A script which acquires uses redis to ensure exactly one server runs a process

redis_lock.sh

#!/bin/bash -e

set -e

# a script which acquires an exclusive lock from a redis server
# and executes a command that needs mutual exclusion across servers
# usage:
# redis_lock.sh <lock_name> <command>

redis_server=redis.example.com
lock_name=$1 ; shift
val=$(uuid -v4)
renew_time=10  # once a second, the TTL on the lock key will be set to this number of seconds
               # used to clean up locks in event of a network/system failure

function acquire() {
	if test "$(redis-cli -h ${redis_server} SET locks:${lock_name} ${val} EX ${renew_time} NX)" = OK ; then 
		echo lock acquired
	else
		echo unable to acquire lock
		exit 1
	fi 
}

function renew() {
	parent_pid=$1
	if test "$(redis-cli -h ${redis_server} GET locks:${lock_name})" = $val ; then
		redis-cli -h ${redis_server} EXPIRE locks:${lock_name} ${renew_time} 
	else
		echo "ERROR!!! unexpexted value for lock. Someone else acquired it!!!" > /dev/fd/2
		kill $parent_pid
		exit 1
	fi
}

function release() {
	if test "$(redis-cli -h ${redis_server} GET locks:${lock_name})" = $val ; then
		redis-cli -h ${redis_server} DEL locks:${lock_name}
	else
		echo "ERROR!!! unexpexted value for lock. Someone else acquired it!!!" > /dev/fd/2
		exit 1
	fi
		
}

function term() {
	kill %1 || /bin/true
	kill %2 || /bin/true
	release
}


acquire
(while true ; do renew $$ ; sleep 1 ; done) &>/dev/null  &  # spawn a lock renewal process
trap "term" TERM INT HUP

$* &  # run the desired command in the background as %2

wait %2
term