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db.watch()

Definition

db.watch(pipeline, options)

For replica sets and sharded clusters only

New in version 4.0: Requires featureCompatibilityVersion (fCV) set to "4.0" or greater. For more information on fCV, see setFeatureCompatibilityVersion.

Opens a change stream cursor for a database to report on all its non-system collections.

Parameter Type Description
pipeline array

Aggregation pipeline consisting of one or more of the following aggregation stages:

Specify a pipeline to filter/modify the change events output.

Starting in MongoDB 4.2, change streams will throw an exception if the change stream aggregation pipeline modifies an event’s _id field.

options document

Optional. Additional options that modify the behavior of db.watch().

You must pass an empty array [] to the pipeline parameter if you are not specifying a pipeline but are passing the options document.

The options document can contain the following fields and values:

Field Type Description
resumeAfter document

Optional. Directs db.watch() to attempt resuming notifications starting after the operation specified in the resume token.

Each change stream event document includes a resume token as the _id field. Pass the entire _id field of the change event document that represents the operation you want to resume after.

resumeAfter is mutually exclusive with startAfter and startAtOperationTime.

startAfter document

Optional. Directs db.watch() to attempt starting a new change stream after the operation specified in the resume token. Allows notifications to resume after an invalidate event.

Each change stream event document includes a resume token as the _id field. Pass the entire _id field of the change event document that represents the operation you want to resume after.

startAfter is mutually exclusive with resumeAfter and startAtOperationTime.

New in version 4.2.

fullDocument string

Optional. By default, db.watch() returns the delta of those fields modified by an update operation, instead of the entire updated document.

Set fullDocument to "updateLookup" to direct db.watch() to look up the most current majority-committed version of the updated document. db.watch() returns a fullDocument field with the document lookup in addition to the updateDescription delta.

batchSize int

Optional. Specifies the maximum number of change events to return in each batch of the response from the MongoDB cluster.

Has the same functionality as cursor.batchSize().

maxAwaitTimeMS int

Optional. The maximum amount of time in milliseconds the server waits for new data changes to report to the change stream cursor before returning an empty batch.

Defaults to 1000 milliseconds.

collation document

Optional. Pass a collation document to specify a collation for the change stream cursor.

If omitted, defaults to simple binary comparison.

startAtOperationTime Timestamp

Optional. The starting point for the change stream. If the specified starting point is in the past, it must be in the time range of the oplog. To check the time range of the oplog, see rs.printReplicationInfo().

startAtOperationTime is mutually exclusive with resumeAfter and startAfter.

Returns:A cursor over the change event documents. See Change Events for examples of change event documents.

Availability

Deployment

db.watch() is available for replica sets and sharded clusters:

  • For a replica set, you can issue db.watch() on any data-bearing member.
  • For a sharded cluster, you must issue db.watch() on a mongos instance.

Storage Engine

You can only use db.watch() with the Wired Tiger storage engine.

Read Concern majority Support

Starting in MongoDB 4.2, change streams are available regardless of the "majority" read concern support; that is, read concern majority support can be either enabled (default) or disabled to use change streams.

In MongoDB 4.0 and earlier, change streams are available only if "majority" read concern support is enabled (default).

Behavior

  • You cannot run db.watch() on the admin, local, or config database.
  • db.watch() only notifies on data changes that have persisted to a majority of data-bearing members.
  • The change stream cursor remains open until one of the following occurs:
    • The cursor is explicitly closed.
    • An invalidate event occurs; for example, a collection drop or rename.
    • The connection to the MongoDB deployment is closed.
    • If the deployment is a sharded cluster, a shard removal may cause an open change stream cursor to close, and the closed change stream cursor may not be fully resumable.
  • You can run db.watch() for a database that does not exist. However, once the database is created and you drop the database, the change stream cursor closes.

Resumability

Unlike the MongoDB drivers, the mongo shell does not automatically attempt to resume a change stream cursor after an error. The MongoDB drivers make one attempt to automatically resume a change stream cursor after certain errors.

db.watch() uses information stored in the oplog to produce the change event description and generate a resume token associated to that operation. If the operation identified by the resume token passed to the resumeAfter or startAfter option has already dropped off the oplog, db.watch() cannot resume the change stream.

See Resume a Change Stream for more information on resuming a change stream.

Note

  • You cannot use resumeAfter to resume a change stream after an invalidate event (for example, a collection drop or rename) closes the stream. Starting in MongoDB 4.2, you can use startAfter to start a new change stream after an invalidate event.
  • If the deployment is a sharded cluster, a shard removal may cause an open change stream cursor to close, and the closed change stream cursor may not be fully resumable.

Resume Token

The resume token _data type depends on the MongoDB versions and, in some cases, the feature compatibility version (fcv) at the time of the change stream’s opening/resumption (i.e. a change in fcv value does not affect the resume tokens for already opened change streams):

MongoDB Version Feature Compatibility Version Resume Token _data Type
MongoDB 4.2 and later “4.2” or “4.0” Hex-encoded string (v1)
MongoDB 4.0.7 and later “4.0” or “3.6” Hex-encoded string (v1)
MongoDB 4.0.6 and earlier “4.0” Hex-encoded string (v0)
MongoDB 4.0.6 and earlier “3.6” BinData
MongoDB 3.6 “3.6” BinData

With hex-encoded string resume tokens, you can compare and sort the resume tokens.

Regardless of the fcv value, a 4.0 or later deployment can use either BinData resume tokens or hex string resume tokens to resume a change stream. As such, a 4.0 or later deployment can use a resume token from a change stream opened on a collection from a 3.6 deployment.

New resume token formats introduced in a MongoDB version cannot be consumed by earlier MongoDB versions.

Full Document Lookup of Update Operations

By default, the change stream cursor returns specific field changes/deltas for update operations. You can also configure the change stream to look up and return the current majority-committed version of the changed document. Depending on other write operations that may have occurred between the update and the lookup, the returned document may differ significantly from the document at the time of the update.

Depending on the number of changes applied during the update operation and the size of the full document, there is a risk that the size of the change event document for an update operation is greater than the 16MB BSON document limit. If this occurs, the server closes the change stream cursor and returns an error.

Access Control

When running with access control, the user must have the find and changeStream privilege actions on the database resource. That is, a user must have a role that grants the following privilege:

{ resource: { db: <dbname>, collection: "" }, actions: [ "find", "changeStream"] }

The built-in read role provides the appropriate privileges.

Example

The following operation in the mongo shell opens a change stream cursor on the hr database. The returned cursor reports on data changes to all the non-system collections in that database.

watchCursor = db.getSiblingDB("hr").watch()

Iterate the cursor to check for new events. Use the cursor.isExhausted() method to ensure the loop only exits if the change stream cursor is closed and there are no objects remaining in the latest batch:

while (!watchCursor.isExhausted()){
   if (watchCursor.hasNext()){
      printjson(watchCursor.next());
   }
}

For complete documentation on change stream output, see Change Events.