mongodb replica set

mongodb replica set allows you to have a backed up data in case one of your server crashes. When data is written to a primary node, the other secondary node will get the same copy of the data that was written to the primary node. Here is how to create replica nodes in mongodb.

Create data directory for each of the mongo replica.

mkdir -p /data/rs1 /data/rs2 /data/rs3

Start 3 mongod instances.

mongod --replSet mongodb123 --logpath "/data/rs1.log" --dbpath /data/rs1 --port 27017 --fork 
mongod --replSet mongodb123 --logpath "/data/rs2.log" --dbpath /data/rs2 --port 27018 --fork
mongod --replSet mongodb123 --logpath "/data/rs3.log" --dbpath /data/rs3 --port 27019 --fork

Connect to mongo instance on port 27017, configure the replication settion, add a test collection and exit.

mongo --port 27017
config = { _id: "mongodb123", members:[
          { _id : 0, host : "localhost:27017"},
          { _id : 1, host : "localhost:27018"},
          { _id : 2, host : "localhost:27019"} ]
         };
rs.initiate(config);
rs.status()
use test;
db.dummycollection.insert({a:1,b:2,c:3});
db.dummycollection.find();
exit

Connect to the other two mongodb replica and verify the test collection dummycollection has the data that was inserted in the above step.

mongo --port 27018
rs.slaveOk();
use test;
db.dummycollection.find();
exit

mongo --port 27019
rs.slaveOk();
use test;
db.dummycollection.find();
exit

Mongodb command help.

mongo --help
mongod help
help #inside mongo shell