Replication is meant to transpose a modification done on one server into the associated servers. We should also insure that a modification done on an entry in more than one server does not lead to inconsistencies.
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.
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.
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 :
Basically, from the user POV, a CSN syntax is [timestamp:replicaId:operationSequence]
We use Java 5 UUID implementation, which is based on variant 2 of RFC 4122
As Mitosis is a multi-master replication system, so each server has to be connected to the server it replicates with, and accept incoming connections from those servers.
We have two components :
The biggest problem we have is to connect to remote servers. As a starting server will have to reconnect to the remote servers, we will two problems :
When a server starts, after having initialized the internal LDAP service, it has to start the network layer. The following algorithm is used :
start the Acceptor set a retry interval to 2 seconds until each remote server is connected do for each not connected remote replica do start a connector if the connection is established remove it from the list of disconnected server done if we have unconnected remote server double the retry interval else exit done
Basically, we try to connect to a remote server, and if we don't success, we wait for an increasing period of time before retrying. When we reach 60 seconds for this interval, we stop increasing the interval and simply try every minute.
Obviously, this is costly, and fragile, as a broken connection has to be detected and immediately restored, otherwise we can't replicate. Plus we don't manage scheduled downtime, as the server still tries to connect to the shutdown server even if it's on purpose.
Plus we have to store the pending operation until the connection is re-established.
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 :
<replicationInterceptor> <configuration> <replicationConfiguration logMaxAge="5" replicaId="instance_a" replicationInterval="2" responseTimeout="10" serverPort="10390"> <s:property name="peerReplicas"> <s:set> <s:value>instance_b@localhost:1234</s:value> <s:value>instance_c@localhost:1234</s:value> </s:set> </s:property> </replicationConfiguration> </configuration> </replicationInterceptor>
Here, for the server instance_a" we have associated two replicas : *instance_b and instance_c. Basically, you just give the list of remote server you want to be connected to.
The MITOSIS service is implemented as an interceptor in the current version (1.5.4). The following operations are handled :
The hasEntry, list, lookup and search operations are only handled to prevent tombstoned (deleted) entries being returned.
We are using Operation objects to manage replications inside the interceptor. Here is the Operation classes hierarchy :
Each of the interceptor's method handling an entry modification will use one of those classes to store the resulting modification.
It creates a AddEntryOperation object, with a ADD_ENTRY operation type (how useful is it, considering that we are already defined a specific class for such an operation ???), an entry and a CSN.
The newly created entry will contain two new AttributeType :
If the added entry already exists in the current server, then we should consider that the entry can't be added.
It creates a CompositeOperation object, which contains a ReplaceAttributeOperation, as the entry is not deleted, but instead a entryDeleted AttributeType is added to the entry, and a ReplaceAttributeOperation containing the injection of a entryCSN AttributeType, with a newly created CSN.
So here are the operation content :