Migrating MongoDB to a Multi-Node Replica Set on Amazon EC2

Overview

High availability and fault tolerance are crucial for any production-grade database system. MongoDB offers built-in support for replication via Replica Sets, which maintains multiple copies of your data across different instances. In this guide, we walk through migrating MongoDB to a multi-node replica set using Amazon EC2 instances, including the setup of primary, secondary, and arbiter nodes. We will also briefly integrate supporting AWS services like AWS IAM, Amazon S3 for backup, and Amazon CloudWatch for monitoring.

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Introduction

Replica sets in MongoDB provide redundancy and high availability. A replica set consists of a primary node receiving all write operations and one or more secondary nodes replicating the primary data. An optional arbiter node can be added to participate in elections without storing data.

This blog details a step-by-step approach to migrate and deploy MongoDB on three Amazon EC2 instances, configure replication, and automate monitoring using AWS-native tools for enhanced observability and security.

Prerequisites

Before you begin, ensure the following:

You have three Ubuntu 20.04+ EC2 instances (Primary, Secondary, and Arbiter) in the same Amazon VPC with appropriate Security Group rules (allowing internal communication on port 27017).
Amazon EC2 instances are launched with AWS IAM roles for secure access to Amazon S3 (optional backup) or Amazon CloudWatch.
SSH access is enabled via a private key, and you have root/sudo privileges.
Basic familiarity with Linux terminal, AWS Console, and MongoDB is helpful.

Step-by-Step Guide

Install MongoDB on Secondary and Other Nodes

Create a user-data script for cloud-init or manual execution on the secondary and arbiter nodes:

#!/bin/bash -xe
# Update the system

# Install MongoDB 6.0
curl -fsSL https://pgp.mongodb.com/server-6.0.asc | sudo gpg --dearmor -o /usr/share/keyrings/mongodb-server-6.0.gpg
apt-get install -y mongodb-org

# Create necessary directories
mkdir -p /var/lib/mongodb
chown -R mongodb:mongodb /var/lib/mongodb

# Configuration file
cat > /etc/mongod.conf << 'EOC'
storage:
  dbPath: /var/lib/mongodb
  journal:
    enabled: true
systemLog:
  destination: file
  path: /var/log/mongodb/mongod.log
  logAppend: true
net:
  port: 27017
  bindIp: 0.0.0.0
replication:
  replSetName: "rs0"
EOC

# Start MongoDB

#!/bin/bash -xe

# Update the system

# Install MongoDB 6.0

curl -fsSL https://pgp.mongodb.com/server-6.0.asc | sudo gpg --dearmor -o /usr/share/keyrings/mongodb-server-6.0.gpg

apt-get install -y mongodb-org

# Create necessary directories

mkdir -p /var/lib/mongodb

chown -R mongodb:mongodb /var/lib/mongodb

# Configuration file

cat > /etc/mongod.conf << 'EOC'

storage:

dbPath: /var/lib/mongodb

journal:

enabled: true

systemLog:

destination: file

path: /var/log/mongodb/mongod.log

logAppend: true

net:

port: 27017

bindIp: 0.0.0.0

replication:

replSetName: "rs0"

EOC

# Start MongoDB

MongoDB Replica Set Configuration Script

Run the following script on all nodes, adjusting the IP addresses:

#!/bin/bash

# Define node IPs
PRIMARY_IP="<primary_private_ip>"
SECONDARY_IP="<secondary_private_ip>"
ARBITER_IP="<arbiter_private_ip>"

# Determine current node IP

# Overwrite MongoDB config
cat > /etc/mongod.conf << EOC
storage:
  dbPath: /var/lib/mongodb
  journal:
    enabled: true
systemLog:
  destination: file
  path: /var/log/mongodb/mongod.log
  logAppend: true
net:
  port: 27017
  bindIp: 0.0.0.0
replication:
  replSetName: "rs0"
EOC

