Deploy a mongodb replica set

Introduction

Replication in MongoDB provides its application with:

High Availability
Disaster Recovery
Data Durability

In most production instances it is crucial that we have all these to ensure as minimal downtime as possible. This artcile will step you through:

How to create 3 Linux servers in Azure
How to configure those servers to be able to communicate
How to install and setup MongoDB
How to configure a 3 node replica set
How to query a replica set to get key information and perform administration tasks

Setup replica set

Setup Servers in azure

In Azure navigate to Virtual Machines and select Create
Create any SKU size you want using a ubuntu image and use ssh for auth
On the networking tab we need to create a new Virtual Network. This Virtual Network we will use for all our nodes so that they can communicate with each other
- Select Create new, give it a name and then accept the defaults for the rest

Select Review + create and create your server
Repeat this process for the other 2 nodes

Make sure you remember to select the same virtual network for all.

Setup inbound network rules

Were in a test environment here, so to save time we are going to setup an any any rule on the network for ease

Navigate to your server and select the Networking Settings tab
Select Create Port Rule

Select Inbound rule and then ensure you set the information like below to allow communication from and to any port

Repeat this for every one of our nodes

Setup DNS names

We should have dns names for ease and to better describe the servers.

Navigate to your server
On the overview page find DNS and select the Not Configured button

Choose a DNS name and press save
If you chose mongo as your DNS name for example your fully qualified domain name would now be:
- mongo.northeurope.cloudapp.azure.com
Repeat these steps for every one of our nodes, ensure you note down the DNS names as you will need them later

Install MongoDB on servers

Install mongo binaries and dependencies

  
sudo apt-get install gnupg curl
curl -fsSL https://www.mongodb.org/static/pgp/server-7.0.asc | \
   sudo gpg -o /usr/share/keyrings/mongodb-server-7.0.gpg \
   --dearmor
echo "deb [ arch=amd64,arm64 signed-by=/usr/share/keyrings/mongodb-server-7.0.gpg ] https://repo.mongodb.org/apt/ubuntu focal/mongodb-org/7.0 multiverse" | sudo tee /etc/apt/sources.list.d/mongodb-org-7.0.list   
sudo apt-get update
sudo apt-get install -y mongodb-org

Enable mongod to start on startup and start the service

  
sudo systemctl start mongod
sudo systemctl enable mongod
sudo systemctl status mongod

Configure Security

Using keyfile as an authentication adds an extra layer of security. It must be specified on each replica set member. If it is then the node can join the replica set, if it is not using the same keyfile then it will be blocked from joining

Create keyfile for use with the nodes

openssl rand -base64 756 > /tmp/keyfile

Copy keyfile contents to other nodes

sudo nano /tmp/keyfile

Configure the keyfile to be able to be used with mongo

  
sudo mkdir -p /etc/mongodb/pki
sudo chmod 0400 /tmp/keyfile
sudo mv /tmp/keyfile /etc/mongodb/pki
sudo chown -R mongodb. /etc/mongodb/pki

Open the configuration file

sudo nano /etc/mongod.conf

Next we need to amend some options in the configuration file to enable us to merge our 3 nodes together.

Find the replication section. It will most likely be commented out.
Change it to look like the below. This will set the replication set name called mongodb-replica-set in our case
1 2 3 replication: replSetName: mongodb-replica-set
Next find the network interfaces section and change the bindIp to 0.0.0.0 this will allow any ip to bind to the replica set

In a production setting you wouldnt bind to all ips, but this is just for testing purposes.

Last thing to change is the security section to add our keyfile authentication. Find the security section and replace with the below

  
security:
  keyFile: /etc/mongodb/pki/keyfile
  authorization: enabled

Your final configuration file should look something like the below
Copy and paste this to every conf file on the 3 nodes.

  
storage:
  dbPath: /var/lib/mongodb

systemLog:
  destination: file
  logAppend: true
  path: /var/log/mongodb/mongod.log

net:
  port: 27017
  bindIp: 0.0.0.0

processManagement:
  timeZoneInfo: /usr/share/zoneinfo

security:
  keyFile: /etc/mongodb/pki/keyfile
  authorization: enabled

replication:
  replSetName:
    mongodb-replica-set 

After you have set all your configurations run the below to restart the service and then check the status of it
1 2 sudo systemctl restart mongod sudo systemctl status mongod

Setup your replica set

We should have a fully working mongo setup now. We just need to initiate the replica set to make it totally up and running.

Open up mongo by running the below on node 1
1 mongosh
Initiate the replica set by running the below
- The replica set name should be as you specified in you configuration file
- The hosts should be the dns names you created in Azure

  
rs.initiate(
  {
     _id: "mongodb-replica-set",
     version: 1,
     members: [
        { _id: 0, host : "<fully qualified node 1>" },
        { _id: 1, host : "<fully qualified node 2>" },
        { _id: 2, host : "<fully qualified node 3>" }
     ]
  }
)

Create yourself as a user on the replica set

  
use admin
db.createUser({
   user: "username",
   pwd: "password",
   roles: [
     {role: "root", db: "admin"}
   ]
 })

Run the below to exit the shell
1 exit
Login to with your newly created replica set, again using your fully qualified host names

  
mongosh --host <fully qualified node 1>:27017,<fully qualified node 2>:27017,<fully qualified node 3>:27017 --username username --password 'password' --authenticationDatabase admin

Querying Replica Sets

Get replica set information

db.hello()

Returns a document that describes the role of the mongod instance. https://www.mongodb.com/docs/manual/reference/method/db.hello/

db.hello()

rs.conf()

Returns a document that contains the current replica set configuration. https://www.mongodb.com/docs/manual/reference/method/rs.conf/

rs.conf()

Change priority on a replica set

The below shows you how to change priority on a replica set so that node 0 has the highest priority and therefore should always be chosen if accessible during an election.

  
config = rs.conf()
config.members[0].priority = 10
config.members[1].priority = 1
config.members[2].priority = 1
config
rs.reconfig(config)