Question

We have configured the Redis server with one master and two slaves. If my master fails, how can we handle the failover without restarting the Redis server.

Answer 1

Update:

Today, I would recommend checking out redis-sentinel, a tool by Redis’ author antirez for monitoring and automatic failover.

Original reply:

Check the SLAVEOF command: http://redis.io/commands/slaveof

When you discover that your master fails, issue a SLAVEOF NO ONE on one of your slaves to promote it to master. Then point your other slave to it’s new master. See also “Upgrading or restarting a Redis instance without downtime”: http://redis.io/topics/admin

For managing configuration files you could do something along these lines (caution: Not tested, meant as an example). The example below assumes two configuration files for each server (/etc/redis/server1.master.conf, /etc/redis/server1.slave.conf, etc), one having that server as a slave of some predefined master:

#!/bin/sh

master()
{
    server_name=$1
    redis-cli slaveof no one
    ln -sf /etc/redis/$server_name.master.conf /etc/redis/$server_name.conf
}

# Usage: slave(server1 server2 6379)
slave()
{
    server_name=$1
    master=$2
    master_port=$3
    redis-cli slaveof $master $master_port
    ln -sf /etc/redis/$server_name.slave.conf /etc/redis/$server_name.conf
}

Instead of having the predefined configuration files, you could edit them on the fly with e.g. sed. Basically, you would make sure to always have a slaveof stanza in the configuration files, either pointing to a master or slaveof no one. Then rewrite the configuration using sed (again, not tested, just meant as food for thought):

#!/bin/sh

master()
{
    server_name=$1
    config=$server_name.conf
    redis-cli slaveof no one
    sed -i "s/^slaveof.*/slaveof no one/" $config
}

# Usage: slave(server1 server2 6379)
slave()
{
    server_name=$1
    config=$server_name.conf
    master=$2
    master_port=$3
    redis-cli slaveof $master $master_port
    sed -i "s/^slaveof.*/slaveof $master $master_port/" $config
}