# EMR WAL cross-cluster replication
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From EMR 7.5, EMR WAL supports HBase cross-cluster replication of write-ahead logs. This topic shows you how to enable the feature and check that it's working. For more details about cluster-replication, see [Cluster Replication](https://hbase.apache.org/book.html#_cluster_replication) in the Apache HBase documentation.

**Note**  
There is extra read cost associated with write-ahead logs, because the replication process reads data from the local EMR WAL. For more details about cost, refer to [About Amazon EMR Releases](https://docs.aws.amazon.com/emr/latest/ReleaseGuide/emr-hbase-wal-metrics.html).

## Cross-cluster replication setup
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In order to have the same user experience, enabling the replication feature on EMR WAL is the same as with native HBase write-ahead logs. The procedure that follows shows a simple example. For more information, see the HBase documentation at [Cluster Replication](https://hbase.apache.org/book.html#_cluster_replication).

1. Start a primary cluster, which is the replication source, with EMR WAL enabled. To enable write-ahead logs, see [Enabling Amazon EMR WAL](emr-hbase-wal-enabling.md). Additionally, start a peer cluster. For this peer cluster, you can choose to enable EMR WAL or not.

1. On both clusters, create a table:

   ```
   HBASE_CMD="sudo -u hbase hbase"
   echo "create 'test_replication_table',{NAME => 'CF'}" | $HBASE_CMD shell
   ```

1. Add peer setup on the primary cluster and enable table replication. During the peer addition, it needs the peer cluster master node hostname, which is **PEER\$1DNS**.

   ```
   HBASE_CMD="sudo -u hbase hbase"
   PEER_DNS="ip-10-1-1-0.ec2.com"
   PEER_NAME="aws"
   TABLE_NAME="test_replication_table"
   
   ## Create peering with the destination cluster
   echo "add_peer '$PEER_NAME', CLUSTER_KEY => '$PEER_DNS:2181:/hbase'" | $HBASE_CMD shell
   
   ## List peers in the primary cluster to confirm peer setup
   echo "list_peers" | $HBASE_CMD shell
   
   ## Enable table replication
   echo "enable_table_replication '$TABLE_NAME'" | $HBASE_CMD shell
   ```

## Confirming cross cluster replication
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After performing the setup steps, replication is enabled between the primary cluster and peer cluster. A test follows that confirms replication is working.

1. Add data on the primary cluster and verify the data replicated to the peer cluster.

   ```
   ## Write on primary cluster with HBase CLI
   
   put 'test_replication_table', 'aaa', 'CF:a', 'aaa_a1'
   put 'test_replication_table', 'bbb', 'CF:b', 'bbb_b1'
   put 'test_replication_table', 'ccc', 'CF:c', 'ccc_c1'
   ```

1. Confirm successful replication on the peer cluster. In this case, you should see replicated data written from the primary to the peer cluster.

   ```
   ### Scan on peer cluster with HBase CLI
   
   scan 'test_replication_table'
   ```