MongoDB TTL Index - Complete Beginner Tutorial

What is a TTL Index?

A TTL index in MongoDB is a special type of index that automatically deletes documents after a specified time.

It works by indexing a date field in documents and removing those whose date value is older than the expiration threshold.

• TTL indexes are created on a single field containing a date or timestamp.
• MongoDB runs a background thread every 60 seconds to delete expired documents.
• TTL indexes help manage data lifecycle without manual intervention.

Creating a TTL Index

To create a TTL index, use the createIndex command with the expireAfterSeconds option.

The indexed field must be a date type that indicates when the document should expire.

• Specify the field to index and set expireAfterSeconds to the number of seconds after which documents expire.
• Example: expireAfterSeconds: 3600 means documents expire one hour after the date in the indexed field.

Use Cases for TTL Indexes

TTL indexes are useful in scenarios where data should only exist temporarily.

Common use cases include session management, caching, and log data retention.

• Automatically expire user sessions after inactivity.
• Remove cached data to free up space.
• Delete old log entries after a retention period.
• Manage temporary tokens or verification codes.

Limitations and Best Practices

While TTL indexes are powerful, they have some limitations and considerations to keep in mind.

• TTL indexes only work on a single field of type date.
• The expiration check runs approximately every 60 seconds, so deletion is not immediate.
• TTL indexes cannot be unique indexes.
• Avoid using TTL indexes on fields with frequently updated dates as it can cause unexpected deletions.
• Monitor disk space and performance when using TTL indexes on large collections.