[ZODB-Dev] Ordering before commit hooks

[Tim Peters]

[Tim]
> ...
> Julien provided links to code that already uses the new feature:
>
> """ As an Indexation Manager :
> http://svn.nuxeo.org/trac/pub/file/CPSCore/trunk/IndexationManager.py
>
> As an Event Manager :
> http://svn.nuxeo.org/trac/pub/file/CPSSubscriptions/trunk/EventManager.py
> """
>
> He appears to use (just) two distinct `order` levels there, and seems
> just to want to make sure one class of hook gets run before the other
> class of hook.  The new scheme does give an easy way to do that.

OTOH, while picking levels of -100 and 100 works for that specific use case,
looks like it threatens to become a mess if multiple subsystems try to use
this scheme simultaneously.  For example, someone who wants to be "the last
kind of hook invoked" has no way to force that, at least not short of
passing sys.maxint as the `order`.  But if that's what's needed,
`sys.maxint` is a strange way to spell it.

Jim still wonders, and he got me wondering too, whether the `order=` gimmick
is really needed.  For example, you could have gotten to the same end here
with the old method, by registering your actions with an object of your own
creation, and registering just one commit hook with the transaction, where
that one hook looked at the actions you registered with your own object and
ran them in whatever order _it_ determined was best.  The ordering logic
would have been out of ZODB then, not limited to what an integer `order` can
express, and might even benefit from "ah, if I have to run A, then there's
no need to also run B or C" kinds of optimizations.

I'm inclined to agree with Jim that `order=` wasn't needed; that it was too
general for the specific use case we've seen; and that it's not general
enough for plausible other use cases.

Should this really go into ZODB 3.5?  The method name change and robustified
signature were good improvements, and I'd certainly like to keep them.  I
think the jury is still out on `order`, though.  Anyone else have strong
feelings for or against it?