[ZODB-Dev] RFC: Pre-commit hooks

[Jim Fulton]

I propose to add a pre-commit hook to ZODB.

Problem

   Sometimes, applications want to execute some code when
   a transaction is committed.  For example, one might want to
   delay object indexing until a transaction commits, rather
   than indexing every time an object is changed. Or, someone
   might want to check invariants only after a set of operations.

Proposal

   We add a new method to transaction objects:

      def beforeCommitHook(hook, *args, **kw):
          """Register a hook to call before the transaction is committed

           The provided hook will be called after the transaction's
           commit method has been called, but before the commit process
           has been started.  The hook will be passed the given
           positional and keyword arguments.

           Multiple hooks can be registered and will be called in order.
           This method can be called from executing hooks.  That is, executing
           hooks can register more hooks.  (Applications should take care
           to avoid creating infinite loops by recursvely registering hooks.)

           Hooks are not called and are discarded if the transaction aborts.
           """

   We would also modify transaction commit methods to call hooks.  Hooks would
   not be called on sub-transaction commit.  (If necessary, we could add
   separate sub-transaction hooks, but I don't think we need this.)

   This hook is very simple to use an implement and, I think, addresses
   the requitement much more directly than implementing special data
   managers.

Thoughts?

Anyone up for implementing this (with a doctest, of course)?

I'd like to get this into ZODB 3.4.

Jim

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