[Freeipa-devel] Move replication topology to the shared tree

[Ludwig Krispenz]

Ticket 4302 is a request for an enhancement: Move replication topology 
to the shared tree


There has been some discussion in comments in the ticket, but I'd like 
to open the discussion to a wider audience to get an agreement on what 
should be implemented, before writing a design spec.

The implementation requires a new IPA plugin for 389 DS and eventually 
an enhancement of the 389 replication plugin (which depends on some 
decisions below). In the following I will use the terms “topology 
plugin” for the new plugin and “replication plugin” for the existing 389 
multimaster replication plugin.


Lets start with the requirements: What should be achieved by this RFE ?

In my opinion there are three different levels of features to implement 
with this request

- providing all replication configuration information consistent across 
all deployed servers on all servers, eg to easily visualize the 
replication topology.

- Allowing to do sanity checks on replication configuration, denying 
modifications which would break replication topology or issue warnings.

- Use the information in the shared tree to trigger changes to the 
replication configuration in the corresponding servers, this means to 
allow to completely control replication configuration with modifications 
of entries in the shared tree


The main questions are

1] which information is needed in the shared tree (eg what params in the 
repl config should be modifiable)

2] how is the information organized and stored (layout of the repl 
config information shared tree)

3] how is the interaction of the info in the shared tree and 
configuration in cn=config and the interaction between the topology 
plugin and the replication plugin


ad 1] to verify the topology, eg connectivity info about all existing 
replication agreements is needed, the replication agreements only 
contain info about the target, and the parameters for connection to the 
target, but not about the origin. If the data have to evaluated on any 
server, information about the origin has to be added, eg replicaID, 
serverID,...

In addition, if agreement config has to be changed based on the shared 
tree all required parameters need to be present, eg 
replicatedAttributeList, strippedAttrs, replicationEnabled, .....

Replication agreements only provide information on connections where 
replication is configured, if connectivity is to be checked independent 
info about all deployed serevers/replicas is needed.

If topology should be validated, do we need params definig requirements, 
eg each replica to be connected to 1,2,3,... others, type of topology 
(ring, mesh, star,.) ?


ad 2] the data required are available in the replicationAgreement (and 
eventually replica) entries, but the question is if there should be a 
1:1 relationship to entries in the shared tree or a condensed 
representation, if there should be a server or connection oriented view.

In my opinion a 1:1 relation is straight forward, easy to handle and 
easy to extend (not the full data of a repl agreement need to be 
present, other attributes are possible). The downside may be a larger 
number of entries, but this is no problem for the directory server and 
replication and the utilities eg to visualize a topology will handle this.

If the number of entries should be reduced information on multiple 
replication agreements would have to be stored in one entry, and the 
problem arises ho to group data belonging to one agreement. LDAP does 
not provide a simple way to group attribute values in one entry, so all 
the info related to one agreement (origin, target, replicated attrs and 
other repl configuration info) could be stored in a single attribute, 
which will make the attribute as nicely readable and managable as acis.

If topology verification and connectivity check is an integral part of 
the feature, I think a connection oriented view is not sufficient, it 
might be incomplete, so a server view is required, the server entry 
would then have the connection information as subentries or as attributes.


Ad 3] The replication configuration is stored under cn=config and can be 
modified either by ldap operations or by editing the dse.ldif. With the 
topology plugin another source of configuration changes comes into play.

The first question is: which information has precendence ? I think if 
there is info in the shared tree it should be used, and the information 
in cn=config should be updated. This also means that the topology plugin 
needs to intercept all mods to the entries in cn=config and have them 
ignored and handle all updates to the shared tree and trigger changes to 
the cn=config entries, which then would trigger rebuilds of the in 
memory replication objects.

Next question: How to handle changes directly done in the dse.ldif, if 
everything should be done by the topology plugin it would have to verify 
and compare the info in cn=config and in the shared tree at every 
startup of the directory server, which might be complicated by the fact 
that the replication plugin might already be started and repl agreemnts 
are active before the topology plugin is started and could do its work. 
(plugin starting order and dependencies need to be checked).

Next next question: should there be a “bootstrapping” of the config 
information in the shared tree ?

I think yes, the topology plugin could check at startup if there is a 
representation of the config info in the shared tree and if not 
construct it, so after deployment and enabling of the topology plugin 
the information in the shared tree would be initialized.


I think that not every part of the feature has to be handled in the 
topology plugin, we could also ask for enhancements in the 389 
replication plugin itself. There could be an extension to the replica 
and replication agreement entries to reference an entry in the shared 
tree. The replication plugin could check at startup if these entries 
contain replication configuration attributes and if so use them, 
otherwise use the values in cn=config. The presence of the reference 
indicates to the topolgy plugin that initialization is done.

In my opinion this would simplify the coordination at startup and avoid 
unnecessary revaluations and other deployments could benefit from this 
new feature in directory server (one could eg have one entry for 
replication argreements containing the fractional replication 
configuration – and it would be identical on all servers)


So my proposal would contain the following components

1] Store replication configuration in the shared tree in a combination 
of server and connection view (think we need both) and map replication 
configuration to these entries. I would prefer a direct mapping (with a 
subset of the cn=config attributes and required additions)

2] provide a topology plugin to do consistency checks and topology 
verification, handle updates to trigger modification changes in 
cn=config, intercept and reject direct mods to cn=config entries At 
startup verify if shared tree opbjects are present, initialize them if 
not, apply to cn=config if required

3] enhance replication plugin to handle config information in the shared 
tree. This would allow to consistently handle config changes either 
applied to the shared config, cn=config mods or des.ldif changes. This 
feature might also be interesting to other DS deployments