Tim Peters wrote:
[Chris McDonough]
There is a wrinkle about performing this merge that eluded my memory until now.
To support multidatabases within Zope, it was reasonable to change ZODB.config.ZODBDatabase to support the heretofore likely-unused-by-real-world-code "databases" and "database_name" options that may now be passed into ZODB.DB's constructor:
http://svn.zope.org/ZODB/branches/blob-merge-branch/src/ZODB/config.py?rev=3...
The current blob-merge-branch code depends on this change being available in the ZODB revision it uses. In case you're interested, the code that actually makes use of this feature in the zodb-blobs-branch is in the Zope2.datatypes.DBTab.getDatabase method.
Is this change acceptable for a merge into the ZODB HEAD?
Turns out that a release of Zope3 has already been made that supports multidatabases, and I'd naturally prefer to follow the lead of a Zope that's already out there. Jim showed me the Zope3 implementation code and an example today. I found the code easily (on Zope3 trunk), but can't for the life of me find anything there that looks like his example. Jim, where is that?
Do you mean an example of a zope.conf that uses it? From a customer engagement: <zodb main> <filestorage> path $DATADIR/Data.fs </filestorage> </zodb> <zodb a> <filestorage> path $DATADIR/A.fs </filestorage> </zodb> We decided to use the section names for the database names. This was to avoid changing ZODB. I'm not sure that that was a good idea. This approach has two disadvantages: - Because section names are case insenstive, database names end up being lower case, whether we want them to be or not. - It may not be obvious that the section name is also the database name. I'm really unsure about whether this is a disadvantage. I'm not sure if: <zodb> name main <filestorage> path $DATADIR/Data.fs </filestorage> </zodb> <zodb> name a <filestorage> path $DATADIR/A.fs </filestorage> </zodb> is better or worse than the first version. I'm inclined to think that any time you have sections of the same type, it is desireable to give them names, in which case we might be tempted to list the names twice.
The Zope3 code in question is in
src/zope/app/appsetup/appsetup.py
function multi_database(). Note that they didn't change any ZODB files, instead they give values to a DB's .databases and .database_name attributes after constructing the DB. While that might be questionable in general <cough>, the implementation of multidatabases was meant to be both concrete and public. It's not an accident that ZODB's tutorial tests/multidb.txt doctest explains and exploits details of the concrete implementation -- it's not meant to be abstract. IOW, poking in new values for these attributes isn't considered to be evil.
I'd be happy to plumb this through the factories open method. It would seem to me that we only need to be able to pass a databases argument. The factory presumably knows it's own name. It could then pass the databases dict and the name to the DB constructor.
I believe (here's where the example I can't find would nail it) they use the name on a <zodb> section as the DB's database_name. Fred points out that ZConfig section names are case-insensitive, forced to lowercase, so that
<zodb CHRIS>
and
<zodb cHris>
have the same name. That's not ideal, and threading these attributes throughout ZODB's config.py instead (as you did) would be a sane way to worm around that.
I haven't looked at Chris's changes. I was pretty happy that the changes we made in Z3 were fairly localized and small.
But for right now, I think doing it differently than Zope3 does it would cause needless confusion more than it would help. Enhancing Zope3 and Zope 2.9 in the same way(s) here could make sense.
OTOH, this feature has hardly been used in Z3. I added to ZODB because I had been meaning to for some time ad because we needed it for a customer. I don't think anyone else has used it, so I don't think there's much established pattern in Z3. Then again, I'm not sure, except for the case insentitivity issue that we didn't do it the best way. I'd much rather revisit the case insenstitivity of section names in ZConfig. I think that if ZConfig section names were case sensitive or at least case preserving, I'd be happy with the approach we took. Jim -- Jim Fulton mailto:jim@zope.com Python Powered! CTO (540) 361-1714 http://www.python.org Zope Corporation http://www.zope.com http://www.zope.org