Tuesday, July 23, 2013

Enable Replication in HBase

HBase does have support for multi-site replication for disaster recovery, it is not a HA solution, the application and solution architecture will need to implement HA. This means that data from one cluster is automatically replicated to a backup cluster, this can within the same data center or across data centers. There are 3 ways to configure this, master-slave, master-master, and cyclic replication. Master slave is the simplest solution for DR as data is written to the master and replicated to the configured slave(s). Master-Master means that the two clusters cross replicate edits, however have means to prevent replication going into an infinite loop by tracking mutations using the HBase cluster ID. Cyclic replication is supported which means you can have multiple clusters replicating to each other these can be in combinations of master-master, master-slave.
Replication relies on the WAL, the WAL edits are replayed from a source region server to a target region server.

A few important points:
1) Version alignment, in a replicated environment the versions of HBase and Hadoop/HDFS must be aligned, this means you must replicate to the same version of HBase/HDFS (> 0.92)
2) Don't use HBase for Zookeeper deployment, deploy separate Zookeeper Quorums for each cluster that are user managed.
3) You need full network connectivity between all nodes in all clusters, that is Node 1 on cluster A must be able to reach Node 2 on Cluster B and so on, this applies to the Zookeeper clusters as well.
4) All tables and their corresponding column families must exist on every cluster within the replication configuration, these must be named identically.
5) Don't enable HLog compression, this will cause replication to fail.

6) Do not use start_replication nor stop_replication, this will cause data loss on the replicated side

Getting Started

Step 1:
To enable replication all clusters in the replication configuration must add the following to their configuration:
property= hbase.replication
value = true

Step 2:

Launch the hbase shell and set the replication:
hbase(main):001:0> alter 'your_table', {NAME => 'family_name', REPLICATION_SCOPE => '1'}

Next add a peer:
hbase(main):002:0> add_peer '1', "zk1,zk4:2181:/hbase-backup

You can list the currently enabled peers by 
hbase(main):003:0> list_peers 

There are some other considerations too, in your slave peers, it would be worth increasing the TTL on the tables, this means that accidental or malicious deletes can be recovered, you can also increase the min versions property so that more history is retained on the slaves to cover more scenarios. As mentioned this is not a cross site HA solution, there is no automated cut over, this means there is work at the application and operational level to facilitate this. To disable the replication, simply use:
hbase(main):004:0> disable_peer("1")

Note, disabling enabling a table at the source cluster will not affect the current replication, it will start from the 0 offset and replicate any entry which is scoped for replication if present in it. To move into a fresh state, you have to roll the logs on the source cluster. This means, after you have removed the peer, you have to force a manual file roll per hbase shell:
hbase(main):009:0> hlog_roll 'localhost,60020,1365483729051'

It takes servername as an argument. You can get the regionservers name from the znode (/hbase/rs).
When you now re-enable the peer, the replication starts with a fresh state.