TTL索引¶
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TTL索引是一种特殊索引，通过这种索引MongoDB会过一段时间后自动移除集合中的文档。这对于某些类型的信息来说是一个很理想的特性，例如机器生成的事件数据，日志，会话信息等，这些数据都只需要在数据库中保存有限时间。
To create a TTL index, use the db.collection.createIndex() method with the expireAfterSeconds option on a field whose value is either a date or an array that contains date values.
For example, to create a TTL index on the lastModifiedDate field of the eventlog collection, use the following operation in the mongo shell:
db.eventlog.createIndex( { "lastModifiedDate": 1 }, { expireAfterSeconds: 3600 } )
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.
如果这个键存储的是一个数组，且在索引中有多个日期类型的数据(和一篇文档关联)，那么当其中 最低 (比如，最早)过期阈值得到匹配时，这篇文档就会过期失效了。
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.
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¶
When you build a TTL index in the background, the TTL thread can begin deleting documents while the index is building. If you build a TTL index in the foreground, MongoDB begins removing expired documents as soon as the index finishes building.
TTL索引不能保证过期数据会被立刻删除。在文档过期和MongoDB从数据库中删除文档之间，可能会有延迟。
删除过期数据的后台任务 每隔60秒 运行一次。所以，在文档过期 之后 和 后台任务运行或者结束 之前 ，文档会依然存在于集合中。
删除操作的持续实际取决于您的 mongod 实例的负载。因此，在两次后台任务运行的间隔间，过期数据可能会继续留在数据库中 超过 60秒。
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.
Record Allocation on MMAPv1¶
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.
Restrictions¶
- TTL indexes are a single-field indexes. Compound indexes do not support TTL and ignores the expireAfterSeconds option.
- The _id field 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 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.
在其他方面，TTL索引是普通索引，并且如果可以的话，MongoDB会使用这些索引来匹配任意查询。