Replication

Redis replication is a very simple to use and configure master-slave replication that allows slave Redis servers to be exact copies of master servers. The following are some very important facts about Redis replication:

• A master can have multiple slaves.

• Slaves are able to accept other slaves connections. Aside from connecting a number of slaves to the same master, slaves can also be connected to other slaves in a graph-like structure.

• Redis replication is non-blocking on the master side, this means that the master will continue to serve queries when one or more slaves perform the first synchronization.

• Replication is non blocking on the slave side: while the slave is performing the first synchronization it can reply to queries using the old version of the data set, assuming you configured Redis to do so in redis.conf. Otherwise you can configure Redis slaves to send clients an error if the link with the master is down. However there is a moment where the old dataset must be deleted and the new one must be loaded by the slave where it will block incoming connections.

• Replications can be used both for scalability, in order to have multiple slaves for read-only queries (for example, heavy SORT operations can be offloaded to slaves, or simply for data redundancy.

• It is possible to use replication to avoid the saving process on the master side: just configure your master redis.conf to avoid saving (just comment all the "save" directives), then connect a slave configured to save from time to time.

How Redis replication works

If you set up a slave, upon connection it sends a SYNC command. And it doesn't matter if it's the first time it has connected or if it's a reconnection.

The master then starts background saving, and collects all new commands received that will modify the dataset. When the background saving is complete, the master transfers the database file to the slave, which saves it on disk, and then loads it into memory. The master will then send to the slave all accumulated commands, and all new commands received from clients that will modify the dataset. This is done as a stream of commands and is in the same format of the Redis protocol itself.

You can try it yourself via telnet. Connect to the Redis port while the server is doing some work and issue the SYNC command. You'll see a bulk transfer and then every command received by the master will be re-issued in the telnet session.

Slaves are able to automatically reconnect when the master <-> slave link goes down for some reason. If the master receives multiple concurrent slave synchronization requests, it performs a single background save in order to serve all of them.

When a master and a slave reconnects after the link went down, a full resync is performed.

Configuration

To configure replication is trivial: just add the following line to the slave configuration file:

slaveof 192.168.1.1 6379

Of course you need to replace 192.168.1.1 6379 with your master IP address (or hostname) and port. Alternatively, you can call the SLAVEOF command and the master host will start a sync with the slave.

Read only slave

Since Redis 2.6 slaves support a read-only mode that is enabled by default. This behavior is controlled by the slave-read-only option in the redis.conf file, and can be enabled and disabled at runtime using CONFIG SET.

Read only slaves will reject all the write commands, so that it is not possible to write to a slave because of a mistake. This does not mean that the feature is conceived to expose a slave instance to the internet or more generally to a network where untrusted clients exist, because administrative commands like DEBUG or CONFIG are still enabled. However security of read-only instances can be improved disabling commands in redis.conf using the rename-command directive.

You may wonder why it is possible to revert the default and have slave instances that can be target of write operations. The reason is that while this writes will be discarded if the slave and the master will resynchronize, or if the slave is restarted, often there is ephemeral data that is unimportant that can be stored into slaves. For instance clients may take information about reachability of master in the slave instance to coordinate a fail over strategy.

Setting a slave to authenticate to a master

If your master has a password via requirepass, it's trivial to configure the slave to use that password in all sync operations.

To do it on a running instance, use redis-cli and type:

config set masterauth <password>

To set it permanently, add this to your config file:

masterauth <password>