This is a simple solution for when you use keepalived with VRRP to failover between two instances of an application (e.g. nginx) that uses Redis/Valkey.
It doesn’t provide a high guarantee of data consistency, so it’s only suitable for caches, user sessions and similar!
keepalived.conf example:
vrrp_instance vi {
...
notify "/etc/keepalived/scripts/notify_valkey.sh"
}
| #!/bin/sh | |
| # This script assumes there are two keepalived and Valkey instances. | |
| set -eu | |
| # Maximum time in milliseconds a master will wait in the waiting-for-sync state | |
| # before aborting the failover attempt. | |
| FAILOVER_TIMEOUT=2000 | |
| # Time in seconds that a replica will wait for a failover initiated by the | |
| # master to occur. | |
| FAILOVER_WAIT=8 | |
| REDIS_CLI='valkey-cli' | |
| REDIS_PORT=6379 | |
| SYSLOG_TAG='keepalived-valkey' | |
| log() { | |
| local level="$1" | |
| local msg="$2" | |
| logger -s -t "$SYSLOG_TAG" -p local0.$level "$msg" | |
| } | |
| redis_cmd() { | |
| local stderr | |
| if ! stderr="$($REDIS_CLI -e "$@" 2>&1 >/dev/null)"; then | |
| log err "Command $1 failed: $stderr" | |
| return 1 | |
| fi | |
| } | |
| repl_info() { | |
| local name="$1" | |
| $REDIS_CLI -e INFO REPLICATION | sed -En "s/^$name:(\S*).*/\1/p" | |
| } | |
| is_master() { | |
| [ "$(repl_info 'role')" = master ] | |
| } | |
| replica_to_master() { | |
| # NOTE: When keepalived on the primary host is started and the secondary is | |
| # MASTER, it enters the BACKUP mode first, so Valkey syncs data from the | |
| # secondary node, then keepalived enters the MASTER mode and Valkey becomes | |
| # the master. | |
| if is_master; then | |
| log info "I'm already master, nothing to be done" | |
| return | |
| fi | |
| if [ "$(repl_info master_link_status)" = 'up' ]; then | |
| log info "Current master is up, waiting $FAILOVER_WAIT seconds for failover" | |
| sleep $FAILOVER_WAIT | |
| fi | |
| if is_master; then | |
| log info "Failover succeeded, I'm master now" | |
| else | |
| log warn 'Failover failed, executing REPLICAOF NO ONE' | |
| redis_cmd REPLICAOF NO ONE | |
| fi | |
| } | |
| master_to_replica() { | |
| local master_host="$1" | |
| if ! is_master; then | |
| log info "I'm already replica, nothing to be done" | |
| return | |
| fi | |
| if [ "$(repl_info connected_slaves)" -gt 0 ]; then | |
| log info 'Starting failover' | |
| if redis_cmd FAILOVER TIMEOUT "$FAILOVER_TIMEOUT"; then | |
| log info "Waiting $FAILOVER_WAIT seconds" | |
| sleep "$FAILOVER_WAIT" | |
| fi | |
| if is_master; then | |
| log warn 'Failover failed' | |
| else | |
| log info "Failover succeeded, I'm replica now" | |
| return | |
| fi | |
| fi | |
| log info "Executing REPLICAOF $master_host $REDIS_PORT" | |
| redis_cmd REPLICAOF "$master_host" $REDIS_PORT | |
| } | |
| if [ $# -lt 3 ]; then | |
| log err "Expected 3 arguments, got $#" | |
| exit 1 | |
| fi | |
| redis_cmd PING || exit 1 | |
| case "$3" in | |
| MASTER) | |
| replica_to_master | |
| ;; | |
| BACKUP | FAULT) | |
| hostname="$(hostname -s)" | |
| domain="$(hostname -d)" | |
| case "$hostname" in | |
| *-01) other_host="${hostname%-*}-02.$domain";; | |
| *-02) other_host="${hostname%-*}-01.$domain";; | |
| esac | |
| master_to_replica "$other_host" | |
| ;; | |
| esac |