[Zope-dev] [Proposal] Drop Mount.py from ZODB 3.5

[Jim Fulton]

Shane Hathaway wrote:
> Tim Peters wrote:
> 
>>    http://www.zope.org/Collectors/Zope/1800
>>
>>describes some of the code problems with Zope's current way of mounting
>>databases.  ZODB 3.4 (still) has a Mount.py module, unused and untested by
>>ZODB.  Jim and I were both surprised today to discover that Zope (2.8) still
>>imports it, so we can't drop it for ZODB 3.4 (Zope 2.8, and the ZODB 3.4 it
>>uses, are both in beta).
>>
>>We'd like to continue getting non-ZODB code out of the ZODB project, so
>>would like to drop Mount.py from ZODB 3.5.  Are any of zodb-dev's
>>"standalone" ZODB users making use of Mount.py?  I would be surprised by
>>that too, since Mount.py relies on other code (like Acquisition) that's
>>already been removed from the ZODB 3.3 and 3.4 lines.  I've been surprised
>>before, though ...
> 
> 
> ZODB/Mount.py is dangerous.  In the past few years, Jim and I have
> discovered necessary constraints on mounted databases, and
> ZODB/Mount.py has not been updated to reflect that thinking.  The main
> problem is that ZODB/Mount.py does not bind each mounted connection with
> a specific root database connection; instead, mounted connections are
> free to be used with any root database connection.  The result is that
> _v_ attributes that cross mount boundaries turn insane.
> 
> I solved the connection binding problem when I wrote the DBTab product,
> but the solution required an ugly monkey patch.  Later, DBTab got folded
> into Zope, but not the way I expected.  By the time I saw the
> integration, it was the wrong time to fix it.  (I don't blame anyone for
> this mistake; DBTab was complex and I didn't explain well enough how it
> worked.)
> 
> So now there's a mess to clean up.  I suggest:
> 
> 1. Products/TemporaryFolder is the only code in Zope that uses
> ZODB/Mount.py.  Before the Zope 2.8.0 release, put a copy of
> ZODB/Mount.py in Products/TemporaryFolder and change the import
> statement to import that copy instead.

I assume we are too late to affect 2.8.

> 2. With the last Zope reference gone, remove ZODB/Mount.py from ZODB
> 3.3, 3.4, and 3.5.  If standalone ZODB authors really depend on it, they
> can get their own copy and change their own import statements.
> 
> 3. Update Products/ZODBMountPoint/Mount.py to use whatever method ZODB
> 3.5 wants to use for mounting databases, removing the ugly
> ConnectionPatches.
 >
> 4. Change TemporaryFolder to use Products/ZODBMountPoint/Mount.py and
> finally remove Products/TemporaryFolder/Mount.py.

Yup, but I think this needs to be a project for Zope 2.9.

Jim

-- 
Jim Fulton           mailto:jim at zope.com       Python Powered!
CTO                  (540) 361-1714            http://www.python.org
Zope Corporation     http://www.zope.com       http://www.zope.org