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).
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. Consider Session class javadoc:
* Session objects may be called by multiple threads. Implementations must
* ensure the integrity of the mutable properties of the session under
such circumstances.
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.
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.
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..).
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..)
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
>