Since I raised this issue on 18 Jan there have been no further views beyond those expressed by Rüdiger and Chris, who
both supported A+B.
So I propose to go ahead with this change. If you object, please say so now. And if you have any suggestions on the
exact wording please let me know.
Wording for B
-------------------
Change 4.4.6. "Conventions for using a session". This currently says
"It is erroneous for client code to use such a session from another thread of control. The only exception to this is the
use of the session or connection close method."
I propose to change it to say:
"It is erroneous for client code to use such a session from another thread of control. The only exception to this is the
use of the consumer, session or connection close method."
Wording for A
-------------------
I propose to change the API doc for the close method on MessageConsumer (and subtypes) and JMSConsumer
In JMS 1.1 this simply says (for MessageConsumer)
"This call blocks until a receive or message listener in progress has completed."
I propose to change it to remove the reference to receive and to say:
"If there is a message listener configured on this consumer, and this method is called from a thread other than the
thread of control which delivers messages to it, then this method must block until that message listener has completed.
This allows close() to be called from any of the sessions's message listeners."
Nigel
On 18/01/2013 18:52, Nigel Deakin wrote:> I think an issue has arisen with JMS_SPEC-48, which clarified the issue of
what should happen if MessageConsumer#close,
> Session#close and Connecton#close was called from within a message listener's onMessage method.
>
> Background
> ----------
>
> Here's a reminder of this issue (which was originally raised by Graham Wallis of IBM):
>
> In JMS 1.1:
>
> * the API docs for Connection#close stated that it "should not return until...all message listeners that may have been
> running have returned"
>
> * the API docs for Session#close stated that "This call will block until a... message listener in progress has completed"
>
> * the API docs for MessageConsumer#close stated that "This call blocks until a... message listener in progress has
> completed.
>
> In all cases, this means that if one of these close methods is called within a message listener's onMessage method then
> it should never return, causing deadlock.
>
> In JMS 2.0 we agreed that this didn't make sense, so we defined that:
>
> * A message listener must not attempt to close its own connection as this would lead to deadlock. The JMS provider must
> detect this and throw a IllegalStateException.
>
> * A MessageListener must not attempt to close its own Session as this would lead to deadlock. The JMS provider must
> detect this and throw a IllegalStateException.
>
> * A MessageListener must not attempt to close its own MessageConsumer as this would lead to deadlock. The JMS provider
> must detect this and throw a IllegalStateException.
>
> Problem
> -------
>
> I think we may have "painted ourself into a corner" here, at least in the case of MessageConsumer#close
>
> Consider an application that creates a MessageConsumer and calls setMessageListener to configure a message listener.
> Once this method has been called (and the conneciton started), then it is prohibited to call MessageConsumer#close from
> any thread other than the one which delivers messages to the message listener.
>
> (JMS 1.1 section 4.4.6 states: "...once the first message listener for a session has been registered, the session is now
> controlled by the thread of control that delivers messages to it. At this point a client thread of control
> cannot be used to further configure the session". This section explicitly states that calling Session#close or
> Connection#close is exempt from this restriction, but does not mention MessageConsumer#close.)
>
> However in JMS 2.0 we have said that it is prohibited to call MessageConsumer#close from the message listener as well.
>
> This means that there is no way of ever calling MessageConsumer#close when a message listener is being used. This is
> clearly a nonsensical state of affairs which we need to resolve.
>
> Possible solutions
> ------------------
>
> I think there are two fairly obvious solutions:
>
> Solution A. Change the specification for MessageConsumer#close to allow it to be called from a message listener's
> onMessage on its own consumer.
>
> Solution B. Add MessageConsumer#close to the list of methods exempted from the "single thread of control" restriction on
> a session, just like Session#close and Connection#close.
>
> Solution C. Both (A) and (B) together.
>
> Discussion: Solution A
> ----------------------
>
> Allowing MessageConsumer#close to be called from a message listener on its own consumer would make it possible to create
> simple applications which need to consume a specific number of messages. After a sufficient number of messages have been
> received, onMessage calls MessageConsumer#close and no further messages are delivered.
>
> Discussion: Solution B
> ----------------------
>
> Allowing MessageConsumer#close to be called from a thread other than the one which is calling onMessage would be
> consistent with Session#close and Connection#close, which are already allowed to be called from a thread other than the
> one which is calling onMessage.
>
> However allowing this on its own does not allow the fine-grained control over the number of messages delivered that is
> offered by (A) .
>
>
>
> Affect of consumer close on message acknowledgement
> ---------------------------------------------------
>
> If we chose (A) then we'd need to decide what happens to the message which was being delivered at the time.
>
> I suggest we define that consumer.close() has no effect on message acknowledgement. After all, acknowledgement is really
> a function of the session rather than the consumer, and the session is still open.
>
> We could clarify the behaviour for each acknowledgement mode as follows:
>
> * Auto-acknowledge: calling consumer.close() has no effect on message acknowledgement. The message will be automatically
> acknowledged when onMessage returns.
>
> * Client-acknowledge: calling consumer.close() has no effect on message
> acknowledgement. If the message listener calls Message#acknowledge (or
> the new JMSContext#acknowledge) then the message will be acknowledged.
> If the message listener does not call acknowledge then the message will
> be redelivered after the session is closed or recovered. It makes no
> difference whether the message is acknowledged before or after the
> consumer is closed.
>
> * Local transaction: calling consumer.close() has no effect on
> transaction commit or rollback. If the message listener calls
> Session#commit (or the new JMSContext#commit) then the transaction will
> be committed. If the message listener calls rollback then the transaction
> will be rolled back and the message will be redelivered. It makes no
> difference whether the transaction is committed or rolled back before or
> after the consumer is closed.
>
> Questions
> ---------
>
> So, which do you prefer? A, B or A+B?
>
> Nigel
>