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Channel sessions

From: Carl Fredrik Hammar
Subject: Channel sessions
Date: Thu, 19 Jul 2007 21:35:01 +0200
User-agent: Gnus/5.1008 (Gnus v5.10.8) Emacs/22.1.50 (gnu/linux)


I've thinking about use-cases for libchannel and started work on
channelio for the last few days and it got me thinking about sessions.

I found that I've been assuming that character devices are
session-less, mostly because they, unlike stores, are not seekable.
Now I'm not so sure that is the case.

Suppose we want to record some audio which we may read from
/dev/sound/audio, which is a channelio translator.  Additionaly we
want to broadcast the audio onto our internet radio station.  These
tasks are handled by seperate processes which both need to read from
/dev/sound/audio.  However we will find that the audio is choppy in
both the recording and the broadcast!

The reason for this is that channelio, upon a io_read call from the
recorder, fills it's output buffer with input from the sound driver
and copies over the requested amount to the client supplied buffer and
discards it from the output buffer.  Later the broadcaster does the
same and receives the audio that follows the audio supplied to the

The right thing to do is to wait until all clients have read the
buffer until its contents are discarded, blocking clients that have
already read it.  Here we also need to impose a time-out, when
processes that can't keep up looses data or gets an error.

However the former is also a valid behavior for some character-device
translators.  Such as /dev/random, where we don't want to all clients
to receive the same random data.

This is all made worse by the fact that channels can be copied to
other processes so that they can do io with it directly, which means
session-state must be shared by all processes holding the channel.

It seems like this will turn out to be a mess.  I want to have a
sanity check before I dig deeper.  Does all this make sense to you
guys or am I thinking to much?

Also how are sessions like above handled by kernel devices?  Are they
handled at all?  Can someone point me to a good example?


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