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TTL Indexes

On this page

  • Create a TTL Index
  • Convert a non-TTL single-field Index into a TTL Index
  • Change the expireAfterSeconds value for 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.

To create a TTL index, use the createIndex() method on a field whose value is either a date or an array that contains date values, and specify the expireAfterSeconds option with the desired TTL value in seconds.

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 } )

Starting in MongoDB 5.1, you can add the expireAfterSeconds option to an existing single-field index. To change a non-TTL single-field index to a TTL index, use the collMod database command:

db.runCommand({
"collMod": <collName>,
"index": {
"keyPattern": <keyPattern>,
"expireAfterSeconds": <number>
}
})

The following example converts a non-TTL single-field index with the pattern { "lastModifiedDate": 1 } into a TTL index:

db.runCommand({
"collMod": "tickets",
"index": {
"keyPattern": { "lastModifiedDate": 1 },
"expireAfterSeconds": 100
}
})

To change the expireAfterSeconds value for a TTL Index, use the collMod database command:

db.runCommand({
"collMod": <collName>,
"index": {
"keyPattern": <keyPattern>,
"expireAfterSeconds": <number>
}
})

The following example changes the expireAfterSeconds value for an index with the pattern { "lastModifiedDate": 1 } on the tickets collection:

db.runCommand({
"collMod": "tickets",
"index": {
"keyPattern": { "lastModifiedDate": 1 },
"expireAfterSeconds": 100
}
})

TTL indexes expire documents after the specified number of seconds has passed since the indexed field value; i.e. 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 (i.e. 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.

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

When the TTL thread is active, you will see delete operations in the output of db.currentOp() or in the data collected by the database profiler.

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 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.

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.

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.

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

←  Index PropertiesExpire Data from Collections by Setting TTL →
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