On Wed, 2008-03-19 at 04:26 -0700, Howard Chu wrote:
Andrew Bartlett wrote:
> Over the past few weeks, I have been testing OpenLDAP as a backend for
> I've been working with the OpenLDAP team on my requirements, and there
> has been some really good outcomes - the memberOf module has been
> improved, as has the refint module.
> However, I seem to have hit a brick wall, in the form of (internal)
> transaction support. I need an LDAP backend to support internal
> transactions - that is, when for example a 'member' modification is
> made, all the memberOf attributes must be updated before the call
> returns. Similarly, if a user or group moves, all the member/memberOf
> attributes that link the user to their groups must also move, before the
> modrdn returns.
> The Samba4 test ldap.js tests this behaviour extensively, because I want
> to be sure it works.
> As understand the discussion I've had with the OpenLDAP team, OpenLDAP
> does not support this, and will not support it for perhaps some time.
I may have overstated the problem in the previous discussion of our refint
module. In fact, RE24 was already changed to work around any potential
deadlock issues a long time ago. But to give some context: the refint module
was originally written to operate synchronously (back in 2004). Some time
later it was changed (2006) to asynchronous mode because users didn't want
their clients to be stuck waiting for all the cascaded updates to complete.
Most clients don't know or care that a particular change has side-effects. We
could introduce a config keyword to select synch vs asynch behavior here, but
I have a feeling that will still leave some group of users unhappy no matter
how you set it.
Great. If run sync, will it error out correctly if I make an invalid
modification (say target not present etc).
> Similarly, from discussions with the Fedora DS team at the CIFS
> developer days, I understand that it is similarly very unlikely that
> Fedora DS will support internal transactions. (It also does not support
> subtree renames, which we also need).
> The fact that LDAP does not expose a transaction API
You mean draft-zeilenga-ldap-txn ?
I suppose I should have said 'The free LDAP implemetnations I'm looking
at don't expose a transaction API'. What did end up happening with
> was always going to
> be a difficult part of having Samba4 use an LDAP backend, but I always
> assumed that if we pushed the really hard bit - updating linked
> attributes - into the LDAP server that we could at least always have a
> consistent DB. (It turns out this is one of the primary uses of
> transactions anyway.)
> But without that consistency, and without knowing as a caller if all the
> updates succeed, I'm worried about how we can safely move forward.
> This is especially disappointing because I was hoping that these free,
> replicating LDAP servers might solve the backed replication problem for
> me, without needing to use AD replication.
> Does anybody have any ideas or suggestions on how I could get around
There are other ways to guarantee consistency. The simplest approach is to
just not store one end of the linked attributes, and always generate them
dynamically when they're referenced.
In the old Symas Connexitor EMS product we used (the equivalent of) a
UUIDAndOptionalName syntax for all references. In that case the DN was
essentially just window-dressing; we always used the ID to actually reference
entries and we updated the DNs whenever we found that they didn't match. As
such, referential integrity was pretty simple - you never had anything
pointing to the wrong entry; the worst that would happen is that you
occasionally had dangling references to deleted entries stored in the DB but
no one ever saw them because they were cleaned out whenever the entry
containing the reference was read.
Do you think the LDAP backend could/should handle this, or will Samba4
have to do the GUID -> DN and DN -> GUID translations before passing
things to the backend?
> Should we drop the LDAP backend as a nice idea, but not going to
> and focus on DRS or some other form of replication?
> Can someone imagine a sane way to reconstruct the DN links, including
> subtree renames, without the help of the LDAP backend? Could we ban
> subtree renames (as Fedora DS does), and try to handle this ourselves
> (with pre/post checks and a good deal of prayer)?
Banning subtree renames seems like a non-starter, and it only eliminates one
case; the overall problem still remains.
Authentication Developer, Samba Team http://samba.org
Samba Developer, Red Hat Inc.