[ZODB-Dev] ZODB4 and DemoStorage

[Barry Warsaw]

On Tue, 2003-04-08 at 06:10, Marius Gedminas wrote:

> I'm trying to build a functional testing framework based on DemoStorage.
> The docstring in demo.py suggests this is an intended use case:

Yes.  The other use case is fronting a read-only storage, say on a
CDROM.

> Here's how I am trying to do it:
> 
>   from zodb.db import DB
>   from zodb.storage.mapping import MappingStorage
>   from zodb.storage.demo import DemoStorage
> 
>   base_storage = MappingStorage("Base")
>      # maybe MemoryFullStorage would be a better choice?

MappingStorage is more like a MemoryMinimalStorage; neither supports
undo or versions.  MemoryFullStorage as its name implies, supports the
full storage interface.

>   db = DB(base_storage)
>   ... bootstrap the DB (create root folder and essential services) ...
>   db.close()
> 
>   ... and for each functional test case do:
> 
>     demo_storage = DemoStorage("Demo", base_storage)
>     db = DB(demo_storage)
>     ... do things with db ...
>     db.close()
> 
> I have some reservations about it.  Closing the DB also closes the
> storage, 

Looking at the code, that's all db.close() does!

> and I am not sure you just pass the same storage to a new DB
> instance as if nothing had happened.

Well, what if you just threw away the DB object without closing it?  I'm
not 100% sure this is kosher, but it would be interesting to try. :)

> BTW there's a bug in DemoStorage: newObjectId() returns oids that are
> already in use by self._back.  DemoStorage.__init__ should set self._oid
> to the last oid used by self._back.  I haven't found a public interface
> method to do that (calling self._back.newObjectId() won't work if
> self._back is read-only), so I think IStorage needs a new method,
> lastObjectId().

Probably so.  It can't call newObjectId() on the backing storage because
that method will raise an exception on a read-only storage.

I'll add a unit test for this case.  Please let me know if you find any
other problems with either the memory storages or demo storage.  There's
a definite dearth of tests for the demo storage.

-Barry