpServletResponse response) {
response.addHeader("Content-Type", "text/event-stream");
response.addHeader("Cache-Control", "no-cache");
response.addHeader("X-Accel-Buffering", "no");
return broadcaster.add();
}
}
public class Broadcaster extends SseBroadcaster {
private final OutboundEvent.Builder eventBuilder =
new OutboundEvent.Builder();
public EventOutput add() {
final EventOutput eventOutput = new EventOutput();
super.add(eventOutput);
return eventOutput;
}
public void sendEvents(Events events, String name) {
final OutboundEvent outboundEvent = eventBuilder.name(name)
.mediaType(MediaType.APPLICATION_JSON_TYPE)
.data(Events.class, events).build();
super.broadcast(outboundEvent);
}
}
For my second attempt I tried modifying how the "Content-Type" header was added as follows:
@Override
public EventOutput listenToBroadcast(HttpServletResponse response) {
response.setContentType("text/event-stream");
response.addHeader("Cache-Control", "no-cache");
response.addHeader("X-Accell-Buffering", no);
return broadcaster.add();
}
This had the same effect, "Cache-Control" and "X-Accell-Bufferig" were added, but "Content-Type" was not.
For the third attempt I replaced HttpServletResponse with ContainerResponse
@Override
public EventOutput listenToBroadcast(ContainerResponse containerResponse)
{
final MultivaluedMap<String, Object> headers =
containerResponse.getHeaders();
headers.add("Content-Type", "text/event-stream");
headers.add("Cache-Control", "no-cache");
headers.add("X-Accel-Buffering", "no");
return broadcaster.add();
}
This solution broke the endpoint and ended up giving me a 406 error, so I can't say definitively that it wouldn't have worked. Could be a bad implementation but I am not seeing anything.
For my fourth attempt I tried adding the headers with a ContainerResponseFilter.
@Provider
@PreMatching
public class SseResponseFilter implements ContainerResponseFilter {
@Override
public void filter(ContainerRequestContext requestContext,
ContainerResponseContext responseContext)
throws IOException {
final MultivaluedMap<String, Object> headers =
responseContext.getHeaders();
final List<Object> contentTypeValues = new ArrayList<Object>();
contentTypeValues.add("text/event-stream");
headers.put("Content-Type", contentTypeValues);
final List<Object> cacheControlValues = new ArrayList<Object>();
cacheControlValues.add("no-cache");
headers.put("Cache-Control", cacheControlValues);
final List<Object> xAccelBufferingValues =
new ArrayList<Object>();
xAccelBufferingValues.add("no");
headers.put("X-Accel-Buffering", xAccelBufferingValues);
}
}
This attempt added the "Cache-Control" and "X-Accel-Buffering" headers to every endpoint in the application, but didn't add "Content-Type" anywhere. Note, some of these other api's were returning a Response object that allowed the "Content-Type" to be set and these REST endpoints were getting the "Content-Type" header set correctly for each particular endpoint. The SSE library returns EventOutput which unfortunately doesn't let me set the Content-Type header like Response does.
For the fifth attempt I replaced the ContainerResponseFilter with a WriterInterceptor
@Provider
public class SseWriterInterceptor implements WriterInterceptor {
@Override
public void arroundWriteTo(WriterInterceptorContext context)
throws IOException, WebApplicationException {
final MultivaluedMap<String, Object> headers =
context.getHeaders();
final List<Object> contentTypeValues = new ArrayList<Object>();
contentTypeValues.add("text/event-stream");
headers.put("Content-Type", contentTypeValues);
final List<Object> cacheControlValues = new ArrayList<Object>();
cacheControlValues.add("no-cache");
headers.put("Cache-Control", cacheControlValues);
final List<Object> xAccelBufferingValues =
new ArrayList<Object>();
xAccelBufferingValues.add("no");
headers.put("X-Accel-Buffering", xAccelBufferingValues);
}
}
This solution worked like the previous where it added "Cache-Control" and "X-Accel-Buffering" to all endpoints in the application but left off "Content-Type" again.
To sum it all up I can't seem to figure out what is preventing me from successfully adding the "Content-Type" header to my SSE endpoint.
This question is similar to: Why does Jersey swallow my "Content-Encoding" header However, the given solution of adding the "Content-Type" to the Response object doesn't work because I am using SSE which returns a EventOutput object that does not have a way to add the "Content-Type" header.
