[ZODB-Dev] Re: POSKeyError

[Bob Horvath]

A long time ago, Tim Peters wrote:
> [Bob Horvath]
> 
>>I have a working site that has several domains all in the same database
>>file, one of which has POSKeyErrors. I have tried the various recipes,
>>but I am very concerned I'll screw things up without a better
>>understanding.
> 
> 
> If you haven't read this, you should:
> 
>     http://zope.org/Wikis/ZODB/FileStorageBackup
> 
> 
>>As I understand it, POSKeyErrors are either objects that have no path
>>from the root, or objects that refer to other objects that don't exist.
> 
> 
> That's too complicated.  Each object has an identifier, called an oid.  A
> storage is essentially a mapping from oids to objects.  POSKeyError occurs
> when a storage is asked to fetch the object associated with an oid, but the
> storage doesn't know about that oid.  It's very similar to the KeyError you
> get when you index a Python dictionary by a key that isn't in the
> dictionary.
> 
> There are really two immediate causes for POSKeyError:
> 
> 1. The oid simply isn't in the database.
> 
> 2. The oid is in the database, but the transaction that created
>    the object was undone.  Then the object still exists, but only
>    in the past, and the object could become visible again if
>    the undo of its creation were itself undone.
> 
> In practice, #2 is at work if and only if "_loadBack_impl" shows up in a
> POSKeyError traceback.



As I am still (months later) trying to figure this out, I noticed that 
my two POSKeyErrors do exhibit the symtoms of #2.

I found the transaction that had the undo of the creation...

2004/10/0621:26:36.214GMT-4/clients/horvath/1/horvath_com/folder_paste
...
   data #00289 oid=000000000000cbc2 class=undo or abort of object creation
   data #00290 oid=000000000000cbc3 class=undo or abort of object creation


... but when I tried to undo I get an error...


Couldn't undo change to '\x00\x00\x00\x00\x00\x00O?' because Some data 
were modified by a later transaction

Does knowing the cause help me at all figure out how to get rid of it?