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An TTL index (time to live index, an index with a lifecycle), which allows you to set a timeout for each document. A document is deleted when it reaches the preset aging level. This type of index is useful for caching issues, such as session saving.

When creating an index, you can create an TTL index by specifying the expireAfterSeconds option, and the collection belongs to the TTL collection.

/ / create a TTL index and set the timeout to 24 hours db.ttl.ensureIndex ({date:1}, {expireAfterSeconds:24*60*60})

This creates a TTL index on the "date" field. If a document's "date" exists and its value is of a date type, the document will be deleted when the server time is expireAfterSeconds seconds later than the document's "date" field.

To prevent active sessions from being deleted, you can update the "date" field to the current time when there is activity on the session. As long as the "date" reaches 24 hours from the current time, the document will be deleted.

MongoDB cleans up the TTL index every minute, so you should not rely on seconds to keep the index alive. You can use the collMod command to change the value of expireAfterSeconds:

Db.runCommand ({collMod: "ttl", index: {keyPattern: {date:1}, expireAfterSeconds:3600}})

There can be multiple TTL indexes on a given collection. An TTL index cannot be a composite index, but it can be used to optimize sorting and queries like a normal index.

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