Create and start

create a master and a slave redis container : docker-compose.yml

version: '3.7'

services:

  redis-master:
    image: redis
    container_name: redis-master
    volumes:
      - "./datamaster:/data"

  redis-slave:
    image: redis
    container_name: redis-slave
    command: redis-server --slaveof redis-master 6379
    volumes:
      - "./dataslave:/data"

write data on master and read on slave

# start redis
docker compose up -d
# set a value on master
docker exec -ti redismaster redis-cli set a 1
# get a value on slave (shows "1")
docker exec -ti redisslave redis-cli get a

wait for slave to be in sync

when you run "info replication" on a redis slave it returns master_link_status: up and master_sync_in_progress: 0 when it is fully synced.
So you can wait for the save to be in sync with this coimmand

function waitslavesync {
    echo waitslavesync $1;

    while true ; do
        output=$(docker exec -ti $1 redis-cli info replication)

        link_status=$(echo "$output" | grep -oP '(?<=master_link_status:)[^[:space:]]+')
        sync_progress=$(echo "$output" | grep -oP '(?<=master_sync_in_progress:)[^[:space:]]+')

        if [ "$link_status" = "up" ] && [ "$sync_progress" = "0" ]; then
            echo "$1 is in sync"
            break;
        else
            sleep 1;
        fi
    done
}

waitslavesync redisslave

kill the slave

i can kill a slave and restart it and it has the updated data

docker rm -f redisslave
docker compose up -d
waitslavesync redisslave
docker exec -ti redisslave redis-cli get a

kill the master

i can kill the master and the slave still has the data. But you cannot write on the slave

if i kill the master the data is gone.
to keep the data i need to activate persistence (run bgsave) and add a volume like ./master:/data
or you can restart the master and make it a slave.

rotate master and slave :

docker rm -f redis-master
docker exec -ti redis-slave redis-cli slaveof no one
docker compose up -d redis-master
docker exec -ti redis-master redis-cli slaveof redis-slave 6379
waitslavesync redis-master

now you can write on the slave. and data is available on master and on slave.
no data have been lost