[ZODB-Dev] Packing to 0 days?

[Toby Dickenson]

On Thursday 09 October 2003 15:15, Chris Withers wrote:
> Shane Hathaway wrote:
> > One more thing: when you pack, do you pack to 0 days?  You probably
> > should.
>
> My memories has something in it about this being "bad"...
>
> Can anyone think why that would be the case?

In FileStorage, this will discard any object that is not referenced by the 
root object, as seen by the state on disk.

There are some corner cases where this type of unreachable object may become 
reachable again, if there is a reference to it in memory that gets written in 
a subsequent transaction. If you happen to pack during the period where it is 
unreachable then the subsequent transaction will write a dangling reference. 
Dangling references will always raise a POSKeyError when accessed.

More details here: 
http://mail.zope.org/pipermail/zodb-dev/2002-May/002602.html

BDBStorage reduces the chances of hitting this by not discarding objects 
written in sufficiently recent transactions. '0 days' doesnt quite mean zero 
time in BDBStorage

DirectoryStorage has this same protection, plus it has code to raise an 
exception when committing a transaction that will write a dangling reference. 
You will get a hard error instead of silently corrupting your storage.

-- 
Toby Dickenson