Oplog and replication mechanism in MongoDB

Introduction

The oplog helps keep copies of data in sync across multiple MongoDB servers. It records changes so all copies stay updated.

You want to keep a backup server updated automatically.

You need to share data across different locations for faster access.

You want to avoid losing data if one server fails.

You want to scale reads by having multiple copies of data.

You want to track changes made to your database over time.

Syntax

MongoDB

No direct query syntax; oplog is a special collection named 'oplog.rs' in the local database used internally by MongoDB replication.

The oplog is a capped collection that stores a rolling log of all operations that modify the data.

Replication uses the oplog to copy changes from the primary server to secondary servers.

Examples

This command shows the last 5 operations recorded in the oplog.

MongoDB

use local
 db.getCollection('oplog.rs').find().limit(5).sort({$natural:-1})

Shows the status of the replica set, including which server is primary and which are secondaries.

MongoDB

rs.status()

Starts a new replica set to enable replication.

MongoDB

rs.initiate()

Sample Program

This sequence starts a replica set, checks its status, and shows the last 3 operations in the oplog.

MongoDB

rs.initiate()
rs.status()
use local
db.getCollection('oplog.rs').find().limit(3).sort({$natural:-1})

Important Notes

The oplog is only accessible on replica set members, not standalone servers.

Oplog entries include the operation type: insert (i), update (u), delete (d), etc.

Replication ensures data is copied from the primary to secondaries by replaying oplog entries.

Summary

The oplog is a special log that records all data changes in MongoDB replica sets.

Replication uses the oplog to keep multiple servers' data in sync automatically.

This helps with data safety, scaling, and availability.