On Jun 19, 2008, at 9:52 AM, Peter Wilkinson wrote:

> At the moment the shelf storage just writes a prefix of "SHELF-1" at
> the start of the file; might it make sense to put an additional bit
> of data after that before the transactions are started which would
> identify when this database was created/packed. A UUID would be
> ideal but a bit rough to require Python 2.5 but maybe a timestamp
> would be enough. This way a script to replicate the database would
> just need to read the first x bytes to compare to what it had to
> know what kind of rsync to perform, no knowledge outside of what is
> in the source and destination database would be needed.

This is a reasonable idea.  Neil Schemenauer recently proposed adding
something like this to the header.  I feel reluctant to make a change
in the file format just now, though, since I hope to get a new release
out in the next month or two.  Maybe if we need to add a new SHELF
format
on a later release, we can use the UUID and make 2.5 the minimum.

I do, however, think we already have an alternative that may work just
as well.
Essentially, we use changes in the stat of the remote ".prepack" file
as a signal that a pack has occurred.
The client runs a remote "stat" on the remote ".prepack" file and
compares
this to a locally saved copy of the last stat output.  If there is no
locally
saved stat info, or if the two stats don't match, we run append with the
"--append-verify" flag.  Otherwise, we just use the "--append" flag.