Replica Sets and Sharding

Remarks#

For those not familiar, a Replica Set is defined as a redundant configuration of three servers. A Sharded Database is defined as a horizintally scalled database, where each Shard is defined as a Replica Set. Therefore, a sharded Mongo cluster involves a minimum of 11 servers for a 2 shard cluster, and increases by three servers for each additional shard. So, a sharded cluster always has 11, 14, 17, 20, 23, etc server instances. That is, there’s 2 shards of 3 servers each, 3 more config controllers, and 2 routers. 11 servers total for a 2 shard cluster.

Replica Set Quickstart

Build yourself three servers using whatever physical or virtual hardware you wish. (This tutorial assumes you’re using Ubuntu as your operating system.) Then repeat the following instructions three times… once for each server.

# add the names of each server to the host file of each server
sudo nano /etc/hosts
  10.123.10.101 mongo-a
  10.123.10.102 mongo-b
  10.123.10.103 mongo-c

# install mongodb on the server
sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv 7F0CEB10
echo 'deb https://downloads-distro.mongodb.org/repo/ubuntu-upstart dist 10gen' | sudo tee /etc/apt/sources.list.d/mongodb.list
sudo apt-get update
sudo apt-get install mongodb-10gen


# create the /data/ directories
sudo mkdir /data
sudo mkdir /data/logs
sudo mkdir /data/db

# make sure the mongodb user and group have access to our custom directories
sudo chown -R mongodb:mongodb /data

# edit the mongo upstart file in /etc/init/mongodb.conf
sudo nano /etc/init/mongodb.conf
  start on started mountall
  stop on shutdown
  respawn
  respawn limit 99 5
  setuid mongodb
  setgid mongodb
  script
    exec /usr/bin/mongod --config /etc/mongodb.conf >> /data/logs/mongo-a.log 2>&1
  end script


# edit mongodb configuration file
sudo nano /etc/mongodb.conf
    dbpath=/data/db
    logpath=/data/logs/mongod.log
    logappend=true
    port=27017
    noauth=true
    replSet=meteor
    fork=true

# add a mongo log-rotation file
sudo nano /etc/logrotate.d/mongod
  /data/logs/*.log {
    daily
    rotate 30
    compress
    dateext
    missingok
    notifempty
    sharedscripts
    copytruncate
    postrotate
        /bin/kill -SIGUSR1 `cat /data/db/mongod.lock 2> /dev/null` 2> /dev/null || true
    endscript
  }

# make sure mongod service is started and running
sudo service mongodb start
sudo reboot

Replica Set Configuration

Then go into the mongo shell and initiate the replica set, like so:

meteor mongo

  > rs.initiate()
  PRIMARY> rs.add("mongo-a")
  PRIMARY> rs.add("mongo-b")
  PRIMARY> rs.add("mongo-c")
  PRIMARY> rs.setReadPref('secondaryPreferred')