[Fedora-directory-devel] Re: Problems creating a Samba4 LDAP Backend

[Howard Chu]

Andrew Bartlett wrote:
> Over the past few weeks, I have been testing OpenLDAP as a backend for
> Samba4.
>
> 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.

> 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 ?

> 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
> this?

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.

> Should we drop the LDAP backend as a nice idea, but not going to work,
> 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.
-- 
   -- Howard Chu
   Chief Architect, Symas Corp.  http://www.symas.com
   Director, Highland Sun        http://highlandsun.com/hyc/
   Chief Architect, OpenLDAP     http://www.openldap.org/project/