Complete guide for MongoDB Replication with docker compose and docker swarm.

• Modify mongo docker image version that suits your environment.
• Change example.env filename to .env, add your own username and password for root user.

1. Create your own keyfile. Key file should be under scripts directory. (or you can change volumes field)

openssl rand -base64 700 > file.key
chmod 400 file.key

2. Copy keyfile into each mongo containers using volumes option.

volumes:
    - ./scripts/file.key:/opt/key/replica.key

3. Run 3 services using docker compose.

docker-compose -f docker-compse-local.yml up -d

4. Enter mongo1 container, run mongo shell command, add replica setting. You can just copy scripts/replica-set.sh.

docker exec -it mongo1 mongo -u <USERNAME> -p <PWD>

rs.status() // check replica setting

rs.initiate({
    "_id": "rs0",
    "members": [
        {
            "_id": 0,
            "host": "mongo1:27017",
            "priority": 2
        },
        {
            "_id": 1,
            "host": "mongo2:27017",
            "priority": 0.5
        },
        {
            "_id": 2,
            "host": "mongo3:27017",
            "priority": 0.5
        }
    ]
});

1. Prepare three different instances.
2. Add docker secret to manage replica key.

$ docker secret create mongo-replica-key ./replica.key

3. Enable docker swarm.
   1. Create a swarm. docker swarm init
   2. Add nodes to the swarm. docker swarm join-token worker will give you a complete command to join with token. Copy and paste to each instance that will become worker nodes.
   3. Add labels to each node. docker node update --label-add LABEL_INFO NODE_NAME For example, run docker node update --label-add name=db MANAGER_NODE command to add name-db label.
4. Deploy service. docker stack deploy -c docker-compose.yml STACK_NAME If you set STACK_NAME to mongo, it will create three services.

   docker service ls
    ID             NAME           MODE         REPLICAS   IMAGE         PORTS
    wg59os2v9ifb   mongo_mongo1   replicated   1/1        mongo:4.2.3   *:27017->27017/tcp
    ioo44nri3zj2   mongo_mongo2   replicated   1/1        mongo:4.2.3   *:27018->27017/tcp
    v0hvhczngyik   mongo_mongo3   replicated   1/1        mongo:4.2.3   *:27019->27017/tcp
   

5. Set Replication setting at Primary Node.

   docker exec -it mongo_mongo1 mongo -u <USERNAME> -p <PWD>
   
   rs.status() // check replica setting
   
   rs.initiate({
       "_id": "rs0",
       "members": [
           {
               "_id": 0,
               "host": "mongo1:27017",
               "priority": 2
           },
           {
               "_id": 1,
               "host": "mongo2:27017",
               "priority": 0.5
           },
           {
               "_id": 2,
               "host": "mongo3:27017",
               "priority": 0.5
           }
       ]
   });