replace Event事件

Document Pre- and Post-Images文档前映像和后映像

Starting in MongoDB 6.0, you see a fullDocumentBeforeChange document with the fields before the document was changed (or deleted) if you perform these steps:从MongoDB 6.0开始，如果执行以下步骤，您将看到一个fullDocumentBeforeChange文档，其中包含更改（或删除）文档之前的字段：

1. Enable the new changeStreamPreAndPostImages field for a collection using db.createCollection(), create, or collMod.使用db.createCollection()、create或collMod为集合启用新的changeStreamPreAndPostImages字段。
2. Set fullDocumentBeforeChange to "required" or "whenAvailable" in db.collection.watch().在db.collection.watch()中将fullDocumentBeforeChange设置为"required"或"whenAvailable"。

Example fullDocumentBeforeChange document in the change stream output:更改流输出中的示例fullDocumentBeforeChange文档：

"fullDocumentBeforeChange" : {
   "_id" : ObjectId("599af247bb69cd89961c986d"),
   "userName" : "alice123",
   "name" : "Alice Smith"
}

For complete examples with the change stream output, see Change Streams with Document Pre- and Post-Images.有关更改流输出的完整示例，请参阅使用文档前后图像更改流。

Pre- and post-images are not available for a change stream event if the images were:如果图像存在以下情况，则前映像和后映像对更改流事件不要用：

• Not enabled on the collection at the time of a document update or delete operation.在文档更新或删除操作时，集合上未启用。

• Removed after the pre- and post-image retention time set in expireAfterSeconds.在expireAfterSeconds中设置的图像保留时间前后删除。

  • The following example sets expireAfterSeconds to 100 seconds on an entire cluster:以下示例将整个集群上的expireAfterSeconds设置为100秒：

    use admin
    db.runCommand( {
       setClusterParameter:
          { changeStreamOptions: {
             preAndPostImages: { expireAfterSeconds: 100 }
          } }
    } )

  • The following example returns the current changeStreamOptions settings, including expireAfterSeconds:以下示例返回当前changeStreamOptions设置，包括expireAfterSeconds：

    db.adminCommand( { getClusterParameter: "changeStreamOptions" } )

  • Setting expireAfterSeconds to off uses the default retention policy: pre- and post-images are retained until the corresponding change stream events are removed from the oplog.将expireAfterSeconds设置为off将使用默认保留策略：保留前后映像，直到从oplog中删除相应的更改流事件。
  • If a change stream event is removed from the oplog, then the corresponding pre- and post-images are also deleted regardless of the expireAfterSeconds pre- and post-image retention time.如果从oplog中删除了更改流事件，则相应的预映像和后映像也将被删除，而不管映像保留时间为expireAfterSeconds之前和之后。

Additional considerations:其他注意事项：

• Enabling pre- and post-images consumes storage space and adds processing time. Only enable pre- and post-images if you need them.启用预映像和后映像会消耗存储空间并增加处理时间。仅在需要时启用前置和后置图像。

• Limit the change stream event size to less than 16 mebibytes. To limit the event size, you can:将更改流事件大小限制为小于16兆字节。要限制事件大小，您可以：

  • Limit the document size to 8 megabytes. You can request pre- and post-images simultaneously in the change stream output if other change stream event fields like updateDescription are not large.将文档大小限制为8 MB。如果其他更改流事件字段（如updateDescription）不大，您可以在更改流输出中同时请求前映像和后映像。
  • Request only post-images in the change stream output for documents up to 16 mebibytes if other change stream event fields like updateDescription are not large.如果其他更改流事件字段（如updateDescription）不大，则仅请求更改流输出中最多16兆字节的文档的后映像。

  • Request only pre-images in the change stream output for documents up to 16 mebibytes if:在以下情况下，仅在更改流输出中请求最多16兆字节的文档的前映像：

    • document updates affect only a small fraction of the document structure or content, and文档更新仅影响文档结构或内容的一小部分，以及
    • do not cause a replace change event. A replace event always includes the post-image.不要引起replace事件。replace事件总是包括后映像。
• To request a pre-image, you set fullDocumentBeforeChange to required or whenAvailable in db.collection.watch(). To request a post-image, you set fullDocument using the same method.要请求预映像，您可以在db.collection.watch()中将fullDocumentBeforeChange设置为required或whenAvailable。要请求后映像，您可以使用相同的方法设置fullDocument。

• Pre-images are written to the config.system.preimages collection.预映像会写入config.system.preimages集合。

  • The config.system.preimages collection may become large. To limit the collection size, you can set expireAfterSeconds time for the pre-images as shown earlier.config.system.preimages集合可能会变大。要限制集合大小，您可以为预映像设置expireAfterSeconds时间，如前所示。
  • Pre-images are removed asynchronously by a background process.前图像由后台进程异步删除。