users@jax-rs-spec.java.net

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

From: Pavel Bucek <pavel.bucek_at_oracle.com>
Date: Fri, 31 Mar 2017 14:44:21 +0200

Hi Sergey,

sorry for not answering that one, I must have missed it :-)

The intention is to have the storage STATIC, accessible from ANY scope.
It must be thread safe.

I'm also thinking about "The returned Broadcaster instance" must be
open, which would imply "isOpen()" check and if that returns false, the
instance would be replaced by a fresh one. (we don't have
"removeBroadcaster(...)" yet and I hope we won't need to have that.

Thanks and regards,
Pavel


On 31/03/2017 14:33, Sergey Beryozkin wrote:
> Hi Pavel
>
> Thanks, it all sounds reasonable, though the question will remain, given
>
> getBroadcaster(String)
>
> how to implement this method - the implementation will not know if it
> is a singleton or per-request resource which is calling it, hence
> how would it know if should do a static storage or not
>
> Re Application.getSingletons - I was only referring to it to imply
> that singleton resources can be easily preferred if needed (it was
> probably off-topic)
>
> Thanks, Sergey
>
> On 30/03/17 18:21, Pavel Bucek wrote:
>>
>> Hi Sergey,
>>
>> please see inline.
>>
>>
>> On 30/03/2017 13:18, Sergey Beryozkin wrote:
>>> 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
>>> (Sse#newBroadcaster())
>>
>> it's a combination, user of that API just has to know what he does.
>>
>> List of valid usecases:
>>
>> per request:
>> - newBroadcaster() with custom broadcaster instances storage
>> - getBroadcaster(ID) with and ID per resource / message domain
>>
>> singleton
>> - newBroadcaster() in constructor, when storing "per-class" or
>> "per message domain" broadcaster to a final field
>> - getBroadcaster(ID) when we don't want to store anything and
>> delegate it to the JAX-RS runtime
>>
>> I believe I could put together couple more..
>>
>>> 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.
>>
>> I'm not exactly sure about the usecase, but Application.getSingletons
>> returns Set<Object>, which is very inconvenient to use by any means
>> other than providing objects to the application bootstrap. (i.e.: I
>> don't see it being used during the runtime to retrieve some concrete
>> singleton). Not to mention that only @Path and @Provider annotated
>> classes should be there.
>>
>> (Not really sure whether there is a requirement for
>> javax.ws.rs.core.Feature implementations to have @Provider (I do see
>> that Jersey doesn't really honor that), but the javadoc of the
>> Application#getSingletons is quite clear about what should be there).
>>
>>>
>>> Now, we will have
>>>
>>> Sse#newBroadcaster()
>>> Sse#getBroadcaster(Class)
>>
>> please consider Class as an ID. I believe I indicated that we
>> consider 2 methods for addition:
>>
>> Sse#getBroadcaster(String)
>> Sse#getBroadcaster(Class).
>>
>> The latter could be something like:
>>
>> default SseBroadcaster getBroadcaster(Class<?> clazz) {
>> return getBroadcaster(clazz.getName());
>> }
>>
>>
>> the overload with class is just a helper method to ease doing the
>> "class scoped" instance, which seems to be the most common case.
>>
>>>
>>> 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.
>>
>> as I already mentioned, both can be used, no matter whether in
>> Singleton or Request scope.
>>
>>>
>>> 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.
>>
>> sure, we can have Sse#getBroadcaster(String).
>>
>>> Likewise, if a singleton calls Sse#getBroadcaster(Class), should Sse
>>> simply return a new instance or keep it in the static storage too ?
>>
>> I wouldn't define the behavior based on the context; Sse instance
>> would be a singleton and the storage would be a singleton map as
>> well, so all methods would behave the same way. Remember, Sse
>> instance can be stored and accessed outside of any scope (for example
>> in external event handler).
>>
>>> 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 ?
>>
>> I was hesitating a little when the proposal came (as I mentioned, it
>> was based on an experience when rewriting existing Jersey SSE
>> examples), but it makes more sense when you compare the code with and
>> without these methods.
>>
>>> 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
>>
>> I see your point and it is an interesting analogy, but I believe that
>> the usecase of AsyncResponse is very different - it doesn't make
>> sense to store AsyncResponse instance as a field in a singleton. I
>> can be mistaken of course :)
>>
>> Thanks and regards,
>> Pavel
>>
>>>
>>> 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
>>>>>>>
>>>
>>
>