Docs Home → Develop Applications → MongoDB Manual
TTL Indexes
On this page
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.
Create a TTL Index
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 } )
Convert a non-TTL single-field Index into a TTL Index
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 } })
Change the expireAfterSeconds value for a TTL Index
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 } })
Behavior
Expiration of Data
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.
Delete Operations
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.
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 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.
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 single-field indexes. Compound indexes do not support TTL and ignore the
expireAfterSecondsoption.The
_idfield does not support TTL indexes.You cannot create a TTL index on a capped collection because MongoDB cannot remove documents from 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 ofexpireAfterSecondsof an existing index. Instead use thecollModdatabase command. See Change theexpireAfterSecondsvalue for a TTL Index.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, use the
collModdatabase command.