[ZODB-Dev] Fixing POSKeyErrors :-)

[Jim Fulton]

On Feb 13, 2008, at 1:45 PM, Binger David wrote:

>
> On Feb 13, 2008, at 11:49 AM, Marius Gedminas wrote:
>
>> Data-loss bugs are *bad*
>
>
> Do all of these unfortunate conditions recently described
> go away if we don't have references to external "persistent"  
> instances?
> Are there benefits of cross-storage references that justify this risk?


Yes, but not unless you understand the semantics.

There's also a problem that we made cross-database references so easy  
to make that they are made accidentally.  We probably need to find a  
way to make them more explicit, at least optionally.  I think where  
people have gotten into trouble is where they've mounted content trees  
into other content trees and then moved objects between trees creating  
cross-database references accidentally.  In the short term, people  
should stop doing that.  When we have an API to prevent accidental  
references, then we can probably support this sort of tree mounting.

In the longer term, multi-database GC is an option. I believe this  
will be doable.

Also, in the medium term, you can use zc.FileStorage to disable GC  
during packing.  My plan is that this will become a built-in (and  
cleaner) FileStorage feature in ZODB 3.9.

Note that, IMO, some of the best use cases for multi databases are  
separating catalog and session data from regular content.  Both of  
these should be pretty safe, although some care would be needed to  
deal with migrating persistent data from session to content databases.

Jim

--
Jim Fulton
Zope Corporation