[ZODB-Dev] Sequences and ZODB conflicts

[Chris McDonough]

This doesn't answer your deeper questiob but FWIW, there is a module in
the BTrees package named Length, which has a class named Length, which
can be used for this application.  It is essentially a marker for an
object that can incremented or decremented as necessary.  It is special
inasmuch as it contains logic to resolve conflicts at the application
level.

On Tue, 2003-01-28 at 20:27, Christian Reis wrote:
> I've got something of an odd problem that I think others that use the
> ZODB in a multi-connection/multi-user environment may have run into
> before. Summarizing my question: How should we handle "locking" of
> user-level data through the ZODB?
> 
> The scenario is a simple one: a domain object has an attribute that we
> will call "code", which is a sequential number that should be
> incremented for each new object instantiated. Let's say the counter is a
> simple persistent integer attached to a counter object in the database,
> and that you can issue next() to it and get the next integer.
> 
> When we have two connections to the database, there is a natural
> concurrency issue. The issue occurs because the two connections see
> their own copies of the counter object, and next() will issue the same
> ID to both. The first connection to commit() will succeed, but the
> second will fail with a ConflictError. 
> 
> Now, just before commit() is called and the exception raised, my
> connection._transaction._objects is chock-full of instances. commit() is
> called, and ZODB.POSException.ConflictError is raised.
> 
> Now the behaviour after this ConflictError is interesting. After it is
> raised, I expected all the instances in my connections to be invalidated
> (reverted to None) and that a subsequent commit() would fail. Wrong!
> 
> The object reference I created (during the conflicting connection) is
> not "uncreated" - the name still points to a valid persistent object.
> However, _transaction._objects is now empty. Have I lost the chance to
> save this instance forever, or can I do something magic, and try again?
> All that conflicted was the Sequence, after all.
> 
> Take care,
> --
> Christian Reis, Senior Engineer, Async Open Source, Brazil.
> http://async.com.br/~kiko/ | [+55 16] 261 2331 | NMFL
> 
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-- 
Chris McDonough <chrism@zope.com>
Zope Corporation