A few contributions to this interesting discussion...
[snip the Zope 3 catalog is not a hack, and clean and simple]
The catalog and index code is not a hack, and is in fact simple,
effective and flexible. Python is the query language, and the lack of
an optimizer is not a reason to go running to an RDBMS index. The
catalog and index code could use polish and even alternate
implementations, but the BTrees, the core code, are fantastic tools.
I have had some opportunity to work with the Zope 3 catalog recently,
and I have a few comments. First of all, I agree with the main idea that
the Zope 3 catalog is not a hack, and is clean and flexible. I believe
the catalog should be invested in, as I think it's cool.
Now as to where I see areas where features are lacking in the Zope 3
Underfeatured query API
I do think that currently the API to query it is woefully underfeatured.
I've tried to work on this problem and am sitting on some code that just
needs a bit of time to polish and release that allows a simple query
language on top of the catalog. It's just building up a tree of python
objects for queries, nothing special, but it is a lot higher level than
what's already there.
Fast, easy batching/sorting
I don't know how to do easy, efficient batching/sorting with the
catalog. I'd like to be able to query *just* a batch of objects, sorted,
for user interface purposes. There doesn't seem to be a straightforward
way to do this yet, and this is a very common use case. The batching
implementation sitting out there in zope.bugtracker.batching is nice,
but doesn't deal with the catalog.
I think this should be fixable with a bit more infrastructure though.
Getting the right batch is just a query on an index, and the result can
be sorted afterwards, though there are tricky issues getting the right
Missing powerful query concepts
Certain powerful query concepts like joins, available in a relational
setting, are missing. I've already run into a scenario where I wanted to
someting like this: given a bunch of version objects with field 'id',
where multiple objects can have the same 'id' to indicate they're
versions of the same object, I want all objects where field
'workflow_state' is 'PUBLISHED' unless there is another object with the
same id that have workflow_state 'NEW', in which case I want that one'.
I think joins would be a way to solve it, though I haven't figured out
the details, nor how to implement them efficiently on top of the
catalog. This kind of thing is where a relational database makes life a
Zope catalog benefits
Now as to benefits of using the ZODB instead of a relational database.
I've seen some mentioned already, and I think there are more that
haven't been mentioned yet. I realize that some of these issues don't
exist so much with 'transparent' maps like Ape, which acts like the
ZODB, though *if* a relational database is used by an application I also
think that those features will be used (otherwise, why do it?), which
will still reduce the portability to non-Ape settings.
Common development platform
I've seen it mentioned elsewhere in this thread that the ZODB can unify
the development community, whereas O/R mapping technologies (in
particular those not transparent to the ZODB) run the risk of scattering
it. I think this is an interesting argument so I'd just like to
Ease of deployment
Right now a Zope application is relatively difficult to deploy compared
to some other solutions like PHP, but, it's probably easier to deploy
than other solutions which require a relational database backend. Now it
might seem that 'enterprise' deployments are big anyway, so we don't
have to worry about making this harder, but:
* enterprises will ask questions like "which relational database does
it support? we standardized on relational database system foo, does it
work with that?" We run the risk of having to say "no", or, if "yes", we
may run the risk of "oops, we cannot test this easily with database
system foo as we don't have it here."
* requiring a RDB for deployment makes it harder to market our
software, as it's harder to just download and install software into your
Zope to try it out. You need to set up a relational database as well. I
may be mistaken, but think Plone would be less popular if a relational
database would be *necessary* in order to play with it.
* closely related, requiring a RDB for deployment makes it harder to
market our open source software to other developers. This ties in
closely to the argument above involving the risk of the community
fracturing. Even inside a company having more software requirements like
a RDB may hinder team development (where each team member runs a
separate instance of the software), as there's simply that much more to
set up and thus harder for someone to get up to speed with a project.
One point I also haven't seen mentioned yet is that I don't want to have
to have a relational database installed in order to run my tests. The
great thing about the ZODB and persistence is that it's very
transparent. Persistent instances are very very similar to
[snip blob support argument]
I agree that the blob argument for RDB mapping is not convincing. There
are other solutions around and this is being improved rapidly.
Anyway, all of the arguments against object/relational mapping aside, I
do think this is an interesting area to explore. You *do* get a whole
lot of power using a relational database, after all. I myself am
actually in two minds concerning very transparent ZODB-style solutions
like Ape or less transparent but more explicit uses of O/R mappings like
SQLObject. While the transparency has many benefits mentioned before,
the more straightforward mapping has the benefits of simplicity, may map
to relational databases more easily, and may expose powerful relational
features more straightforwardly.
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