Expire Data from Collections by Setting TTL通过设置TTL使集合中的数据过期

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This document provides an introduction to MongoDB’s “time to live” or TTL collection feature. 本文档介绍MongoDB的“生存时间”或TTL集合功能。TTL collections make it possible to store data in MongoDB and have the mongod automatically remove data after a specified number of seconds or at a specific clock time.TTL集合可以将数据存储在MongoDB中，并让mongod在指定的秒数或特定的时钟时间后自动删除数据。

Data expiration is useful for some classes of information, including machine generated event data, logs, and session information that only need to persist for a limited period of time.数据过期对于某些类别的信息很有用，包括机器生成的事件数据、日志和会话信息，这些信息只需要持续一段有限的时间。

A special TTL index property supports the implementation of TTL collections. 一个特殊的TTL索引属性支持TTL集合的实现。The TTL feature relies on a background thread in mongod that reads the date-typed values in the index and removes expired documents from the collection.

Procedures过程

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.

Note

The TTL index is a single field index. Compound indexes do not support the TTL property. TTL索引是单字段索引。复合索引不支持TTL属性。For more information on TTL indexes, see TTL Indexes.有关TTL索引的更多信息，请参阅TTL索引。

You can modify the expireAfterSeconds of an existing TTL index using the collMod command.

Expire Documents after a Specified Number of Seconds在指定的秒数后使文档过期

To expire data after a specified number of seconds has passed since the indexed field, create a TTL index on a field that holds values of BSON date type or an array of BSON date-typed objects and specify a positive non-zero value in the expireAfterSeconds field. 要在索引字段之后经过指定的秒数后使数据过期，请在保存BSON date类型值或BSON date类型对象数组的字段上创建TTL索引，并在expireAfterSeconds字段中指定非零值。A document will expire when the number of seconds in the expireAfterSeconds field has passed since the time specified in its indexed field. 当expireAfterSeconds字段中的秒数超过其索引字段中指定的时间后，文档将过期。[1]

For example, the following operation creates an index on the log_events collection’s createdAt field and specifies the expireAfterSeconds value of 3600 to set the expiration time to be one hour after the time specified by createdAt.

db.log_events.createIndex( { "createdAt": 1 }, { expireAfterSeconds: 3600 } )

When adding documents to the log_events collection, set the createdAt field to the current time:

db.log_events.insert( {
   "createdAt": new Date(),
   "logEvent": 2,
   "logMessage": "Success!"
} )

MongoDB will automatically delete documents from the log_events collection when the document’s createdAt value [1] is older than the number of seconds specified in expireAfterSeconds.

[1](1, 2) If the field contains an array of BSON date-typed objects, data expires if at least one of BSON date-typed object is older than the number of seconds specified in expireAfterSeconds.如果该字段包含BSON date类型的对象数组，则如果至少有一个BSON date类型的对象早于expireAfterSeconds中指定的秒数，则数据将过期。

Expire Documents at a Specific Clock Time在特定时间使文档过期

To expire documents at a specific clock time, begin by creating a TTL index on a field that holds values of BSON date type or an array of BSON date-typed objects and specify an expireAfterSeconds value of 0. For each document in the collection, set the indexed date field to a value corresponding to the time the document should expire. 对于集合中的每个文档，将索引日期字段设置为与文档到期时间对应的值。If the indexed date field contains a date in the past, MongoDB considers the document expired.如果索引日期字段包含过去的日期，MongoDB将认为该文档已过期。

For example, the following operation creates an index on the log_events collection’s expireAt field and specifies the expireAfterSeconds value of 0:

db.log_events.createIndex( { "expireAt": 1 }, { expireAfterSeconds: 0 } )

For each document, set the value of expireAt to correspond to the time the document should expire. For instance, the following insert() operation adds a document that should expire at July 22, 2013 14:00:00.

db.log_events.insert( {
   "expireAt": new Date('July 22, 2013 14:00:00'),
   "logEvent": 2,
   "logMessage": "Success!"
} )

MongoDB will automatically delete documents from the log_events collection when the documents’ expireAt value is older than the number of seconds specified in expireAfterSeconds, i.e. 0 seconds older in this case. 当文档的expireAt值早于expireAfterSeconds中指定的秒数时，MongoDB将自动从log_events集合中删除文档，在本例中为0秒。As such, the data expires at the specified expireAt value.因此，数据将在指定的expireAt值处过期。