[jax-rs-spec users] Re: Early feedback about JAX-RS 2.1 / SSE specification

From: Sergey Beryozkin <>
Date: Wed, 8 Feb 2017 16:42:09 +0000

Hi Pavel
On 08/02/17 16:01, Pavel Bucek wrote:
> Hello Sergey,
> ad synchronization / thread safety: SseBroadcaster must be thread safe
> anyway. The field I put into my resource is final, so no
> synchronization is necessary in that case.
Yes, sorry, I was actually referring to my code example, where the
synchronization of some sort is needed, not to your one where it is
obviously is not needed
> ad param injection: the case you mentioned is indeed valid and doesn't
> provide maybe expected behavior, but consider something like:
Right, so do we really want JAX-RS SSE users do this code for keeping
SSEBroadcaster, similar to what you typed below ?
The reason the users would have to do is because a non-natural
SSEContext creating SSEBroadcaster is there even though
a single SSEBroadcaster is about broadcasting the events to one to many
connections represented by SSEContexts.

Can you please give some thought to having a standalone SSEBroadcaster
context introduced ? May be what we have now should stay as is but IMHO
we do need to at least consider the alternative as far as dealing with
broadcasters is concerned
Thanks, Sergey
> private AtomicReference<SseBroadcaster>broadcasterReference =new AtomicReference<>();
> @GET @Path("eventStream")
> @Produces(MediaType.SERVER_SENT_EVENTS)
> public SseEventOutput eventStream(@Context SseContext sseContext) {
> SseEventOutput sseEventOutput = sseContext.newOutput();
> SseBroadcaster broadcaster = getBroadcaster(sseContext::newBroadcaster);
> broadcaster.subscribe(sseEventOutput);
> broadcaster.broadcast(sseContext.newDataEvent("new event stream created!"));
> return sseEventOutput;
> }
> private SseBroadcaster getBroadcaster(Supplier<SseBroadcaster> supplier) {
> SseBroadcaster result =broadcasterReference.get();
> if (result ==null) {
> result = supplier.get();
> if (!broadcasterReference.compareAndSet(null, result)) {
> return broadcasterReference.get();
> }
> }
> return result;
> }
> I know it might seem little artificial, but I hope it conveys the
> message that creating the broadcaster from context which is received
> as a method parameter might be valid option, so forbidding that
> explicitly would fix the usecase you mentioned, but disallow usecase
> above. (Also, there is no way how to get a information about how the
> SseContext instance was retrieved - there is no
> "SseContext#canProvideBroadcaster()" method).
> (note to a random reader - AtomicReference could really use
> computeIfAbsent...)
> Did this convince you? :)
> Best regards, Pavel
> On 08/02/2017 15:47, Sergey Beryozkin wrote:
>> Hi Pavel Thanks for the fast feedback, SSEContext name is fine IMHO,
>> nearly everything it creates is specific to a given client SSE
>> connection, so I'd keep the name, the only thing that it creates and
>> is not specific to a current SSEContext is SSEBroadcaster. The code
>> you typed below is fine, which is what I was implying. The following
>> code though looks strange: @GET public Response(@Context SSEContext
>> sseContext) { SSEBroadcaster broadcaster =
>> sseContext.newBroadcaster()
>> broadcaster.subscribe(sseContext.newOutput()); } It is a valid JAXRS
>> SSE code but it does not make sense given that the broadcaster is a
>> method local property which indeed defeats the idea of the
>> broadcaster and if it is made a class level property then it will
>> have to be synchronized ? Thus the code you typed below is OK, but
>> the code I typed above is not hence I suggested to make it invalid by
>> having sseContext.newBroadcaster() throwing ISE if this method is
>> called from a SSEContext injected as a method parameter (same as for
>> example in your code below you'd try to subscribe a
>> sseContext.newOutput to the broadcaster during the property injection
>> time) Or may be introduce SSEBroadcaster context Thanks, Sergey On
>> 08/02/17 13:37, Pavel Bucek wrote:
>>> Hi Sergey,
>>> thanks for digging in your email archive and bringing this up.
>>> I just tried to write minimal SseBroadcaster usecase and this was
>>> the result:
>>> @Singleton @Path("/") public class SseResource { private final
>>> SseContext sseContext; private final SseBroadcaster sseBroadcaster;
>>> @Inject public SseResource(SseContext sseContext) { this.sseContext
>>> = sseContext; this.sseBroadcaster = sseContext.newBroadcaster(); }
>>> @GET @Path("eventStream") @Produces(MediaType.SERVER_SENT_EVENTS)
>>> public SseEventOutput eventStream() { final SseEventOutput
>>> eventOutput = sseContext.newOutput();
>>> sseBroadcaster.subscribe(eventOutput); return eventOutput; } public
>>> void onEvent(String event) {
>>> sseBroadcaster.broadcast(sseContext.newDataEvent(event)); } }
>>> I see what You and Andriy mean about broadcaster not being really
>>> contextual, but .. I believe that's valid for all methods. Let's
>>> review SseContext:
>>> public interface SseContext { SseEventOutput
>>> newOutput();OutboundSseEvent.Builder newEvent();SseBroadcaster
>>> newBroadcaster(); // this is not pushed to the public repo yet :)
>>> default OutboundSseEvent newDataEvent(String data) {}default
>>> OutboundSseEvent newDataEvent(String data, String name) {}}
>>> It's not really a context, it feels more like a factory. I don't see
>>> anything related to current context - thread/request/application. I
