Re: non-undoable storage using directorystorage?

On Tuesday 23 November 2004 19:59, Tim Hicks wrote:

> In thinking about all this, I realised that I still haven't quite groked
> what needs to be changed to make Full._pack work as Minimal._pack.  All we
> are trying to achieve is getting rid of 'deleted' objects from the
> database.  I mean:
> 
> 1) Full._pack will currently remove any file (containing a pickle) that is
> not referenced by any other object and is therefore unreachable from the
> root object.  These are the 'deleted' objects right?  This seems to be
> what we want for Minimal._pack.
> 
> 2) Full._pack will search back for previous revisions, but would
> presumably not find any for a Minimal storage, so that would be harmless.
> 
> So why can't I just do the following in the Minimal.Minimal class
> declaration and be done with?::
> 
>   _pack = Full._pack

For the same reason that it is not possible to take data files creates using a 
Full storage and access them using Minimal, hoping to only see the current 
revisions of everything.... Minimal is a subset of Full in functionality, but 
it is *not* a subset of its implementation or disk layout. 

Some examples:

The file layout for object data is different. For each object:
* Minimal has one file only.
* Full has one 8 byte file defining the current revision number, plus one file 
per revision holding the real data. None of these files have the same 
filename as Minimal's one.

Full stores a log of transaction metadata, but Minimal does not. Full's 
packing code:
*  will raise errors if the log does not exist. 
* (pass 2b) makes use of this log to work around some corner cases in ZODB 
that can lead to data loss. 
* Ensures that it preseves enough of this log for use in replication.

> Sorry, I think I'm being a bit slow.

I suggest:
1. create a small Minimal storage using the 'flat' format, containing a few 
objects and a few transactions. Pick apart by hand the directory structure 
and file content.
2. do the same for Full.
3. dont hesitate to ask questions.

-- 
Toby Dickenson

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