[jax-rs-spec users] Re: Minor SSE API changes

From: Sergey Beryozkin <>
Date: Thu, 30 Mar 2017 12:18:32 +0100

Hi Pavel

Thanks, sorry it was not a good argument on my side, you are right,
per-request subscriber instances do not make sense.
I suppose what I'm trying to say is that we will have 2 methods for
creating the subscribers, one which makes sense for per-request resources
(Sse#getBroadcaster(Class)) and one for the singletons

As a side note, note sure about the RI community but I think (possibly
mistakenly) that singletons are primarily used by CXF users;
irrespectively of that though, Application.getSingletons is here to
support them.

Now, we will have


these methods are siblings at the API level hence either of them should
make sense for the service code working with Sse but we know that one
method will actually make sense if the service is running as a
singleton, another one - as a per-request.

The other thing (I thought about it before you started this thread)
that the broadcasters can be grouped by URIs, ex, given 4 services
methods which can support the SSE connections, 2 of those methods will
have its own broadcaster, and 2 other methods - its own broadcaster.
Sse#getBroadcaster(Class) will not help.

Likewise, if a singleton calls Sse#getBroadcaster(Class), should Sse
simply return a new instance or keep it in the static storage too ?

Having typed all of it, I'm thinking now, may be it is OK to have 2 of
these methods - the side-effects will be documented, ex, that
getBroadcaster(Class) will create a static instance, though it is a bit
unusual ?

Do you think that per-request apps can simply have a static
ConcurrentHashMap<Class, SseBroadcaster> in their code if needed and
use Sse#newBroadcaster() would be unreasonable ? Ex, check
AsyncResponse docs, API does not offer any support for keeping
AsyncResponse instances, and I guess if it is per-request then the same
issue applies

Thanks Sergey

On 30/03/17 08:51, Pavel Bucek wrote:
> 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 Ssesse;
>>>>> private static SseBroadcasterbroadcaster;// ... 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 Ssesse;
>>>>> 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