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 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.
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 that draft?
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.
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 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.
Indeed.
Andrew Bartlett