Configuring a MongoDB Cluster - mongodb

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.