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db.collection.watch()

On this page

  • Definition
  • Availability
  • Deployment
  • Storage Engine
  • Read Concern majority Support
  • Behavior
  • Resumability
  • Full Document Lookup of Update Operations
  • Access Control
  • Cursor Iteration
  • Examples
  • Open a Change Stream
  • Change Stream with Full Document Update Lookup
  • Change Streams with Document Pre- and Post-Images
  • Change Stream with Aggregation Pipeline Filter
  • Resuming a Change Stream
db.collection.watch( pipeline, options )

Important

mongosh Method

This page documents a mongosh method. This is not the documentation for database commands or language-specific drivers, such as Node.js.

For the database command, see the aggregate command with the $changeStream aggregation stage.

For MongoDB API drivers, refer to the language-specific MongoDB driver documentation.

For the legacy mongo shell documentation, refer to the documentation for the corresponding MongoDB Server release:

For replica sets and sharded clusters only

Opens a change stream cursor on the collection.

Parameter
Type
Description
pipeline
array

Optional. An 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 watch()

The options document can contain the following fields and values:

Field
Type
Description
resumeAfter
document

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

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

Starting in MongoDB 6.0, you can set fullDocument to:

  • "whenAvailable" to output the document post-image, if available, after the document was inserted, replaced, or updated.

  • "required" to output the document post-image after the document was inserted, replaced, or updated. Raises an error if the post-image is not available.

fullDocumentBeforeChange
string

Optional.

Starting in MongoDB 6.0, you can use the new fullDocumentBeforeChange field and set it to:

  • "whenAvailable" to output the document pre-image, if available, before the document was replaced, updated, or deleted.

  • "required" to output the document pre-image before the document was replaced, updated, or deleted. Raises an error if the pre-image is not available.

  • "off" to suppress the document pre-image. "off" is the default.

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.

Starting in MongoDB 4.2, defaults to simple binary comparison if omitted. In earlier versions, change streams opened on a single collection would inherit the collection's default collation.

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 that remains open as long as a connection to the MongoDB deployment remains open and the collection exists. See Change Events for examples of change event documents.

Tip

db.collection.watch() is available for replica set and sharded cluster deployments :

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

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).

  • db.collection.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.

Unlike the MongoDB Drivers, mongosh 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.collection.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.collection.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.

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.

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.

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

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

The built-in read role provides the appropriate privileges.

MongoDB provides multiple ways to iterate on a cursor.

The cursor.hasNext() method blocks and waits for the next event. To monitor the watchCursor cursor and iterate over the events, use hasNext() like this:

while (!watchCursor.isClosed()) {
if (watchCursor.hasNext()) {
firstChange = watchCursor.next();
break;
}
}

The cursor.tryNext() method is non-blocking. To monitor the watchCursor cursor and iterate over the events, use tryNext() like this:

while (!watchCursor.isClosed()) {
let next = watchCursor.tryNext()
while (next !== null) {
printjson(next);
next = watchCursor.tryNext()
}
}

The following operation opens a change stream cursor against the data.sensors collection:

watchCursor = db.getSiblingDB("data").sensors.watch()

Iterate the cursor to check for new events. Use the cursor.isClosed() method with the cursor.tryNext() 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.isClosed()) {
let next = watchCursor.tryNext()
while (next !== null) {
printjson(next);
next = watchCursor.tryNext()
}
}

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

Set the fullDocument option to "updateLookup" to direct the change stream cursor to lookup the most current majority-committed version of the document associated to an update change stream event.

The following operation opens a change stream cursor against the data.sensors collection using the fullDocument : "updateLookup" option.

watchCursor = db.getSiblingDB("data").sensors.watch(
[],
{ fullDocument : "updateLookup" }
)

Iterate the cursor to check for new events. Use the cursor.isClosed() method with the cursor.tryNext() 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.isClosed()) {
let next = watchCursor.tryNext()
while (next !== null) {
printjson(next);
next = watchCursor.tryNext()
}
}

For any update operation, the change event returns the result of the document lookup in the fullDocument field.

For an example of the full document update output, see change stream update event.

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

Starting in MongoDB 6.0, you can use change stream events to output the version of a document before and after changes (the document pre- and post-images):

  • The pre-image is the document before it was replaced, updated, or deleted. There is no pre-image for an inserted document.

  • The post-image is the document after it was inserted, replaced, or updated. There is no post-image for a deleted document.

  • Enable changeStreamPreAndPostImages for a collection using db.createCollection(), create, or collMod

Pre- and post-images are not available for a change stream event if the images were:

  • Not enabled on the collection at the time of a document update or delete operation.

  • Removed after the pre- and post-image retention time set in expireAfterSeconds.

    • The following example sets expireAfterSeconds to 100 seconds:

      use admin
      db.runCommand( {
      setClusterParameter:
      { changeStreamOptions: { preAndPostImages: { expireAfterSeconds: 100 } } }
      } )
    • The following example returns the current changeStreamOptions settings, including 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.

    • 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.

Additional considerations:

  • Enabling pre- and post-images consumes storage space and adds processing time. Only enable pre- and post-images if you need them.

  • Limit the change stream event size to less than 16 megabytes. To limit the event size, you can:

    • 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.

    • Request only post-images in the change stream output for documents up to 16 megabytes if other change stream event fields like updateDescription are not large.

    • Request only pre-images in the change stream output for documents up to 16 megabytes if:

      • document updates affect only a small fraction of the document structure or content, and

      • do not cause a replace change event. A replace event always includes the post-image.

  • 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.

  • Pre-images are written to the config.system.preimages collection.

    • 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.

    • Pre-images are removed asynchronously by a background process.

Important

Backward-Incompatible Feature

Starting in MongoDB 6.0, if you are using document pre- and post-images for change streams, you must disable changeStreamPreAndPostImages for each collection using the collMod command before you can downgrade to an earlier MongoDB version.

