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TTL Indexes

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  • Create a TTL Index
  • Behavior
  • Restrictions

Note

If you are removing documents to save on storage costs, consider Online Archive in MongoDB Atlas. Online Archive automatically archives infrequently accessed data to fully-managed S3 buckets for cost-effective data tiering.

TTL indexes are special single-field indexes that MongoDB can use to automatically remove documents from a collection after a certain amount of time or at a specific clock time. Data expiration is useful for certain types of information like machine generated event data, logs, and session information that only need to persist in a database for a finite amount of time.

You can create and manage TTL indexes in the UI for deployments hosted in MongoDB Atlas.

Create a TTL Index

Warning

After you create a TTL index, it might have a very large number of qualifying documents to delete at once. This large workload might cause performance issues on the server. To avoid these issues, plan to create the index during off hours, or delete qualifying documents in batches before you create the index for future documents.

To create a TTL index, use createIndex(). Specify an index field that is either a date type or an array that contains date type values. Use the expireAfterSeconds option to specify a TTL value in seconds.

The TTL index expireAfterSeconds value must be within 0 and 2147483647 inclusive.

For example, to create a TTL index on the lastModifiedDate field of the eventlog collection, with a TTL value of 3600 seconds, use the following operation in mongosh:

db.eventlog.createIndex( { "lastModifiedDate": 1 }, { expireAfterSeconds: 3600 } )

Important

Consider the following before updating the expireAfterSeconds parameter of a TTL index:

  • Changing the expireAfterSeconds parameter does not trigger a complete index rebuild. However, reducing the expireAfterSeconds value can make many documents eligible for immediate deletion, potentially causing performance issues due to the increased delete operations.

  • The recommended approach is to manually delete documents in small batches before updating the TTL index. This helps control the impact on your cluster.

  • Deleting many documents can fragment storage files, additionally impacting performance. You may need to run the compact command on your collection or perform a Initial Sync to reclaim space and optimize storage.

Behavior

Expiration of Data

TTL indexes expire documents after the specified number of seconds has passed since the indexed field value. The expiration threshold is the indexed field value plus the specified number of seconds.

If the field is an array, and there are multiple date values in the index, MongoDB uses lowest (earliest) date value in the array to calculate the expiration threshold.

If the indexed field in a document is not a date or an array that holds one or more date values, the document will not expire.

If a document does not contain the indexed field, the document will not expire.

Delete Operations

A background thread in mongod reads the values in the index and removes expired documents from the collection.

In progress delete operations performed by the TTL thread appear in db.currentOp() output. As the TTL thread deletes documents, the metrics.ttl.deletedDocuments server status metric is incremented.

If a time series collection contains documents with timeField timestamps before 1970-01-01T00:00:00.000Z or after 2038-01-19T03:14:07.000Z, no documents are deleted from the collection by the TTL "time to live" feature.

Timing of the Delete Operation

MongoDB begins removing expired documents as soon as the index finishes building on the primary. For more information on the index build process, see Index Builds on Populated Collections.

The TTL index does not guarantee that expired data will be deleted immediately upon expiration. There may be a delay between the time that a document expires and the time that MongoDB removes the document from the database.

The background task that removes expired documents runs every 60 seconds. As a result, documents may remain in a collection during the period between the expiration of the document and the running of the background task. MongoDB starts deleting documents 0 to 60 seconds after the index completes.

Because the duration of the removal operation depends on the workload of your mongod instance, expired data may exist for some time beyond the 60 second period between runs of the background task.

The delete operations initiated by the TTL task run in the foreground, like other deletes.

Replica Sets

On replica set members, the TTL background thread only deletes documents when a member is in state primary. The TTL background thread is idle when a member is in state secondary. Secondary members replicate deletion operations from the primary.

Support for Queries

A TTL index supports queries in the same way non-TTL indexes do.

Restrictions

  • TTL indexes are a single-field indexes. Compound indexes do not support TTL and ignore the expireAfterSeconds option.

  • The _id field does not support TTL indexes.

  • You cannot create a TTL index on a capped collection.

  • You cannot create a TTL index on a time series collection. Similar functionality is provided through automatic removal on time series collections instead.

  • You cannot use createIndex() to change the value of expireAfterSeconds of an existing index. Instead use the collMod database command in conjunction with the index collection flag. Otherwise, to change the value of the option of an existing index, you must drop the index first and recreate.

  • If a non-TTL single-field index already exists for a field, you cannot create a TTL index on the same field since you cannot create indexes that have the same key specification and differ only by the options. To change a non-TTL single-field index to a TTL index, you must drop the index first and recreate with the expireAfterSeconds option.

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