On Wed, 28 May 2008, Howard Chu wrote:

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A case I have is where a consumer replicates a glue'ed database, with the exception of one subordinate backend where the consumer is the master. The subordinate backend is replicated back to the master of the glue'ed database. With the current code the master would send the content of the subordinate db back to its master.

Understood. In fact, having multiple sources of data in a glued tree is really a form of multi-master. (The separate glued branches cannot cause inconsistencies with each other, but still their contextCSNs must be managed individually.)

Yes, this is a sort of multi-master configuration, but not what I think of when I hear it (and read the doc). The doc. could be clearer that different serverID values are always required when there are multiple master servers, even if the configuration ensures that there will never be more than one master for any backend.

Btw, if I have understood things correctly there cannot safely be more then one subordinate db (in a glued environment) that replicates from any single master, but it is OK as long as all the subordinates replicates from different masters. That's why the syncrepl consumer in this case is used on the glue db itself, not the subordinates. And it definitely tries to interfere with what goes on in the subordinates (as it should).

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A patch that fixes this is at the referenced URL. As I am not sure of the consequences if a defaulted serverID=0 value was included in the syncCookie the patch changes the internal default slap_serverID value to -1 to make it possible to differentiate between a configured and defaulted serverID=0.

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Btw, there are potential problems with using serverID=0, so it would be best if that value was reserved for the default unconfigured case. I.e, a default serverID=0 value could be chosen be slapadd when the two-argument form of serverID is used in the config, as resolving the URL needs the listener argument to slapd to succeed.

You mean the three-argument form? The two-argument form only allows a single serverID to be configured anyway, so there is no ambiguity there. But you're right, in tool mode when multiple serverIDs are configured, there's no way for it to choose the right serverID. That's a problem regardless of whether the default is 0 or -1 though.

Yes, I didn't count the serverID itself as an argument.

If it was clear that different serverID values are always required if there are more than one master server in a replication setup then it should be safe to always include the sid in the syncrepl cookie. And the internal distinction between a configured and defaulted serverID=0 value this patch introduces would probably not be needed.

For now I think this is a doc issue; we could simply recommend that slapadd always be performed on the node with ID 0, and you manually change the serverID config if you need to slapadd on some other node.

My thought was that with serverID=0 reserved for the true single-master configuration and tool mode we could safely ignore a contextCSN with 0 in the sid field when serverID>0 is in use (to get rid of values already in the databases). At startup syncprov could update its own contextCSN value with newer entryCSN values that has 0 in the sid field. Ref ITS#5537. Slapadd'ing on more than one master without synchronizing them between each run could still be a potential problem though..

Rein