Configuring a MongoDB Cluster

Question

I should configure a mongodb cluster with two nodes. Each node should have all data replicated. If one server die, the other should assume as primary.

Before I configure the cluster, I'm doing a local test using docker instances to configure it. As I saw in the documentation, I should have at least 3 instances of MongoDB. Is that correct?

First I created the tree instance with docker, the I configured the instance one to be the primary. The code bellow is my docker compose and the configure script.

The Docker compose:

version: '3'
services:
    mongo-2:
        container_name: mongo-2
        image: mongo:4
        ports:
            - 30102:27017
        command: mongod --replSet cnf-serv --port 27017 --oplogSize 16 --bind_ip_all
        restart: always

    mongo-3:
        container_name: mongo-3
        image: mongo:4
        ports:
            - 30103:27017
        command: mongod --replSet cnf-serv --port 27017 --oplogSize 16 --bind_ip_all
        restart: always 

    mongo-1:
        container_name: mongo-1
        image: mongo:4
        ports:
            - 30101:27017
        command: mongod --replSet cnf-serv --port 27017 --oplogSize 16 --bind_ip_all
        links:
            - mongo-2:mongo-2
            - mongo-3:mongo-3
        restart: always

    mongo-setup:
        container_name: mongo-setup
        image: mongo:4
        depends_on:
            - mongo-1
            - mongo-2
            - mongo-3
        links:
            - mongo-1:mongo-1
            - mongo-2:mongo-2
            - mongo-3:mongo-3
        volumes:
            - ./scripts:/scripts
        environment: 
            - MONGO1=mongo-1
            - MONGO2=mongo-2
            - MONGO3=mongo-3
            - RS=cnf-serv
            - PORT=27017
        entrypoint: [ "/scripts/setup.sh" ]

The configure script:

#!/bin/bash 

mongodb1=`getent hosts ${MONGO1} | awk '{ print $1 }'`
mongodb2=`getent hosts ${MONGO2} | awk '{ print $1 }'`
mongodb3=`getent hosts ${MONGO3} | awk '{ print $1 }'`

port=${PORT:-27017}

echo "Waiting for startup.."
until mongo --host ${mongodb1}:${port} --eval 'quit(db.runCommand({ ping: 1 }).ok ? 0 : 2)' &>/dev/null; do
printf '.'
sleep 1
done

echo "Started.."

echo setup.sh time now: `date +"%T" `
mongo --host ${mongodb1}:${port} <<EOF
var cfg = {
        "_id": "${RS}",
        "protocolVersion": 1,
        "members": [
            {
                "_id": 100,
                "host": "${mongodb1}:${port}"
            },
            {
                "_id": 101,
                "host": "${mongodb2}:${port}"
            },
            {
                "_id": 102,
                "host": "${mongodb3}:${port}"
            }
        ]
    };
    rs.initiate(cfg, { force: true });
    rs.reconfig(cfg, { force: true });
EOF

When I try to connect in any external container port, I have the same error message: Could not connect to MongoDB on the provided host and port

How can I configure a internal cluster using docker?

Answer 1

A 2-node replica set won't be very much help; if one of the nodes goes down, then the other node can't take over as primary because it can't command a majority of votes in the election. You get some benefit in that you have a full copy of the data on the second node, but to get the full benefit of your replica set (both redundancy and high-availability), you would need to add a 3rd node.