$changeStream (aggregation)

Definition定义

$changeStream

Returns a Change Stream cursor on a collection, a database, or an entire cluster. Must be used as the first stage in an aggregation pipeline.返回集合、数据库或整个集群上的更改流游标。必须用作聚合管道中的第一个阶段。

The $changeStream stage has the following syntax:$changeStream阶段具有以下语法：

{
  $changeStream: {
    allChangesForCluster: <boolean>,
    fullDocument: <string>,
    fullDocumentBeforeChange: <string>,
    resumeAfter: <int>
    showExpandedEvents: <boolean>,
    startAfter: <document>
    startAtOperationTime: <timestamp>
  }
}

Parameter参数	Description描述
allChangesForCluster	Optional: 可选的。Sets whether the change stream should include all changes in the cluster. May only be opened on the admin database.设置更改流是否应包括集群中的所有更改。只能在admin数据库上打开。
fullDocument	Optional: 可选的。Specifies whether change notifications include a copy of the full document when modified by update operations. 指定在通过update操作进行修改时，更改通知是否包括完整文档的副本。 default: Change notifications do not include the full document for update operations.：更改通知不包括用于update操作的完整文档。 required: Change notifications includes a copy of the modified document as it appeared immediately after the change. ：更改通知包括更改后立即出现的修改文档的副本。If the document cannot be found, the change stream throws an error.如果找不到文档，更改流将抛出一个错误。 To use this option, you must first use the collMod command to enable the changeStreamPreAndPostImages option. 要使用此选项，必须首先使用collMod命令启用changeStreamPreAndPostImages选项。 New in version 6.0. 6.0版新增。 updateLookup: Change notifications includes a copy of the document modified by the change. ：更改通知包括更改所修改的文档的副本。This document is the current majority-committed document or null if it no longer exists.此文档是当前多数提交的文档，如果已不存在，则为null。 whenAvailable: Change notification includes a copy of the modified document as it appeared immediately after the change or null if the document is unavailable.：更改通知包括更改后立即出现的修改文档的副本，如果文档不可用，则为null。 To use this option, you must first use the collMod command to enable the changeStreamPreAndPostImages option. 要使用此选项，必须首先使用collMod命令启用changeStreamPreAndPostImages选项。 New in version 6.0. 6.0版新增。 In the case of partial updates, the change notification also provides a description of the change. 在部分更新的情况下，更改通知还提供对更改的描述。
fullDocumentBeforeChange	Include the full document from before the change. 包括更改前的完整文档。This field accepts the following values: 此字段接受以下值： off: Disables inclusion of the document from before the change.：禁止包含更改前的文档。 whenAvailable: Includes document from before the change. ：包括更改前的文档。The query does not fail if the unmodified document is not available.如果未修改的文档不可用，则查询不会失败。 required: Includes document from before the change. The query fails if the unmodified document is not available.：包括更改前的文档。如果未修改的文档不可用，则查询失败。
resumeAfter	Specifies a resume token as the logical starting point for the change stream. 指定恢复令牌作为更改流的逻辑起点。Cannot be used with startAfter or startAtOperationTime fields.不能与startAfter或startAtOperationTime字段一起使用。
showExpandedEvents	Specifies whether to include additional change events, such as such as DDL and index operations. 指定是否包括其他更改事件，例如DDL和索引操作。 New in version 6.0. 6.0版新增。
startAfter	Specifies a resume token as the logical starting point for the change stream. 指定恢复令牌作为更改流的逻辑起点。Cannot be used with resumeAfter or startAtOperationTime fields.不能与resumeAfter或startAtOperationTime字段一起使用。
startAtOperationTime	Specifies a time as the logical starting point for the change stream. 指定一个时间作为更改流的逻辑起点。Cannot be used with resumeAfter or startAfter fields.不能与resumeAfter或startAfter字段一起使用。

Examples实例

To create a change stream cursor using the aggregation stage, run the aggregate command.要使用聚合阶段创建更改流游标，请运行aggregate命令。

var cur = db.names.aggregate( [
   { $changeStream: {} }
] )

To open the cursor, run cur.要打开游标，请运行cur。

When the change stream detects a change, the next() method returns a change event notification. 当变更流检测到变更时，next()方法返回一个变更事件通知。For example, after running cur.next(), MongoDB returns a document similar to the following:例如，在运行cur.next()后，MongoDB返回一个类似于以下内容的文档：

{
   "_id": {
      _data: "8262E2EE54000000022B022C0100296E5A100448E5E3DD01364019AE8FE8C6859527E046645F6964006462E2EE54C8756C0D5CF6F0720004"
   },
   "operationType": "insert",
   "clusterTime": Timestamp({ t: 1659039316, i: 2 }),
   "wallTime": ISODate("2022-07-28T20:15:16.148Z"),
   "fullDocument": {
      "_id": ObjectId("62e2ee54c8756c0d5cf6f072"),
      "name": "Walker Percy"
   },
   "ns": {
      "db": "test",
      "coll": "names"
   },
   "documentKey": { _id: ObjectId("62e2ee54c8756c0d5cf6f072") }
}

For more information on change stream notifications, see Change Events.有关更改流通知的更多信息，请参阅更改事件。