# Restart MongoDB

# Wait a bit
# Only on PRIMARY node
if [ "$CURRENT_IP" == "$PRIMARY_IP" ]; then
  echo "Initializing replica set..."
  cat > /tmp/init_replica.js << EOC
rs.initiate({
  _id: "rs0",
  members: [
    { _id: 0, host: "${PRIMARY_IP}:27017", priority: 2 },
    { _id: 1, host: "${SECONDARY_IP}:27017", priority: 1 },
    { _id: 2, host: "${ARBITER_IP}:27017", arbiterOnly: true }
  ]
})
EOC

  mongosh < /tmp/init_replica.js

  # Wait and create users
  
  ADMIN_PASS=$(openssl rand -base64 12)
  TEST_PASS=$(openssl rand -base64 12)

#Create User

#!/bin/bash

# Define node IPs

PRIMARY_IP="<primary_private_ip>"

SECONDARY_IP="<secondary_private_ip>"

ARBITER_IP="<arbiter_private_ip>"

# Determine current node IP

# Overwrite MongoDB config

cat > /etc/mongod.conf << EOC

storage:

dbPath: /var/lib/mongodb

journal:

enabled: true

systemLog:

destination: file

path: /var/log/mongodb/mongod.log

logAppend: true

net:

port: 27017

bindIp: 0.0.0.0

replication:

replSetName: "rs0"

EOC

# Restart MongoDB

# Wait a bit

# Only on PRIMARY node

if [ "$CURRENT_IP" == "$PRIMARY_IP" ]; then

echo "Initializing replica set..."

cat > /tmp/init_replica.js << EOC

rs.initiate({

_id: "rs0",

members: [

{ _id: 0, host: "${PRIMARY_IP}:27017", priority: 2 },

{ _id: 1, host: "${SECONDARY_IP}:27017", priority: 1 },

{ _id: 2, host: "${ARBITER_IP}:27017", arbiterOnly: true }

]

})

EOC

mongosh < /tmp/init_replica.js

# Wait and create users

ADMIN_PASS=$(openssl rand -base64 12)

TEST_PASS=$(openssl rand -base64 12)

#Create User

Add Monitoring Script

On all nodes:

cat > /home/ubuntu/check_mongodb_status.sh << 'EOC'
#!/bin/bash
mongosh --eval "rs.status()" | tee /tmp/rs_status.txt
if grep -q "PRIMARY" /tmp/rs_status.txt; then
  echo "Replica set healthy"
else
  echo "WARNING: No PRIMARY found"
fi

if grep -q "unreachable" /tmp/rs_status.txt; then
  echo "Some members unreachable"
fi
EOC

cat > /home/ubuntu/check_mongodb_status.sh << 'EOC'

#!/bin/bash

mongosh --eval "rs.status()" | tee /tmp/rs_status.txt

if grep -q "PRIMARY" /tmp/rs_status.txt; then

echo "Replica set healthy"

else

echo "WARNING: No PRIMARY found"

if grep -q "unreachable" /tmp/rs_status.txt; then

echo "Some members unreachable"

EOC

Advantages

High Availability: The replica set ensures the database remains available even if one node goes down.
Data Redundancy: Automatically replicates data across nodes to prevent data loss.
Automatic Failover: MongoDB automatically elects a new primary if the primary node fails.
Scalability: You can add more secondary nodes or convert to sharded clusters for horizontal scaling.
Read Distribution: Secondary nodes can serve read traffic if configured, improving performance.

Conclusion

Setting up a MongoDB replica set on Amazon EC2 ensures a production-ready architecture with high availability and resilience. This guide introduced essential setup concepts while abstracting sensitive commands to avoid misapplication.

You can further enhance this setup by:

Automating instance provisioning via AWS CloudFormation or Terraform.
Securing credentials using AWS IAM Roles and AWS Secrets Manager.
Enabling daily backups to Amazon S3 via a cron job or AWS Lambda.

Drop a query if you have any questions regarding Amazon EC2 and we will get back to you quickly.

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FAQs

1. What is the role of the arbiter in a replica set?

ANS: – Arbiters participate in elections but do not store data. They help break ties and maintain an odd number of votes for consistent elections.

2. Can I add more secondaries later?

ANS: – Yes, MongoDB supports dynamic reconfiguration of the replica set to add or remove members.

WRITTEN BY Dhruv Rajeshbhai Patel

Dhruv Patel is a Research Intern at CloudThat. He has completed his Master's in Computer Application and Cloud Certification in Azure and AWS. His area of interest lies in Cloud and Mobile Development Solutions. He loves to take ownership of the work that he is doing.