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Postfix Archives: Re: Postfix multi-queue support

Re: Postfix multi-queue support

Subject: Re: Postfix multi-queue support
From: Wietse Venema (wietseporcupine.org)
Date: Tue Jan 04 2000 - 19:51:01 CST

James Youngman:
> wietseporcupine.org (Wietse Venema) writes:
>
> > If you want to start a discussion on multiple queues, that is fine
> > by me. Issues to consider:
> >
> > - One message arrives that must end up in multiple queues.
> > Unfortuately, the system crashes after Postfix has inserted the
> > message into some queues but before it has finished updating all
> > queues.
>
>
> I'll speak up, despite not knowing anything of importance about the
> file formats used for the Postfix queue files. I've solved this
> problem before (and tested the solution) in another existence...
> Here, I'm reconstructing my earlier design from memory. The actual
> design was thoroughly tested, but this is just a description from
> memory. It's quite possible that I've not remembered it correctly.

The only remarkable detail about Postfix queue files is that there's
only file one per message, that files are write-once, with one-byte
updates when a recipient is done, and that a file is either complete
(chmod +x) or it can be removed when it is too old.

This approach was chosen to minimize file creation/deletion overhead
(reading/writing is much, much, cheaper) and to avoid having to undo
or finish an incomplete operation, because that is what makes error
recovery so complex. With Postfix, there's no error to recover from,
as far as queue files are concerned.

> A failrly generic case is for the copying of a file, with optional
> modification, into two other directories. Let's assume that they're
> all on the same filesystem (are we willing to assume this for the
> purposes of the larger discussion?)

[use temporary work file that says what output files need to exist
when the operation is complete]

That is an elaborate scheme that would work. In fact, it is more
robust than most of Postfix, where it is considered unacceptable
to lose mail, but where it is considered acceptable to allow for
a small probability of duplicate delivery after system crash.

        Wietse

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