db.collection.watch()
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Definition
db.collection.watch( pipeline, options )
Important
mongo Shell Method
This page documents a
mongo
method. This is not the documentation for database commands or language-specific drivers, such as Node.js. To use the database command, see theaggregate
command with the$changeStream
aggregation stage.For MongoDB API drivers, refer to the language-specific MongoDB driver documentation.
For replica sets and sharded clusters only
Opens a change stream cursor on the collection.
ParameterTypeDescriptionpipeline
arrayAggregation pipeline consisting of one or more of the following aggregation stages:
$replaceWith
(Available starting in MongoDB 4.2)$set
(Available starting in MongoDB 4.2)$unset
(Available starting in MongoDB 4.2)
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
documentOptional. Additional options that modify the behavior of
watch()
.You must pass an empty array
[]
to thepipeline
parameter if you are not specifying a pipeline but are passing theoptions
document.The
options
document can contain the following fields and values:FieldTypeDescriptionresumeAfter
documentOptional. 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 withstartAfter
andstartAtOperationTime
.startAfter
documentOptional. 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 withresumeAfter
andstartAtOperationTime
.New in version 4.2.
fullDocument
stringOptional. 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 directwatch()
to look up the most current majority-committed version of the updated document.watch()
returns afullDocument
field with the document lookup in addition to theupdateDescription
delta.batchSize
intOptional. 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
intOptional. 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
documentOptional. 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
TimestampOptional. 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 withresumeAfter
andstartAfter
.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.
Availability
Deployment
db.collection.watch()
is available for replica set and
sharded cluster deployments :
For a replica set, you can issue
db.collection.watch()
on any data-bearing member.For a sharded cluster, you must issue
db.collection.watch()
on amongos
instance.
Storage Engine
You can only use db.collection.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
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 closes or times out. See Cursor Behaviors for more information.
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.
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.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.
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 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.
Examples
Open a Change Stream
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.
Note
You cannot use cursor.isExhausted()
with change
streams.
Change Stream with Full Document Update Lookup
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.
Change Stream with Aggregation Pipeline Filter
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.
Resuming a Change Stream
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.