Existing_Mongo_AEM.sh

cd <Mongo bin directory>
use admin
db.shutdownServer()
#Take Backup 
./mongodump --dbpath ../data/<NAME OF DB> -o dataout
# Now set up new Mongo DB servers that you want to add in replica set Instruction above
# Now start your existing Mongo Using following command, Note that this time we are using replSet
# You can also do this using Mongo DB configuration file
./mongod --port 27017 --dbpath <your DB path> --replSet <Replica set Name> &
#Connect to the mongodb instance
./mongod --port 27107
rs.initiate()
#Make sure that other MongoDB are are up you can now add them to replica set
rs.add("<HOST2>:<PORT2>")
# To make sure that you have odd number of instances
rs.add("<HOST3>:<PORT3>")
#If you don't have space or don't want third instance to store data then you can do something like
# rs.addArb("<HOST4>:<PORT4>")
# Check config again
rs.conf()
# Check status of Replica
rs.status()

#If something goes wrong you can do something like
cd <Mongo bin location>
mongorestore --dbpath <database path> <path to the backup>