>>> can imagine an app (war) which would create new outbout event and
>>> pass it to another war. And it would still work. Also, the naming of
>>> the methods (new*) suggests that the "SseContext" always produces
>>> new instances, it doesn't do a lookup in thread local or any other
>>> store, it just creates instances. Would rename to SseFactory (or any
>>> better name), help with the issue raised by Andriy? TBH, it doesn't
>>> feel right to limit the API in a way you've proposed - a method with
>>> SseContext as a parameter would work / not work based on from where
>>> it was called. I'm not saying there are no schemes like that
>>> already, but here it feels more artificial and doesn't really help
>>> (the error is still visible in the runtime, not before). Also, I
>>> belive the sole purpose of having SseContext is to not have builders
>>> for OutboundEvent, Broadcaster and EventOutput. We might even
>>> consider removing SseContext and introduce those builders, but it
>>> would make the connection from the API to the implementation
>>> slightly more difficult. Or we can improve that, make it a
>>> "broadcaster store" - something like
>>> "SseContext#getOrCreateBroadcaster(String Id)".. (that's just me
>>> thinking out loud :). What do you think? Thanks and regards, Pavel
>>> On 08/02/2017 12:47, Sergey Beryozkin wrote:
>>>> Hi Pavel Can you review the thread text below please (see a note
>>>> from Marek). SSEContexts can be passed as method parameters, so
>>>> when one creates a broadcaster it gives the impression the
>>>> broadcaster is the current SSEContext specific Would it make sense
>>>> to enforce that SSEBroadcaster can only be created if SSEContext is
>>>> injected as a thread local proxy as opposed to a case where the
>>>> broadcaster is created from SSEContext passed as a parameter
>>>> (SSEContext.newBroadcaster() would throw the IllegalStateException
>>>> is this case) ?. Thanks, Sergey On 15/08/16 12:58, Sergey
>>>> Beryozkin wrote:
>>>>> Hi Marek, Andriy, All FYI I tried to 'justify' the current design
>>>>> by the spec leads by the requirement to have an implementation
>>>>> specific SSEBroadcaster introduced without having to introduce an
>>>>> SSEBroadcaster Context - the latter would make it possible to pass
>>>>> SSEBroadcaster as a method parameter which might not be ideal. I
>>>>> agree with the Andriy's observation though that it does look like
>>>>> it is a current SSEContext/event stream dependent if SSEContext is
>>>>> passed as a method parameter. Perhaps a neater way to work with
>>>>> broadcasters can be found :-) Sergey On 15/08/16 08:49, Marek
>>>>> Potociar wrote:
>>>>>> Hi Andriy,
>>>>>> This is a good observation - thank you for spotting this. I’ll
>>>>>> give it some thought and come up with a fix proposal.
>>>>>> Cheers,
>>>>>> Marek
>>>>>>> On 14 Aug 2016, at 04:36, Andriy Redko <
>>>>>>> <>> wrote:
>>>>>>> Early feedback about JAX-RS 2.1 / SSE specification
>>>>>>> Hey JAX-RS leads, As some of you may already know, the Apache
>>>>>>> CXF project is one of the early adopters of the JAX-RS 2.1
>>>>>>> specification. In particular, we are currently actively working
>>>>>>> on implementing SSE support, based on drafted JAX-RS 2.1 API.
>>>>>>> Along the way we stumbled upon some design decisions which may
>>>>>>> require a bit of clarification or (hopefuly) a second thought.
>>>>>>> The use case in question is related to SseBroadcaster.
>>>>>>> Essentially, the idea sounds pretty simple: events into
>>>>>>> multiple SSE streams could be easily broadcasted. F.e. here is a
>>>>>>> pseudo-code for typical scenario: SseBroadcaster broadcaster =
>>>>>>> new SseBroadcasterImlp(); broadcaster.register(stream1);
>>>>>>> broadcaster.register(stream2); ...
>>>>>>> broadcaster.broadcast(event1); broadcaster.broadcast(event2); In
>>>>>>> nutshell, current Jersey's SSE implementation does exactly that.
>>>>>>> However, in JAX-RS 2.1 API draft SseBroadcaster could be
>>>>>>> created only from SseContext, f.e.: Response stream(@Context
>>>>>>> SseContext context) { SseBroadcaster broadcaster =
>>>>>>> context.newBroadcaster();
>>>>>>> broadcaster.register(context.newOutput()); ... } It kind of
>>>>>>> works but totally defeats the idea of broadcaster as being
>>>>>>> independent from SseContext or/and streams it serves. Currently,
>>>>>>> every time new instance of the SseBroadcaster will be created,
>>>>>>> with no straightforward way to share it across multiple SSE
>>>>>>> streams. How it could work though may look like that (the
>>>>>>> implementation details could vary, just trying to present the
>>>>>>> idea): private SseBroadcaster broadcaster =
>>>>>>> SseBroadcaster.newBroadcaster(); Response stream1(@Context
>>>>>>> SseContext context) { broadcaster.register(context.newOutput());
>>>>>>> ... } Response stream2(@Context SseContext context) {
>>>>>>> broadcaster.register(context.newOutput()); ... } In this case,
>>>>>>> the SseBroadcaster is truly context-independent, and serves it
>>>>>>> purpose very well. We were trying to come up with some realistic
>>>>>>> scenarios for SseBroadcaster utilizing the current specification
>>>>>>> draft but did not really succeeded. Surely, there are a few way
>>>>>>> to make it work but they do not look good. Does it make sense?
>>>>>>> Whould you mind guys to give some clarifications/guidance on
>>>>>>> that? Thank you a lot! Best Regards, Andriy Redko