There are cases where you need to synchronize multiple clusters to the same state. Synchronization of clusters, also known as WAN (Wide Area Network) Replication, is mainly used for replicating stats of different clusters over WAN environments like the Internet.
Imagine you have different data centers in New York, London and Tokyo each running an independent Hazelcast cluster. Every cluster would be operating at native speed in their own LAN (Local Area Network) settings but you also want some or all recordsets in these clusters to be replicated to each other: updates to Tokyo cluster also go to London and New York, in the meantime updates from New York cluster are synchronized to Tokyo and London.
The current WAN Replication implementation supports two different operation modes.
Active-Passive: This mode is mostly used for failover scenarios where you want to replicate only one active cluster to one or more non-active ones for backup reasons.
Active-Active: Every cluster is fully equal and all clusters replicate to all others. This is normally used to connect different clients to different clusters for the sake of the shortest path between client and server.
Let's see how we can set up WAN Replication for London and Tokyo clusters:
<hazelcast> <wan-replication name="my-wan-cluster"> <target-cluster group-name="tokyo" group-password="tokyo-pass"> <replication-impl>com.hazelcast.wan.impl.WanNoDelayReplication</replication-impl> <end-points> <address>10.2.1.1:5701</address> <address>10.2.1.2:5701</address> </end-points> </target-cluster> <target-cluster group-name="london" group-password="london-pass"> <replication-impl>com.hazelcast.wan.impl.WanNoDelayReplication</replication-impl> <end-points> <address>10.3.5.1:5701</address> <address>10.3.5.2:5701</address> </end-points> </target-cluster> </wan-replication> ... </hazelcast>
Using this configuration, the cluster running in New York is replicating to Tokyo and London. The Tokyo and London clusters should have a similar configurations if you want to run in Active-Active mode.
If the New York and London cluster configurations contain the
wan-replication element and the Tokyo cluster does not, it means
New York and London are active endpoints and Tokyo is a passive endpoint.
By using an Active-Active Replication setup, you might end up in situations where multiple clusters simultaneously update the same entry in the same distributed data structure. Those situations will cause conflicts, which makes it sufficient for you to provide merge-policies to resolve those conflicts.
<hazelcast> <wan-replication name="my-wan-cluster"> <merge-policy>com.hazelcast.map.merge.PassThroughMergePolicy</merge-policy> ... </wan-replication> ... </hazelcast>
As noted earlier, you can have Hazelcast replicating only some or all of the data in your cluster. Imagine you have 5 different
distributed maps but you might want only one of these maps replicating across clusters. To achieve this, you mark the maps to be
replicated by adding the
wan-replication-ref element in the map configuration as shown below.
<hazelcast> <wan-replication name="my-wan-cluster"> ... </wan-replication> <map name="my-shared-map"> <wan-replication-ref name="my-wan-cluster"> <merge-policy>com.hazelcast.map.merge.PassThroughMergePolicy</merge-policy> ... </wan-replication-ref> </map> ... </hazelcast>
You see that we have
my-shared-map configured to replicate itself to the cluster targets defined in the earlier
You will also have to define a
merge policy for merging replica entries and resolving conflicts during the merge
as mentioned before.
For huge clusters or high data mutation rates, you might need to increase the replication queue size. The default queue
size for replication queues is
100000. This means, if you have heavy put/update/remove rates, you might exceed the queue size
so that the oldest, not yet replicated, updates might get lost.
To increase the replication queue size, a Hazelcast Enterprise user can use the
You can do this by setting the property on the command line (where xxx is the queue size):
or by setting the properties inside the
hazelcast.xml (where xxx is the requested queue size):
<hazelcast> <properties> <property name="hazelcast.enterprise.wanrep.queuesize">xxx</property> </properties> </hazelcast>
You can download the white paper Hazelcast on AWS: Best Practices for Deployment from Hazelcast.com.
Please refer to the WAN Replication Configuration section for a full description of Hazelcast WAN Replication configuration.