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.