Simultaneous invocation of handlers

[Dan D]

Thanks, that's helpful.  I was most interested in (3), a stateful Handler
that's created per-channel with no ExecutionHandler.  It sounds like in this
case I can assume that there will only be one outstanding Channel event at a
time, and that all events will happen in order.  That makes sense; keeping
state right in a per-channel, stateful handler would get complicated if
events could happen simultaneously.

Are there any visibility guarantees across invocations of Handlers? E.g. if
I handle an event in my per-Channel handler and change a member variable of
the Handler, will that change be visible when the same Handler instance is
invoked for the next event in a different thread?

Date: Sun, 18 Oct 2009 00:37:46 -0700 (PDT)
> From: Frederic Bregier <fredbregier at free.fr>
> Subject: Re: Simultaneous invocation of handlers
> To: netty-users at lists.jboss.org
> Message-ID: <1255851466682-3843496.post at n2.nabble.com>
> Content-Type: text/plain; charset=us-ascii
>
>
> Hi Dan,
>
> It depends on the way you wrote your handler:
>
> 1) If you create only one Handler for any connected Channel (so not using a
> ChannelPipelineFactory, and if using it not creating a new handler as
> business handler - generally the last one): then any connected channel can
> send a message and thus giving two messageReceived calls at the same time,
> except if an OrderedMemoryAwareThreadPoolExecutor is added in the pipeline.
>
> 2) If you have one handler by channel but using a
> MemoryAwareThreadPoolExecutor, then for one client, if two sends occur, it
> may lead to two messageReceived at the same time (note the "may" since I'm
> not totally sure, what I'm sure is that message can arrive in any order
> like
> the API says).
>
> 3) If you have one handler by channel and you don't use any
> MemoryAwareThreadPoolExecutor or if one is used it is the Ordered one, then
> message will arrived in order and one by one for this handler (= one
> channel). (Note that the OrderedMemoryAwareThreadPoolExecutor is not
> necessary to get this behaviour).
>
> 4) If you have an handler shared among all connected channel but you use a
> OrderedMemoryAwareThreadPoolExecutor in the pipeline before this handler,
> then the order is respected for at least one channel, but still two
> channels
> can received each one a message at the same time.
>
> So first it depends on if your logic is to have one handler for all
> channel,
> or one handler by channel.
> Then try to follow those guidelines (note they are not official, just
> mines,
> they surely exist other ways ;-)
>
> Cheers,
> Frederic
>
>
> Dan D wrote:
> >
> > Say I've got a ChannelHandler that acts upstream of the  the HTTP code
> > handlers.   Within messageReceived() I'm handling an HTTP request.  While
> > I'm in messageReceived(), can messageReceived be invoked *for the same
> > Channel* in another thread (a pipelined HTTP request comes in on the same
> > connection while I'm still processing the first one)? Or will the second
> > messageReceived() not be invoked until the first completes?
> >
> > Thanks.
> >
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> >
> >
>
>
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> Hardware/Software Architect
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