# Multi Cluster Replication

This package provides replication between HBase clusters.

## Table Of Contents

1. Status
2. Requirements
3. Deployment
4. Verifying Replicated Data

## Status

This package is experimental quality software and is only meant to be a base for future developments. The current implementation offers the following features:

1. Master/Slave replication.
2. Master/Master replication.
3. Cyclic replication.
4. Replication of scoped families in user tables.
5. Start/stop replication stream.
6. Supports clusters of different sizes.
7. Handling of partitions longer than 10 minutes.
8. Ability to add/remove slave clusters at runtime.
9. MapReduce job to compare tables on two clusters

Please report bugs on the project's Jira when found.

## Requirements

Before trying out replication, make sure to review the following requirements:

1. Zookeeper should be handled by yourself, not by HBase, and should always be available during the deployment.
2. All machines from both clusters should be able to reach every other machine since replication goes from any region server to any other one on the slave cluster. That also includes the Zookeeper clusters.
3. Both clusters should have the same HBase and Hadoop major revision. For example, having 0.90.1 on the master and 0.90.0 on the slave is correct but not 0.90.1 and 0.89.20100725.
4. Every table that contains families that are scoped for replication should exist on every cluster with the exact same name, same for those replicated families.
5. For multiple slaves, Master/Master, or cyclic replication version 0.92 or greater is needed.

## Deployment

The following steps describe how to enable replication from a cluster to another.

1. Edit ${HBASE_HOME}/conf/hbase-site.xml on both cluster to add the following configurations:

       <property>
         <name>hbase.replication</name>
         <value>true</value>
       </property>

   deploy the files, and then restart HBase if it was running.
2. Run the following command in the master's shell while it's running

       add_peer

   This will show you the help to setup the replication stream between both clusters. If both clusters use the same Zookeeper cluster, you have to use a different zookeeper.znode.parent since they can't write in the same folder.
3. Once you have a peer, you need to enable replication on your column families. One way to do it is to alter the table and to set the scope like this:

             disable 'your_table'
             alter 'your_table', {NAME => 'family_name', REPLICATION_SCOPE => '1'}
             enable 'your_table'

   Currently, a scope of 0 (default) means that it won't be replicated and a scope of 1 means it's going to be. In the future, different scope can be used for routing policies.
4. To list all configured peers run the following command in the master's shell

       list_peers

   (as of version 0.92)

You can confirm that your setup works by looking at any region server's log on the master cluster and look for the following lines;

    Considering 1 rs, with ratio 0.1
    Getting 1 rs from peer cluster # 0
    Choosing peer 10.10.1.49:62020

In this case it indicates that 1 region server from the slave cluster was chosen for replication.

## Verifying Replicated Data

Verifying the replicated data on two clusters is easy to do in the shell when looking only at a few rows, but doing a systematic comparison requires more computing power. This is why the VerifyReplication MR job was created, it has to be run on the master cluster and needs to be provided with a peer id (the one provided when establishing a replication stream) and a table name. Other options let you specify a time range and specific families. This job's short name is "verifyrep" and needs to be provided when pointing "hadoop jar" to the hbase jar.