Am 07.02.2013 17:11, schrieb Jim Fulton:
On Thu, Feb 7, 2013 at 10:48 AM, Jürgen Herrmann
<> wrote:
Am 06.02.2013 15:05, schrieb Jürgen Herrmann:

Hi there!

I hav a relstorage with mysql backend that grew out of bounds
and we're looking into different backend solutions now. Possibly
also going back to FileStorage and using zeo...

Anyway we'll have to convert the databases at some point. As these
are live DBs we cannot shut them down for longer than the
ususal maintenance interval during the night, so for maybe 2-3h.

a full conversion process will never complete in this time so
we're looking for a process that can split the conversion into
two phases:

1. copy transactions from backup of the source db to the destination
   db. this can take a long time, we don't care. note the last
   timestamp/transaction_id converted.
2. shut down the source db
3. copy transactions from the source db to the destination db, starting at the last converted transaction_id. this should be fast, as only
   a few transactions need to be converted, say < 1% .

if i would reimplement copyTransactionsFrom() to accept a start
transaction_id/timestamp, would this result in dest being an exact
copy of source?

source = open_my_source_storage()
dest = open_my_destination_storage()
last_txn_id = source.lastTransaction()

source = open_my_source_storage()
# add some transactions

source = open_my_source_storage()
dest = open_my_destination_storage()
dest.copyTransactionsFrom(source, last_txn_id=last_txn_id)

I will reply to myself here :) This actually works, tested with a
modified version of FileStorage for now. I modified the signature
of copyTransactionsFrom to look like this:

def copyTransactionsFrom(self, source, verbose=0, not_before_tid=None):

``start`` would be better to be consistent with the iterator API.

not_before_tid is a packed tid or None, None meaning "copy all"
(the default, so no existing API usage would break).

Is there public interest in modifying this API permamently?


This "API" is a bit of an attractive nuisance.  I'd rather people
learn how to use iterators in their own scripts, as they are very
useful and powerful.  This API just hides that.

The second part, replaying old transactions is a bit more subtle,
but it's still worth it for people to be aware of it.

If I were doing this today, I'd make this documentation
rather than API. But then, documentation ... whimper.

Anybody want to look at the actual code changes?

Sure, if they have tests.  Unfortunately, we can only accept
pull requests from zope contributors. Are you one?
Wanna be one? :)


1. SUCCESS. I migrated to FileStorage/ZEO successfully with the modified copyTransactionsFrom() with downtime of only 10 minutes. Very cool :) DB sizes are reasonable again and repozo backups are set up, just like before the migration to RelStorage. Feels robust again, a good feeling.
   I'd advise anybody thinking about migrating to RelStorage to think
about a proper (incremental) backup strategy for the underlying sql db
   first, this might become crucial! repozo makes your life easy there
   when using FileStorage.

2. Regarding the tests: I did not find any test for copyTransactionsFrom()
   in the current test suite, am I blind? If there isn't any test yet,
maybe you could suggest a proper file for the test, maybe even a similar
   one to start from?

3. Regarding my request to have this in ZODB 3.10.5:
After reading my own statement I quickly realized that changing an API can only happen in a major version. Anyway, this was a one-shot action
   so I'll probably never need my own code again :)

best regards,
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