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As the remote servers may not be available, due to network conditions, we also have to wait for the synchronization to be done before we can validate a full replication for an entry. For instance, if we delete an entry on server A, it can be deleted for real only when all the remote servers has confirmed that the deletion was successful.

Data structure

CSN and UUID

We will use two tags, stored within each entry, to manage the replication. The CSN (Change Sequence Number) stores when and where (which server) the entry was last modified. A replicated entry on 3 servers will have the same CSN. Before replication they may be different. The UUID (Universal Unique Identifier) is associated with an entry, and only one. So if we have an entry replicated on 3 servers, it will have one CSN (as the entry is at the same version for all servers) and only one UUID (as it's the same entry). The UUID is not currently used. The CSN stored in the entry is used to prevent older modifications overwriting newer ones. Unfortunately this leads to inconsistent servers (see DIRSERVER-894) - we need to check the CSN for each attribute instead. Once this is fixed the CSN stored on each entry will no longer be used.

CSN structure

A CSN is a composition of a timestamp, a replica ID and a operation sequence number. It's described in The LDAP Change Sequence Number. We have defined a simpler version, as the current RFC is still a draft, where we use a unique operationSequence instead of two integers (timeCount and changeCount) to disambiguate entries changed at the same time.

As the timestamp is computed using a System.currentTimeMillis() call, the accuracy is around 10 ms. We may have at hundreds of changes done in this interval. This is the reason we have a additional operationSequence number.

The CSN class structure is described by the following schema :

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Basically, from the user POV, a CSN syntax is *\[timestamp:replicaId:operationSequence\]*

UUID structure

Network

Configuration

The replication system is a Multi-Master replication, ie, each server can update any server it is connected to. The way you tell a server to replicate to others is simple :

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