A replica set in MongoDB is a group of mongod processes that maintain the same data set. Replica sets provide redundancy and high availability and are the basis for all production deployments. Replication provides redundancy and increases data availability. With multiple copies of data on different database servers, replication provides a level of fault tolerance against the loss of a single database server. A replica set contains several data bearing nodes and optionally one arbiter node. Of the data-bearing nodes, one and only one member is deemed the primary node, while the other nodes are deemed secondary nodes.

The primary node receives all write operations. A replica set can have only one primary capable of confirming writes with { w: "majority" } write concern. By default, clients read from the primary [1]; however, clients can specify a read preference to send read operations to secondaries.

Doing it with docker-compose is pretty simple. The first step is to create the docker-compose.yml configuration file:

version: "3"
services:
  mongo1:
    hostname: mongo1
    container_name: localmongo1
    image: mongo:latest
    volumes:
      - mongodb1-data:/data/db
      - mongodb1-config:/data/configdb
    expose:
    - 27017
    ports:
      - 27011:27017
    restart: always
    entrypoint: [ "/usr/bin/mongod", "--bind_ip_all", "--replSet", "rs0" ]
  mongo2:
    hostname: mongo2
    container_name: localmongo2
    image: mongo:latest
    volumes:
      - mongodb2-data:/data/db
      - mongodb2-config:/data/configdb
    expose:
    - 27017
    ports:
    - 27012:27017
    restart: always
    entrypoint: [ "/usr/bin/mongod", "--bind_ip_all", "--replSet", "rs0" ]
  mongo3:
    hostname: mongo3
    container_name: localmongo3
    image: mongo:latest
    volumes:
      - mongodb3-data:/data/db
      - mongodb3-config:/data/configdb
    expose:
    - 27017
    ports:
    - 27013:27017
    restart: always
    entrypoint: [ "/usr/bin/mongod", "--bind_ip_all", "--replSet", "rs0" ]

volumes:
  mongodb1-data: {}
  mongodb1-config: {}
  mongodb2-data: {}
  mongodb2-config: {}
  mongodb3-data: {}
  mongodb3-config: {}
  proftpd-home: {}

At this point, docker must be started:

$ docker-compose up 
# or
$ docker-compose up -d

At this point, enter in one mongo bash and access to mongo console:

$ docker exec -it localmongo1 /bin/bash
$ mongo

The last step is to run the DB replica set initialization:

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

Now, mongo is ready to accept a connection on port 27011 and, as soon as a DB / collection / document is created or updated, it will be replicated on secondary servers.