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.
For example, to create a TTL index on the
lastModifiedDate field of the
eventlog collection, use the following operation in the
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 a date value(s), the document will not expire.
If a document does not contain the indexed field, the document will not expire.
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.
With the MMAPv1 storage engine, a collection with a TTL index has
usePowerOf2Sizes automatically enabled. You cannot modify this setting for the collection. As a result of enabling
usePowerOf2Sizes, MongoDB must allocate more disk space relative to data size. This approach helps mitigate the possibility of storage fragmentation caused by frequent delete operations and leads to more predictable storage use patterns.
- TTL indexes are a single-field indexes. Compound indexes do not support TTL and ignore 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 use
createIndex()to change the value of
expireAfterSecondsof an existing index. Instead use the
collModdatabase command in conjunction with the
indexcollection 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