On 12/01/2015 18:53, Pavel Bucek wrote:
> Hi Mark,
>
> seems like there might be some misinterpretation in the original thread
> - I read the reply from Danny as "container has to send the whole
> message before sending another one"; it does not seem to concern async
> messages, because container already has the message and it will be sent
> (or @OnError invoked).
The thread looks clear to me (with some edits to get to the key bits).
Me: If an application sends a message using a Future or a SendHandler,
is it permitted to send a second message before the first has
completed?
Danny: I think one message needs to finish before you try sending the
next.
Me: My original reason for suggesting this is no longer valid. That
said, I'm happy keeping this restriction in place so that the
application must wait for the previous message to complete before
sending the next.
Bill: +1
So, this discussion:
a) applied to async messages
b) applied to the application sending the message
> Your sample can be reduced to:
>
> session.getAsyncRemote().sendText(message);
> session.getBasicRemote().sendText(message);
>
>
> and I don't think this should end up in an exception. There must be some
> kind of queue implemented in the container and second call will wait for
> first message to be sent, but that's all.
Yes, there is a queue. Yes the above sequence could be supported.
However, in the discussion above the EG decided that it should not be
supported.
The problem with allowing applications to send another message before
the previous one has finished is that there is no mechanism for the
container to push back on the application if it is sending messages too
quickly. Requiring the previous message to finish before the next is
sent provides (in a simple way) a mechanism for the container to push back.
> There is no API for users to get the state of current Session instance
> like "SENDING" or "READY_TO_SEND". When shared and used from multiple
> threads, it would not be easily possible to reliably send a message, if
> the sending state of individual messages could cause an exception.
Yes, if an application wants to send form multiple threads the
application has to deal with the concurrency issues.
> The other thing is the original usecase. I believe we should signal
> something when user tries to send a message in the middle of sending
> another partial message. I will have to investigate little bit more
> about this, but I can say that the RI does not do this at the moment.
Given the above discussion, this is already handled. An ISE should be
thrown (and Tomcat does this).
> Looking at this issue from broader perspective, it could be nice to have
> the alternative to this, something as "force send", since we might get
> to the state when we want to send some message, no matter what is
> happening in other threads. Just for the sake of completeness, here is
> the usecase:
>
> session.getBasicRemote().sendText(message, false); // partial
> session.getBasicRemote().sendText(message);
>
> (Currently, RI sends the second (whole) message as part of the previous
> one; or not really, it is just interpreted in that way..).
Tomcat throws an ISE if an application tries that.
> Following is even more evil:
>
> session.getBasicRemote().sendText(m, false); // partial text
> session.getBasicRemote().sendBinary(ByteBuffer.wrap(m.getBytes()));
>
> (this is interpreted in the same way as previous; if you think of the
> frame content and order, it makes sense that the other side interpreted
> it as one text message. Problem is that we might need to detect this on
> the sender side and throw an exception probably..)
Again, Tomcat throws an ISE in this case.
As I stated in the original thread, I'd be happy to lift this
restriction (in a new version of the spec) but we'd need to think about
how the container could prevent the application placing too much data in
the queue.
Mark
>
>
> Thanks and regards,
> Pavel
>
>
> On 12/01/15 11:21, Mark Thomas wrote:
>> Looking back though the EG discussions, we discussed concurrency and
>> agreed that one message had to complete before the next was sent [1]. No
>> distinction was made between synchronous and asynchronous messages.
>>
>> The discussion in [1] happened before the split between
>> RemoteEndpoint.[Basic|Async] and after the split restriction described
>> in [1] only appears in the Javadoc for RemoteEndpoint.Basic.
>>
>> My question is this:
>>
>> Does the restriction agreed in [1] apply to async messages or not?
>>
>> I believe that the restriction does apply. Without it, there is no
>> mechanism for the container to signal to the application to stop sending
>> messages if the client isn't reading them fast enough.
>>
>> By way of example, consider the following:
>> @OnMessage
>> public String onMessage(String message, Session session) {
>> received.add(message);
>> session.getAsyncRemote().sendText(MESSAGE_ONE);
>> return message;
>> }
>>
>>
>> My expectation is that, unless the async message completes very quickly
>> (unlikely) then an ISE will be thrown because the async message is still
>> in progress when the sync message send is triggered by the return value.
>>
>> That said, if batching is enabled then it is much more likely that the
>> async message would complete instantly and that the sync message would
>> then be sent.
>>
>> For bonus points, assuming that the batch buffer is big enough for both
>> the async and sync message, is the use of the return value to send the
>> message a trigger to flush the batch? I can seen no reason why this
>> should be the case but the question popped up on a Tomcat list so I
>> wanted to check here.
>>
>> Mark
>>
>>
>> [1]
>> https://java.net/projects/websocket-spec/lists/jsr356-experts/archive/2013-02/message/48
>>
>>