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- To: OpenLDAP Devel <email@example.com>
- Subject: LDAP transactions
- From: Howard Chu <firstname.lastname@example.org>
- Date: Wed, 23 Apr 2008 16:00:37 -0700
- User-agent: Mozilla/5.0 (X11; U; Linux i686; rv:1.9pre) Gecko/2008041618 SeaMonkey/2.0a1pre
Just thinking aloud right now about what's needed here.
Basically all incoming requests participating in a transaction are queued
until the Commit is received. Aside from basic parsing/validation, no other
processing is performed until then.
If an Abort is received, the queued requests are simply discarded.
When the Commit request is received, all of the queued requests are performed
sequentially by a single thread and the results are gathered. If any
individual request fails, any preceding completed requests must be rolled back.
Since I'm considering this for both back-bdb and back-ndb, it appears we're
going to need some transaction-specific hooks to be added to the BackendInfo
1) start a TXN, get a TXN handle
2) end a TXN: Commit or Abort
For back-bdb the tricky part is exposing the updates atomically in the caches.
I think the fact that entry caching uses BDB locks helps somewhat; we can keep
entry cache items locked until the overall transaction completes. But for
Aborts we'll either need to keep a copy of the previous cache info, or just
discard it all. For now, discarding it all seems simpler.
For back-ndb things are currently easy since this backend does no caching of
its own. As such, once the backend issues a Commit or Abort request, there's
no further (backend) work to be done.
It's tempting to think about this for backglue, but we'd need a cross-database
lock manager of some kind for detecting deadlocks. That implies that we really
need an LDAP-level lock request, to handle distributed locking, and that the
Transaction handling ought to be built on top of that. Currently the
Transaction layer says nothing at all about locking, and it's up to the
underying LDAP database to take care of it.
I guess another approach would just be to have backglue fully serialize all
transactions; if only one is outstanding at any time there can be no deadlocks.
This brings up a question about whether slapd in general should fully
serialize them. I was thinking, at the least, that we should only allow one
active transaction per connection, though that was mainly a matter of
-- Howard Chu
CTO, Symas Corp. http://www.symas.com
Director, Highland Sun http://highlandsun.com/hyc/
Chief Architect, OpenLDAP http://www.openldap.org/project/