DATABASE » NOSQL » MONGODB

Replication

Last updated:
Reference:docs.mongodb.com

Replica Set ​

Create 3 separate folders for each instance, then start 3 instances of mongod. Hostname is the local IP address.

shell
mkdir -p mongodb/rs0-0  mongodb/rs0-1 mongodb/rs0-2

mongod --replSet rs0 --port 27018 --bind_ip localhost,<private IP> --dbpath mongodb/rs0-0 --fork --logpath mongodb-0.log
mongod --replSet rs0 --port 27019 --bind_ip localhost,<private IP> --dbpath mongodb/rs0-1 --fork --logpath mongodb-1.log
mongod --replSet rs0 --port 27020 --bind_ip localhost,<private IP> --dbpath mongodb/rs0-2 --fork --logpath mongodb-2.log

--fork --logpath ~/mongod-rs0.log  # For running in background

Use --smallfiles --oplogSize 128 for debug purposes.

Connect to one instance.

shell
mongo --port 27018

Configure the replica set.

js
rsconf = {
  _id: "rs0",
  members: [
    {
     _id: 0,
     host: "<hostname>:27018"
    },
    {
     _id: 1,
     host: "<hostname>:27019"
    },
    {
     _id: 2,
     host: "<hostname>:27020"
    }
   ]
}

rs.initiate(rsconf)

Check the current configuration with the following command.

js
rs.conf()

Adjust priority ​

js
cfg = rs.conf()
cfg.members[0].priority = 2    // Primary
cfg.members[1].priority = 0.5  // Secondary
cfg.members[2].priority = 0.5  // Secondary

rs.reconfig(cfg)

Read from Secondary ​

Set this on every mongo instance

js
db.getMongo().setReadPref('secondaryPreferred')
db.getMongo().setSlaveOk()