Apache Hadoop High Availability
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HBase Cluster Replication Overview

Cluster replication uses a source-push methodology. An HBase cluster can be a 'source' cluster, which means it is the source of the new data (also known as a 'master' or 'active' cluster), a 'destination' cluster, which means that it is the cluster that receives the new data by way of replication (also known as a 'slave' or 'passive' cluster), or an HBase cluster can fulfill both roles at once. Replication is asynchronous, and the goal of replication is eventual consistency. When the source receives an edit to a column family with replication enabled, that edit is propagated to all destination clusters using the WAL for that column family on the RegionServer that manages the relevant region.

When data is replicated from one cluster to another, the original source of the data is tracked by using a cluster ID which is part of the metadata. In HBase 0.96 and newer (HBASE-7709), all clusters that have already consumed the data are also tracked. This prevents replication loops.

The WALs for each RegionServer must be kept in HDFS as long as they are needed to replicate data to a slave cluster. Each RegionServer reads from the oldest log it needs to replicate and keeps track of its progress by processing WALs inside ZooKeeper to simplify failure recovery. The position marker which indicates a slave cluster’s progress, as well as the queue of WALs to process, may be different for every slave cluster.

The clusters participating in replication can be of different sizes. The master cluster relies on randomization to attempt to balance the stream of replication on the slave clusters. It is expected that the slave cluster has storage capacity to hold the replicated data, as well as any data it is responsible for ingesting. If a slave cluster runs out of room, or is inaccessible for other reasons, it throws an error, the master retains the WAL, and then retries the replication at intervals.