TTL indexes for auto-expiry in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a TTL index in MongoDB?

A TTL (Time To Live) index in MongoDB automatically deletes documents from a collection after a specified amount of time has passed since a date field's value.

beginner

How do you create a TTL index on a field named 'createdAt' that expires documents after 3600 seconds?

Use the command: db.collection.createIndex({ createdAt: 1 }, { expireAfterSeconds: 3600 })

beginner

Which data type must the field used for a TTL index be?

The field must be a date type (ISODate) for TTL indexes to work correctly.

intermediate

What happens if you update the date field used in a TTL index?

Updating the date field resets the countdown for document expiry based on the new date value.

intermediate

Can TTL indexes be used to expire documents based on a timestamp in the future?

No, TTL indexes expire documents based on the time elapsed since the date field's value, so the date must be in the past or present.

What does the 'expireAfterSeconds' option specify in a TTL index?

A The number of seconds to keep the index active

B The number of seconds before documents are indexed

C The number of seconds to delay index creation

D The number of seconds after which documents expire

The 'expireAfterSeconds' option sets how many seconds after the date field's value a document should be deleted.

Which MongoDB command creates a TTL index on the 'lastModified' field with a 24-hour expiry?

A db.collection.createTTLIndex({ lastModified: 1 }, 86400)

B db.collection.createIndex({ lastModified: 1 }, { expireAfterSeconds: 86400 })

C db.collection.createIndex({ lastModified: 1 }, { expireAfterMinutes: 1440 })

D db.collection.createIndex({ lastModified: 1 }, { expireAfterSeconds: 1440 })

The correct syntax uses 'expireAfterSeconds' with the number of seconds (86400 for 24 hours).

What type of field is required for a TTL index to work properly?

A Date

B String

C Number

D Boolean

TTL indexes require a date field to calculate expiry times.

If a document's date field used in a TTL index is updated to a newer date, what happens?

A The document is never deleted

B The document expires immediately

C The expiry countdown resets based on the new date

D The TTL index is removed

Updating the date field resets the expiry countdown from the new date.

Can TTL indexes be used to expire documents exactly at a future date and time?

A No, TTL indexes only expire documents based on elapsed time since the date field

B Yes, but only if the date is stored as a string

C Yes, TTL indexes support future expiry dates

D No, TTL indexes expire documents immediately

TTL indexes expire documents based on how much time has passed since the date field value, not at a specific future timestamp.

Explain how TTL indexes work in MongoDB and how you would set one up to auto-delete documents after 1 hour.

Think about how MongoDB uses a date field and a time interval to remove old documents.

What are the limitations and important considerations when using TTL indexes for auto-expiry in MongoDB?

Consider data types, timing behavior, and how MongoDB cleans expired documents.

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