W3C home > Mailing lists > Public > public-xml-processing-model-wg@w3.org > April 2007


From: Henry S. Thompson <ht@inf.ed.ac.uk>
Date: Wed, 04 Apr 2007 20:24:09 +0100
To: public-xml-processing-model-wg <public-xml-processing-model-wg@w3.org>
Message-ID: <f5bslbgx7s6.fsf@hildegard.inf.ed.ac.uk>

Hash: SHA1

Thinking more about Norm's dependency example, I realise I think we
need to be _very_ modest in what we require in terms of guarantees of
order of execution.  As an illustration, I propose a thought
experiment about implementation -- suppose I am worried about
deadlocks and buffering and all that, and so I take the simplest
possible approach: I create a new thread for _every_ step in a
pipeline, connect up the inputs and outputs, and start all the threads
running simultaneously.  Is there anything in the spec. which rules
this out?  I don't think so, and in fact I think it's a pretty good
implementation strategy.

What follows from this is that if you have an out-of-band dependency,
you _must_ either synchronise it oob, or use the writing of outputs
and the reading of inputs to do so.

So, that ends up being another argument against explicit step-on-step
dependency annotations:  that would require my imaginery
implementation to add an explicit thread management layer, where
before the normal mechanisms of blocking for input would have done the
job without any explicit control flow management at all.

I'm reminded of a point I've made before about pipelines --  they are
_much_ easier to understand in terms of dataflow.  Adding a story
about control flow just makes things messy -- I'm very loathe to go
there just to allow side-effects to be synchronised.

- -- 
 Henry S. Thompson, HCRC Language Technology Group, University of Edinburgh
                     Half-time member of W3C Team
    2 Buccleuch Place, Edinburgh EH8 9LW, SCOTLAND -- (44) 131 650-4440
            Fax: (44) 131 650-4587, e-mail: ht@inf.ed.ac.uk
                   URL: http://www.ltg.ed.ac.uk/~ht/
[mail really from me _always_ has this .sig -- mail without it is forged spam]
Version: GnuPG v1.2.6 (GNU/Linux)

Received on Wednesday, 4 April 2007 19:24:18 UTC

This archive was generated by hypermail 2.3.1 : Tuesday, 6 January 2015 21:32:42 UTC