Tip

See also:

Create a temperatureSensor collection that has changeStreamPreAndPostImages enabled:

db.createCollection(
"temperatureSensor",
{ changeStreamPreAndPostImages: { enabled: true } }
)

Populate the temperatureSensor collection with temperature readings:

db.temperatureSensor.insertMany( [
{ "_id" : 0, "reading" : 26.1 },
{ "_id" : 1, "reading" : 25.9 },
{ "_id" : 2, "reading" : 24.3 },
{ "_id" : 3, "reading" : 22.4 },
{ "_id" : 4, "reading" : 24.6 }
] )

The following sections show change stream examples for document pre- and post-images that use the temperatureSensor collection.

You use the fullDocumentBeforeChange: "whenAvailable" setting to output the document pre-image, if available. The pre-image is the document before it was replaced, updated, or deleted. There is no pre-image for an inserted document.

The following example creates a change stream cursor for the temperatureSensor collection using fullDocumentBeforeChange: "whenAvailable":

watchCursorFullDocumentBeforeChange = db.temperatureSensor.watch(
[],
{ fullDocumentBeforeChange: "whenAvailable" }
)

The following example uses the cursor to check for new change stream events:

while ( !watchCursorFullDocumentBeforeChange.isClosed() ) {
if ( watchCursorFullDocumentBeforeChange.hasNext() ) {
printjson( watchCursorFullDocumentBeforeChange.next() );
}
}

In the example:

  • The while loop runs until the cursor is closed.

  • hasNext() returns true if the cursor has documents.

The following example updates the reading field for a temperatureSensor document:

db.temperatureSensor.updateOne(
{ _id: 2 },
{ $set: { reading: 22.1 } }
)

After the temperatureSensor document is updated, the change event outputs the document pre-image in the fullDocumentBeforeChange field. The pre-image contains the temperatureSensor document reading field before it was updated. For example:

{
"_id" : {
"_data" : "82624B21...",
"_typeBits" : BinData(0,"QA==")
},
"operationType" : "update",
"clusterTime" : Timestamp(1649090957, 1),
"ns" : {
"db" : "test",
"coll" : "temperatureSensor"
},
"documentKey" : {
"_id" : 2
},
"updateDescription" : {
"updatedFields" : {
"reading" : 22.1
},
"removedFields" : [ ],
"truncatedArrays" : [ ]
},
"fullDocumentBeforeChange" : {
"_id" : 2,
"reading" : 24.3
}
}

Tip

See also:

You use the fullDocument: "whenAvailable" setting to output the document post-image, if available. The post-image is the document after it was inserted, replaced, or updated. There is no post-image for a deleted document.

The following example creates a change stream cursor for the temperatureSensor collection using fullDocument: "whenAvailable":

watchCursorFullDocument = db.temperatureSensor.watch(
[],
{ fullDocument: "whenAvailable" }
)

The following example uses the cursor to check for new change stream events:

while ( !watchCursorFullDocument.isClosed() ) {
if ( watchCursorFullDocument.hasNext() ) {
printjson( watchCursorFullDocument.next() );
}
}

In the example:

  • The while loop runs until the cursor is closed.

  • hasNext() returns true if the cursor has documents.

The following example updates the reading field for a temperatureSensor document:

db.temperatureSensor.updateOne(
{ _id: 1 },
{ $set: { reading: 29.5 } }
)

After the temperatureSensor document is updated, the change event outputs the document post-image in the fullDocument field. The post-image contains the temperatureSensor document reading field after it was updated. For example:

{
"_id" : {
"_data" : "8262474D...",
"_typeBits" : BinData(0,"QA==")
},
"operationType" : "update",
"clusterTime" : Timestamp(1648840090, 1),
"fullDocument" : {
"_id" : 1,
"reading" : 29.5
},
"ns" : {
"db" : "test",
"coll" : "temperatureSensor"
},
"documentKey" : {
"_id" : 1
},
"updateDescription" : {
"updatedFields" : {
"reading" : 29.5
},
"removedFields" : [ ],
"truncatedArrays" : [ ]
}
}

Tip

See also:

Note

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

The following operation opens a change stream cursor against the data.sensors collection using an aggregation pipeline to filter only insert events:

watchCursor = db.getSiblingDB("data").sensors.watch(
[
{ $match : {"operationType" : "insert" } }
]
)

Iterate the cursor to check for new events. Use the cursor.isClosed() method with the cursor.hasNext() 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.isClosed()){
if (watchCursor.hasNext()){
printjson(watchCursor.next());
}
}

The change stream cursor only returns change events where the operationType is insert. For complete documentation on change stream output, see Change Events.

Every document returned by a change stream cursor includes a resume token as the _id field. To resume a change stream, pass the entire _id document of the change event you want to resume from to either the resumeAfter or startAfter option of watch()

The following operation resumes a change stream cursor against the data.sensors collection using a resume token. This assumes that the operation that generated the resume token has not rolled off the cluster's oplog.

let watchCursor = db.getSiblingDB("data").sensors.watch();
let firstChange;
while (!watchCursor.isClosed()) {
if (watchCursor.hasNext()) {
firstChange = watchCursor.next();
break;
}
}
watchCursor.close();
let resumeToken = firstChange._id;
resumedWatchCursor = db.getSiblingDB("data").sensors.watch(
[],
{ resumeAfter : resumeToken }
)

Iterate the cursor to check for new events. Use the cursor.isClosed() method with the cursor.hasNext() 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 (!resumedWatchCursor.isClosed()){
if (resumedWatchCursor.hasNext()){
print(resumedWatchCursor.next());
}
}

See Resume a Change Stream for complete documentation on resuming a change stream.

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