MongoDB replica set on single machine - mongodb

Question

I have a single instance (I.e. no virtualisation) of Linux running on a server. I have a single mongod instance that is running. Moving to production I want to implement replica sets. Everything I've read talks about running mongod on multiple machines.
I understand it might not be best practice however is it possible to run replica sets of the same machine. Also, the machine has two hard drives. I want the primary DB to be on the first HD and the replica set on the second hard drive.
Is this setup possible?

Answer 1

Here's how I did it on Ubuntu. I wanted to use systemd for the second instance.
Copy our existing MongoDB conf:
sudo cp /etc/mongod.conf /etc/mongod_repl.conf
sudo pico /etc/mongod_repl.conf
Change these lines:
storage:
dbPath: /var/lib/mongodb_repl
systemLog:
path: /var/log/mongodb/mongod_repl.log
net:
port: 27018
replication:
replSetName: rs0
Create the dirs and files, and assign permissions
sudo mkdir /var/lib/mongodb_repl
sudo chown mongodb:mongodb /var/lib/mongodb_repl
sudo touch /var/log/mongodb/mongod_repl.log
sudo chown mongodb:mongodb /var/log/mongodb/mongod_repl.log
Copy our existing startup file:
sudo cp /lib/systemd/system/mongod.service /etc/systemd/system/mongod_repl.service
If you can't find mongod.service, sudo systemctl status mongod. It looks like:
Loaded: loaded (/lib/systemd/system/mongod.service; disabled; vendor preset: enabled)
sudo pico /etc/systemd/system/mongod_repl.service
Change these lines:
ExecStart=/usr/bin/mongod --config /etc/mongod_repl.conf
PIDFile=/var/run/mongodb/mongod_repl.pid
Let systemctl know we've made some changes:
sudo systemctl daemon-reload
Try start it our second server:
sudo systemctl start mongod_repl
sudo systemctl status mongod_repl
If it's not running, have a look at the logs:
sudo tail -n100 /var/log/mongodb/mongod_repl.log
sudo journalctl -n100 -u mongod_repl
(You can add a -f to either of those lines to tail the log)
Log in
mongo --port 27018
Great! Now you have a second MongoDB server up and running.
Configure your original server to use the same replication set:
sudo pico /etc/mongod.conf
Add this:
replication:
replSetName: rs0
Restart MongoDB
sudo systemctl restart mongod
Now you should be able to go through the process of rs.initiate() on your primary, and then add 127.0.0.1:27018 as your secondary.

Answer 2

Yes it is possible to get the replica set up and running on a single machine and this is how I do it for testing:
Install mongodb-org on your machine by following the official instructions and make sure it is starting fine.
Then stop the instance process: sudo systemctl stop mongod
You can now start a new instance with --replSet option by following link. Also, in order to have it running in the background you can use --fork and --logpath to create a child process:
mongod --fork --port 27017 --dbpath /var/lib/mongodb --logpath /var/log/mongodb/mongo_0.log --replSet rs0 --bind_ip localhost,<Your machine's IP address>
Then connect to the instance and initiate the replica set in Mongo shell:
mongo --port 27017
>rs.initiate()
In case you want to stop the process do ps aux | grep mongod and then kill -9 <pid>

Answer 3

Yes, it is possible.
Create a script: sudo nano mongod.sh
Copy and paste:
sudo mongod --port 27017 --dbpath /var/lib/mongodb --replSet rs0 --bind_ip_all &
sudo mongod --port 27018 --dbpath /var/lib/mongodb2 --replSet rs0 --bind_ip_all &
sudo mongod --port 27019 --dbpath /var/lib/mongodb3 --replSet rs0 --bind_ip_all &
Permit the script to execute:
sudo chmod +x mongod.sh
Run the script:
./mongod.sh
Initialize replica set in primary mongo server:
$ mongo
> rs.initiate()
output
{
"info2" : "no configuration specified. Using a default configuration for the set",
"me" : "<<YOUR_IP>>:27017",
"ok" : 1
}
rs0:SECONDARY> rs.add('<<YOUR_IP>>:27018')
{ "ok" : 1 }
rs0:PRIMARY> rs.add('<<YOUR_IP>>:27019')
{ "ok" : 1 }
Now, test with your connection string.
Connection String
mongodb://<YOUR_IP>:27017,<YOUR_IP>:27018,<YOUR_IP>:27019/<DB>?replicaSet=rs0&readPreference=secondary
Note: If you're testing in a local system, then remove --bind_ip_all option for security reasons.
Options
--port: Specify port
--dbpath: Path where the mongod instance stores its data
--replSet: Replica set name
--bind_ip_all: Allow remote connections