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TTL Indexes¶
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TTL indexes are special indexes that MongoDB can use to automatically remove documents from a collection after a certain amount of time. This is ideal for some types of information like machine generated event data, logs, and session information that only need to persist in a database for a limited amount of time.
Considerations¶
TTL indexes have the following limitations:
- Compound indexes are not supported.
- The indexed field must be a date type.
- If the field holds an array, and there are multiple date-typed data in the index, the document will expire when the lowest (i.e. earliest) matches the expiration threshold.
The TTL index does not guarantee that expired data will be deleted immediately. 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 after they expire but before the background task runs or completes.
The duration of the removal operation depends on the workload of
your mongod instance. Therefore, expired data may exist
for some time beyond the 60 second period between runs of the
background task.
In all other respects, TTL indexes are normal indexes, and if appropriate, MongoDB can use these indexes to fulfill arbitrary queries.