Capped collections for fixed-size data in MongoDB

Introduction

Capped collections keep data in a fixed size. They automatically remove old data when full, like a rolling log.

Storing logs that only need recent entries.

Keeping a fixed number of recent messages in a chat app.

Tracking sensor data where only the latest readings matter.

Caching temporary data that should not grow endlessly.

Syntax

MongoDB

db.createCollection(name, { capped: true, size: maxSizeInBytes, max: maxDocuments })

The 'size' option sets the maximum storage size in bytes.

The optional 'max' limits the number of documents stored.

Examples

Creates a capped collection named 'logs' with a max size of 100,000 bytes.

MongoDB

db.createCollection('logs', { capped: true, size: 100000 })

Creates a capped collection 'messages' with max 50,000 bytes and max 1000 documents.

MongoDB

db.createCollection('messages', { capped: true, size: 50000, max: 1000 })

Sample Program

This creates a capped collection 'recentEvents' with max 5 documents. After inserting 6, the oldest is removed automatically.

MongoDB

db.createCollection('recentEvents', { capped: true, size: 10240, max: 5 })

// Insert 6 documents
for (let i = 1; i <= 6; i++) {
  db.recentEvents.insertOne({ eventNumber: i, description: `Event ${i}` })
}

// Retrieve all documents
printjson(db.recentEvents.find().toArray())

Important Notes

Capped collections maintain insertion order and do not allow document deletion or updates that increase size.

They are very fast for inserts and reads because of their fixed size and structure.

Once created, you cannot convert a normal collection to capped; you must create a new capped collection.

Summary

Capped collections store data with a fixed size limit.

Old data is removed automatically when space runs out.

Useful for logs, caches, and recent data storage.