TTL indexes for auto-expiry in MongoDB

Introduction

TTL indexes help automatically remove old data from your database after a set time. This keeps your data fresh and saves space without manual cleanup.

You want to delete user sessions after they expire.

You need to remove temporary logs older than a day.

You want to clear cached data automatically after some time.

You want to keep only recent sensor readings and discard old ones.

You want to auto-delete expired coupons or offers.

Syntax

MongoDB

db.collection.createIndex({ <field>: 1 }, { expireAfterSeconds: <seconds> })

The field must be a date type.

expireAfterSeconds sets how many seconds after the date the document expires.

Examples

This removes documents 1 hour after the createdAt date.

MongoDB

db.sessions.createIndex({ "createdAt": 1 }, { expireAfterSeconds: 3600 })

This deletes log entries 24 hours after their timestamp.

MongoDB

db.logs.createIndex({ "timestamp": 1 }, { expireAfterSeconds: 86400 })

Sample Program

This example creates a TTL index that deletes documents 10 seconds after their createdAt time. It inserts one document and shows it immediately.

MongoDB

use mydb

// Create a TTL index on 'createdAt' field to expire documents after 10 seconds
db.tempData.createIndex({ "createdAt": 1 }, { expireAfterSeconds: 10 })

// Insert a document with current date
db.tempData.insertOne({ "name": "test", "createdAt": new Date() })

// Query documents immediately
printjson(db.tempData.find().toArray())

Important Notes

TTL cleanup runs in the background every 60 seconds, so documents may not disappear exactly at the expiration time.

TTL indexes only work on fields with BSON date type.

You cannot update the TTL value without dropping and recreating the index.

Summary

TTL indexes automatically delete documents after a set time.

They help keep data storage clean without manual work.

Use a date field and expireAfterSeconds option to set expiry.

Practice

1. What is the main purpose of a TTL index in MongoDB?

easy

A. Automatically delete documents after a specified time

B. Encrypt documents for security

C. Create backups of collections

D. Improve query speed by sorting data

Solution

1. Step 1: Understand TTL index function

   A TTL index is designed to remove documents automatically after a certain time period.

2. Step 2: Compare options with TTL purpose

   Only Automatically delete documents after a specified time describes automatic deletion after time, which matches TTL index behavior.

3. Final Answer:

   Automatically delete documents after a specified time -> Option A

4. Quick Check:

   TTL index = auto-delete documents [OK]

Hint: TTL means auto-delete documents after time [OK]

Common Mistakes:

• Thinking TTL encrypts data
• Confusing TTL with backup or sorting
• Assuming TTL speeds up queries

2. Which of the following is the correct way to create a TTL index on a field named createdAt that expires documents after 3600 seconds?

easy

A. db.collection.createIndex({ createdAt: 1 }, { expireAfterSeconds: 3600 })

B. db.collection.createIndex({ createdAt: 1 }, { ttl: 3600 })

C. db.collection.createTTLIndex({ createdAt: 1 }, 3600)

D. db.collection.createIndex({ createdAt: 1 }, { expireAfter: 3600 })

Solution

1. Step 1: Recall correct TTL index syntax

   The correct syntax uses createIndex with the option expireAfterSeconds set to the expiry time in seconds.

2. Step 2: Match options with correct syntax

   Only db.collection.createIndex({ createdAt: 1 }, { expireAfterSeconds: 3600 }) uses createIndex and expireAfterSeconds correctly.

3. Final Answer:

   db.collection.createIndex({ createdAt: 1 }, { expireAfterSeconds: 3600 }) -> Option A

4. Quick Check:

   expireAfterSeconds option = correct TTL syntax [OK]

Hint: Use expireAfterSeconds option in createIndex [OK]

Common Mistakes:

• Using wrong option names like ttl or expireAfter
• Using non-existent createTTLIndex method
• Forgetting to specify expireAfterSeconds

3. Given a collection with documents having a timestamp field and a TTL index created as db.logs.createIndex({ timestamp: 1 }, { expireAfterSeconds: 120 }), what happens to a document inserted with { timestamp: new Date() } after 3 minutes?

medium

A. The document remains indefinitely

B. The document is automatically deleted

C. The document is archived but not deleted

D. The document causes an error on insertion

Solution

1. Step 1: Understand TTL expiry timing

   The TTL index deletes documents after expireAfterSeconds from the timestamp field value.

2. Step 2: Calculate expiry for 3 minutes

   3 minutes = 180 seconds, which is more than 120 seconds, so the document will be deleted.

3. Final Answer:

   The document is automatically deleted -> Option B

4. Quick Check:

   TTL expiry after 120s means deletion after 3 minutes [OK]

Hint: TTL deletes after expireAfterSeconds from date field [OK]

Common Mistakes:

• Thinking document stays forever
• Assuming TTL archives instead of deletes
• Believing TTL causes insertion errors

4. You created a TTL index with expireAfterSeconds: 0 on a field expireAt, but documents are not being deleted. What is the likely cause?

medium

A. TTL indexes only work on _id field

B. expireAfterSeconds cannot be zero

C. You must restart MongoDB for TTL to work

D. The expireAt field is not a date type

Solution

1. Step 1: Check TTL index requirements

   TTL indexes require the indexed field to be a date type to calculate expiry.

2. Step 2: Analyze why documents not deleted

   If expireAt is not a date, TTL cannot determine expiry time, so documents stay.

3. Final Answer:

   The expireAt field is not a date type -> Option D

4. Quick Check:

   TTL needs date field type to delete documents [OK]

Hint: Ensure TTL field is a date type [OK]

Common Mistakes:

• Thinking expireAfterSeconds cannot be zero
• Believing TTL only works on _id
• Assuming MongoDB restart is needed

5. You want documents in a collection to expire exactly at a specific date and time stored in the expireAt field. Which TTL index configuration achieves this?

hard

A. Create TTL index on createdAt with expireAfterSeconds set to document age

B. Create TTL index on expireAt with expireAfterSeconds: 3600

C. Create TTL index on expireAt with expireAfterSeconds: 0

D. Use a background job to delete documents at expireAt

Solution

1. Step 1: Understand TTL with exact expiry date

   Setting expireAfterSeconds to 0 means documents expire exactly at the date/time in the field.

2. Step 2: Compare options for exact expiry

   Only Create TTL index on expireAt with expireAfterSeconds: 0 uses expireAfterSeconds: 0 on expireAt field to expire exactly at that time.

3. Final Answer:

   Create TTL index on expireAt with expireAfterSeconds: 0 -> Option C

4. Quick Check:

   expireAfterSeconds: 0 = expire exactly at date field [OK]

Hint: Use expireAfterSeconds: 0 for exact expiry date [OK]

Common Mistakes:

• Using non-zero expireAfterSeconds for exact expiry
• Confusing createdAt with expireAt field
• Relying on manual background jobs unnecessarily