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

From: Pavel Bucek <>
Date: Wed, 8 Feb 2017 17:08:33 +0100

// for the reference, this would also most likely work:

SseBroadcaster sseBroadcaster =broadcasterReference .updateAndGet(sseBroadcaster -> sseBroadcaster ==null ?sseContext.newBroadcaster() : sseBroadcaster);

On 08/02/2017 17:01, 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.
> ad param injection: the case you mentioned is indeed valid and doesn't
> provide maybe expected behavior, but consider something like:
> 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