Docker Swarm Service DNS Names

General
,
1

Hi,
I have a Docker Swarm Stack, that consists of a Frontend Service, MongoDB Replicaset with 3 replicas for HA reasons and a service that runs one time on deploy and sets up the replicaset.
For the MongoDB Replicaset I need to specify the DNS Names of the other MongoDBs so that they can communicate with each other. Simplified my docker compose looks like this:

version: '3.8'
services:
  rocketchat:
    image: Frontend
    environment:
      - MONGO_URL=mongodb://mongo1:27017,mongo2:27017,mongo3:27017/dbname?replicaSet=rs0&readPreference=primaryPreferred&w=majority
      - MONGO_OPLOG_URL=mongodb://mongo1:27017,mongo2:27017,mongo3:27017/local?replicaSet=rs0&readPreference=primaryPreferred

  mongo1:
    image: mongo
    command: mongod --smallfiles --oplogSize 128 --replSet rs0

  mongo2:
    image: mongo
    command: mongod --smallfiles --oplogSize 128 --replSet rs0

  mongo3:
    image: mongo
    command: mongod --smallfiles --oplogSize 128 --replSet rs0

  init-replica1:
    image: mongo
    deploy:
      restart_policy:
        condition: on-failure
    command: 'bash -c "for i in `seq 1 30`; do mongo mongo1/dbname --eval \"rs.initiate({ _id: ''rs0'', members: [ { _id: 0, host: ''mongo1:27017'' },  { _id: 1, host: ''mongo2:27017'' }, { _id: 2, host: ''mongo3:27017'' }  ]})\" && s=$$? && break || s=$$?; echo \"Tried $$i times. Waiting 5 secs...\"; sleep 5; done; (exit $$s)"'

This works fine, but when I need to update my MongoDB container (Verision update etc.) then all of them restart at once when I do:

docker stack deploy my-stack

The other option to my knowledge is to have only one Mongo Service with three replicas, so i can specify the update_config but then I dont have a dns name for each replica (here mongo1,mongo2, mongo3) so my replicaset fails:

version: '3.8'
services:
  rocketchat:
    image: Frontend
    environment:
      - MONGO_URL=mongodb://mongo1:27017,mongo2:27017,mongo3:27017/dbname?replicaSet=rs0&readPreference=primaryPreferred&w=majority
      - MONGO_OPLOG_URL=mongodb://mongo1:27017,mongo2:27017,mongo3:27017/local?replicaSet=rs0&readPreference=primaryPreferred

  mongo1:
    image: mongo
    command: mongod --smallfiles --oplogSize 128 --replSet rs0
    deploy:
      replicas: 3
      update_config:
        delay: 1m
        failure_action: rollback

  init-replica1:
    image: mongo
    deploy:
      restart_policy:
        condition: on-failure
    command: 'bash -c "for i in `seq 1 30`; do mongo mongo1/dbname --eval \"rs.initiate({ _id: ''rs0'', members: [ { _id: 0, host: ''mongo1:27017'' },  { _id: 1, host: ''mongo2:27017'' }, { _id: 2, host: ''mongo3:27017'' }  ]})\" && s=$$? && break || s=$$?; echo \"Tried $$i times. Waiting 5 secs...\"; sleep 5; done; (exit $$s)"'

Does anyone have an idea how to configure this so that only one db container at a time is down, in case I have an update for my mongoDbs?

3

This is not a problem with exposed port, the first example i wrote works, but my problem is when updating the service I am unable to do a rolling update because you can only specify update_config on service level and not stack level.

The MonogoDB’s are replicating their status and electing a Primary. So they ned to have the excat DNS/IP in their config. So what needs to be in the MongoDB Replicaset looks like this:

"members" : [

                {
                        "_id" : 0,
                        "name" : "mongo1:27017",
                        "stateStr" : "SECONDARY",
                },
                {
                        "_id" : 1,
                        "name" : "mongo2:27017",
                        "stateStr" : "PRIMARY",
                },
               {
                        "_id" : 2,
                        "name" : "mongo3:27017",
                        "stateStr" : "SECONDARY",
                }