Redis replication: couldn't be any easier

Redis [1] supports master-slave replication [2] and it’s extremely easy to setup. And the beauty of setting up replication is that all you have to do is fire up a slave instance and have it point to some other Redis instance (which then becomes the master node).

Accordingly, open up the redis.conf file for an intended secondary (or slave node) and find the section containing a slaveof phrase. Uncomment it and add the IP address & port of the master node:

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slaveof ec2-23-21-a8-21.compute-1.amazonaws.com 6379

Then cycle the secondary node (so the redis.conf file is re-read). That’s it.

When you fire up the slave node and run the INFO command, you’ll see a few new items, namely:

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role:slave
master_host:ec2-23-21-a8-21.compute-1.amazonaws.com
master_port:6379
master_link_status:down
master_last_io_seconds_ago:-1
master_sync_in_progress:1
master_sync_left_bytes:-1
master_sync_last_io_seconds_ago:0
master_link_down_since_seconds:14

You’ll note that it takes a few moments for a data sync to occur, thus, after a few seconds, rerun the INFO command to see:

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role:slave
master_host:ec2-23-21-a8-21.compute-1.amazonaws.com
master_port:6379
master_link_status:up
master_last_io_seconds_ago:6
master_sync_in_progress:0

Running the same INFO command on the master node will yield a new field as well:

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role:master
slave0:10.255.3.143,34647,online

A master node can have 0..N slaves (hence the naming scheme of slaveN); what’s more, slaves can become a master by commenting out or unsetting the SLAVEOF command in their corresponding configuration.

It should be noted that Redis does support configuration changes at runtime via the CONFIG SET command [3], which means you don’t even need to cycle a particular node to synchronize it with a master.