Replica Set Initialization and Management

Advanced20 min

Deploy a MongoDB replica set for high availability with automatic failover, data redundancy, and read scaling across multiple nodes.

Prerequisites

-MongoDB 6+ installed on 3 servers (or 3 local instances on different ports)
-Network connectivity between all nodes

Steps

Start MongoDB instances with replication enabled

Launch three mongod instances with the same replica set name.

$ mongod --replSet rs0 --port 27017 --dbpath /data/rs0-0 --bind_ip localhost --fork --logpath /var/log/mongodb/rs0-0.log

Repeat for ports 27018 and 27019 with different dbpath and logpath values.

Initialize the replica set

Connect to one instance and initialize the replica set with all members.

$ mongosh --port 27017 --eval 'rs.initiate({_id: "rs0", members: [{_id: 0, host: "localhost:27017"}, {_id: 1, host: "localhost:27018"}, {_id: 2, host: "localhost:27019"}]})'

Check replica set status

View the current state of all replica set members and identify the primary.

$ mongosh --port 27017 --eval 'rs.status().members.forEach(m => print(m.name + ": " + m.stateStr + " (health: " + m.health + ")"))'

Verify replication is working

Write data to the primary and read it from a secondary to confirm replication.

$ mongosh --port 27017 --eval 'db.test.insertOne({msg: "replication test", ts: new Date()})' mydb && mongosh --port 27018 --eval 'db.getMongo().setReadPref("secondary"); db.test.find().sort({ts: -1}).limit(1)' mydb

View replication lag

Check how far behind each secondary is from the primary.

$ mongosh --port 27017 --eval 'rs.printSecondaryReplicationInfo()'

Full Script

full-script.sh

#!/bin/bash
# MongoDB Replica Set Status and Health Check
set -euo pipefail

RS_PRIMARY="localhost:27017"

echo "=== Replica Set Configuration ==="
mongosh --port 27017 --quiet --eval "
const config = rs.conf();
print('Replica Set: ' + config._id);
config.members.forEach(m => {
  print('  ' + m.host + ' (priority: ' + m.priority + ', votes: ' + m.votes + ')');
});
"

echo ""
echo "=== Member Status ==="
mongosh --port 27017 --quiet --eval "
const status = rs.status();
status.members.forEach(m => {
  const lag = m.optimeDate ? (status.date - m.optimeDate) / 1000 : 'N/A';
  print(m.name + ': ' + m.stateStr + ' | health: ' + m.health + ' | lag: ' + lag + 's');
});
"

echo ""
echo "=== Replication Lag Detail ==="
mongosh --port 27017 --quiet --eval "rs.printSecondaryReplicationInfo()"

echo ""
echo "=== Oplog Size ==="
mongosh --port 27017 --quiet --eval "
const oplog = db.getSiblingDB('local').oplog.rs.stats();
print('Size: ' + Math.round(oplog.size / 1024 / 1024) + 'MB');
print('Max Size: ' + Math.round(oplog.maxSize / 1024 / 1024) + 'MB');
print('Docs: ' + oplog.count);
"

echo ""
echo "=== Connection String ==="
mongosh --port 27017 --quiet --eval "
const config = rs.conf();
const hosts = config.members.map(m => m.host).join(',');
print('mongodb://' + hosts + '/mydb?replicaSet=' + config._id);
"

FAQ

Discussion

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#!/bin/bash # MongoDB Replica Set Status and Health Check set -euo pipefail RS_PRIMARY="localhost:27017" echo "=== Replica Set Configuration ===" mongosh --port 27017 --quiet --eval " const config = rs.conf(); print('Replica Set: ' + config._id); config.members.forEach(m => { print(' ' + m.host + ' (priority: ' + m.priority + ', votes: ' + m.votes + ')'); }); " echo "" echo "=== Member Status ===" mongosh --port 27017 --quiet --eval " const status = rs.status(); status.members.forEach(m => { const lag = m.optimeDate ? (status.date - m.optimeDate) / 1000 : 'N/A'; print(m.name + ': ' + m.stateStr + ' | health: ' + m.health + ' | lag: ' + lag + 's'); }); " echo "" echo "=== Replication Lag Detail ===" mongosh --port 27017 --quiet --eval "rs.printSecondaryReplicationInfo()" echo "" echo "=== Oplog Size ===" mongosh --port 27017 --quiet --eval " const oplog = db.getSiblingDB('local').oplog.rs.stats(); print('Size: ' + Math.round(oplog.size / 1024 / 1024) + 'MB'); print('Max Size: ' + Math.round(oplog.maxSize / 1024 / 1024) + 'MB'); print('Docs: ' + oplog.count); " echo "" echo "=== Connection String ===" mongosh --port 27017 --quiet --eval " const config = rs.conf(); const hosts = config.members.map(m => m.host).join(','); print('mongodb://' + hosts + '/mydb?replicaSet=' + config._id); "