I wonder what sense it actually makes to have broadcasters being scoped non-application? I mean, I cannot imaginate a scenario where a broadcaster shall be created later than an application starts, or die earlier than an application ends. Also I think that broadcasters are shared often between resource classes. So wouldn't it make sense to simply do this:
class MyResource {
@Context @Named("MyBroadcaster") SseBroadcaster b; // application-scoped, injected at resource instantiation
}
-Markus
From: Pavel Bucek [mailto:pavel.bucek_at_oracle.com]
Sent: Donnerstag, 30. März 2017 09:52
To: jsr370-experts_at_jax-rs-spec.java.net
Subject: Re: Minor SSE API changes
Hi Sergey,
please see inline.
On 29/03/2017 16:34, Sergey Beryozkin wrote:
Hi Pavel
With the newBroadcaster() method I thought one would usually create it at the injection time and save it. If it is a singleton - Sse will allocate a single instance only. if it is per-request - Sse will allocate an instance of SseBroadcaster per request, from the service code's point of view it is still an instance of SseBroadcaster.
that might be our disconnect. Does it make sense to use "request-scoped" broadcaster? That would imply that it will contain only single SseEventSink and if not stored, it will be discarded after resource method finishes. Which is not that useful - you could use SseEventSink only and it would achieve the same functionality; opt1 is equivalent to opt2
// consider standard, request-scoped JAX-RS resource
@GET
@Path("subscribe")
@Produces(MediaType.SERVER_SENT_EVENTS)
public void subscribe(@Context SseEventSink eventSink) {
// opt1
eventSink.onNext(sse.newEvent("msg"));
// opt2
SseBroadcaster broadcaster = sse.newBroadcaster();
broadcaster.subscribe(eventSink);
// broadcaster has only single subsrciber and will be discarded
broadcaster.broadcast(sse.newEvent("msg"));
}
If we have additional methods like Sse#getBroadcaster(String ID) which would get-or-create the broadcaster, it could be different:
@GET
@Path("subscribe")
@Produces(MediaType.SERVER_SENT_EVENTS)
public void subscribe(@Context SseEventSink eventSink) {
// broadcaster has "x" subscribers
SseBroadcaster broadcaster = sse.getBroadcaster(this.getClass());
// broadcaster has "x+1" subscribers, the instance is preserved
broadcaster.subscribe(eventSink);
broadcaster.broadcast(sse.newEvent("msg"));
}
I'm just curious, what side-effects will be waiting to happen if we introduce effectively a dedicate support for the static storage of the broadcasters.
I might not see something here, not sure whether there could be any unwanted side-effect. The "Sse" instance would only become "real context" instead of just a factory for creating broadcasters and outbound events. Everything else can stay the same - in our prototype implementation, Sse instance already is a singleton and there are generally no issues with injecting singletons into any-scoped beans, as long as the injected object is thread safe.
I can see how it can help to support per-request service instances reuse the same SseBroadcaster instance, but what exactly does that achieve. save the runtime to create an instance of SseBroadcaster per request. A method like sse.getBroadcaster(Class) will not really make sense for the singletons, so I can see some ambiguity being introduced into Sse API ?
I don't see that as an ambiguity, maybe just as another usecase. JAX-RS resources are by default request scoped, so it actually makes more sense to have "get or create store", since SseBroadcaster instances are expected to be retained for more than just a single request. The "newBroadcaster" is still valid for some usecases, even when it could be always replaced by "getOrCreate" with random id, but we are not removing that now. Or at least I don't think about this like that. Thanks and regards, Pavel
Thanks, Sergey On 29/03/17 13:07, Pavel Bucek wrote:
Ah, I did not mean it like anything is going away - Sse#newBroadcaster will stay, only two method could be added to ease "storing" and retrieving the broadcasters.
It's meant as an addition, existing Sse methods will stay as they are for now.
Regards, Pavel
On 29/03/2017 13:28, Sergey Beryozkin wrote:
Hi Pavel In your original example you had a non static SseBroadcaster created from Sse.newBroadcaster. Yes, I recall it was also a singleton in your example, but as I said, I don't understand what difference does it make, @Context also works well for per-request resources. Is the actual problem is that SseBroadcasters are rather be 'static' in nature ? May be we can have sse.newBroadcaster() but add sse.getBroadcaster(Class) ? Cheers, Sergey On 29/03/17 12:14, Pavel Bucek wrote:
Hi Sergey,
that is still true, but with these new methods on Sse "context", we'd remove the need for having JAX-RS Resources as singleton, since the "store" on the Sse would be application-scoped.
(let me add a code example to be extra sure we are on the same page)
current state:
(the synchronization could be done in a better way, please take it only as an example)
@Path("server-sent-events")
public class ServerSentEventsResource {
private final Sse sse;
private static SseBroadcaster broadcaster; // ... per-class broadcaster storage
public ServerSentEventsResource(@Context Sse sse) {
this.sse = sse;
}
private static synchronized SseBroadcaster getBroadcaster(Sse sse) {
if (broadcaster == null) {
broadcaster = sse.newBroadcaster();
}
return broadcaster;
}
@POST
public void sendMessage(final String message) throws IOException {
getBroadcaster(sse).broadcast(sse.newEvent(message));
}
// ...
}
future?:
@Path("server-sent-events")
public class ServerSentEventsResource {
private final Sse sse;
public ServerSentEventsResource(@Context Sse sse) {
this.sse = sse;
}
@POST
public void sendMessage(final String message) throws IOException {
sse.getBroadcaster(this.getClass()).broadcast(sse.newEvent(message));
}
// ...
}
Thanks, Pavel
On 29/03/2017 12:34, Sergey Beryozkin wrote:
HI Pavel That works for us. Recall that you said that 'Sse' object can be injected with a JAX-RS @Context and assuming the service resource is called DemoResource this works for either per-request or singleton, public class DemoResource { public DemoResource(@Context Sse sse) {} } For ex, if DemoService is per-request then the following works right now: public DemoResource(@Context Application app) {} Both 'contexts' are very much similar in that both are effectively the non-request specific factories Thanks, Sergey On 29/03/17 11:10, Pavel Bucek wrote:
Dear experts, we did a couple of changes in SSE API: - removed SseEventSource.Builder#named(...) the intention was to name threads used by the event source, but in the light of future change, which should introduce "common" per-client thread pool(s) for async operation, this won't be feasible to have there (since it would require creating another threadpool per SseEventSource instance) - SseEventSource now implements Flow.Source<InboundSseEvent> seems like we'd need to have some form of Publisher and Subscriber, currently named as Source and Sink in the API. The main reason would be integration with other frameworks - without it, it would be harder to do without a good reason. Also, during the implementation and rewriting "old" Jersey examples, one difficulty was discovered. It used to be possible to create a broadcaster statically, which simulates "per resource class" scope. The problem is that currently the only way how to create a new SseBroadcaster is injecting Sse object. But that is not available during static initialization and since JAX-RS resources are by default request-scoped, storing the SseBroadcaster instances might be a challenge for some users. Does anyone see the same issue as we do? Would someone object if we add methods like Sse#getBroadcaster(String) Sse#getBroadcaster(Class<?>) which would serve as a storage for broadcasters? (semantically it could be "get-or-create", passed String or Class would serve as a key). Thanks and regards, Pavel