Navigation

Change Streams

Database Triggers and Change Streams

Database triggers, based on change streams, are available via MongoDB Stitch. Please review the Database Triggers documentation for more information.

New in version 3.6.

Change streams allow applications to access real-time data changes without the complexity and risk of tailing the oplog. Applications can use change streams to subscribe to all data changes on a single collection, a database, or an entire deployment, and immediately react to them. Because change streams use the aggregation framework, applications can also filter for specific changes or transform the notifications at will.

Change stream is available for replica sets and sharded clusters that use WiredTiger storage engine and replica set protocol version 1 (pv1). Change streams can also be used on deployments which employ MongoDB’s encryption-at-rest feature.

Watch Collection/Database/Deployment

You can open change streams against:

Target Description
A collection

You can open a change stream cursor for a single collection (except system collections, or any collections in the admin, local, and config databases).

The examples on this page use the MongoDB drivers to open and work with a change stream cursor for a single collection. See also the mongo shell method db.collection.watch().

A database

Starting in MongoDB 4.0, you can open a change stream cursor for a single database (excluding admin, local, and config database) to watch for changes to all its non-system collections.

For the MongoDB driver method, refer to your driver documentation. See also the mongo shell method db.watch().

A deployment

Starting in MongoDB 4.0, you can open a change stream cursor for a deployment (either a replica set or a sharded cluster) to watch for changes to all non-system collections across all databases except for admin, local, and config.

For the MongoDB driver method, refer to your driver documentation. See also the mongo shell method Mongo.watch().

Change Stream Examples

The examples on this page use the MongoDB drivers to illustrate how to open a change stream cursor for a collection and work with the change stream cursor.

Open A Change Stream

For a replica set, you can open change stream for any of the data-bearing members.

For a sharded cluster, you must issue the open change stream operation against the mongos.

The following example opens a change stream for a collection and iterates over the cursor to retrieve the change stream documents. [1] While the connection to the MongoDB deployment remains open, the cursor remains open until one of the following occurs:

  • The cursor is explicitly closed.
  • An invalidate event occurs.
  • 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.

The Python examples below assume that you have connected to a MongoDB replica set and have accessed a database that contains an inventory collection.

cursor = db.inventory.watch()
document = next(cursor)

The Java examples below assume that you have connected to a MongoDB replica set and have accessed a database that contains an inventory collection.

MongoCursor<ChangeStreamDocument<Document>> cursor = inventory.watch().iterator();
ChangeStreamDocument<Document> next = cursor.next();

The Node.js examples below assume that you have connected to a MongoDB replica set and have accessed a database that contains an inventory collection.

const collection = db.collection('inventory');
const changeStream = collection.watch();
const next = await changeStream.next();

The examples below assume that you have connected to a MongoDB replica set and have accessed a database that contains an inventory collection.

$changeStream = $db->inventory->watch();
$changeStream->rewind();

$firstChange = $changeStream->current();

$changeStream->next();

$secondChange = $changeStream->current();

The examples below assume that you have connected to a MongoDB replica set and have accessed a database that contains an inventory collection.

cursor = db.inventory.watch()
document = await cursor.next()

The C examples below assume that you have connected to a MongoDB replica set and have accessed a database that contains an inventory collection.

mongoc_collection_t *collection;
bson_t *pipeline = bson_new ();
bson_t opts = BSON_INITIALIZER;
mongoc_change_stream_t *stream;
const bson_t *change;
bson_iter_t iter;
bson_error_t error;

collection = mongoc_database_get_collection (db, "inventory");
stream = mongoc_collection_watch (collection, pipeline, NULL /* opts */);
mongoc_change_stream_next (stream, &change);
if (mongoc_change_stream_error_document (stream, &error, NULL)) {
   MONGOC_ERROR ("%s\n", error.message);
}

mongoc_change_stream_destroy (stream);

The C# examples below assume that you have connected to a MongoDB replica set and have accessed a database that contains an inventory collection.

var enumerator = inventory.Watch().ToEnumerable().GetEnumerator();
enumerator.MoveNext();
var next = enumerator.Current;
enumerator.Dispose();

The examples below assume that you have connected to a MongoDB replica set and have accessed a database that contains an inventory collection.

cursor = inventory.watch.to_enum
next_change = cursor.next

To retrieve the data change event notifications, iterate the change stream cursor.

Note

The lifecycle of an unclosed cursor is language-dependent.

See Change Events for more information on the change stream response document format.

[1]Starting in MongoDB 4.0, you can specify a startAtOperationTime to open the cursor at a particular point in time. If the specified starting point is in the past, it must be in the time range of the oplog.

Modify Change Stream Output

You can control change stream output by providing an array of one or more of the following pipeline stages when configuring the change stream:

pipeline = [
    {'$match': {'fullDocument.username': 'alice'}},
    {'$addFields': {'newField': 'this is an added field!'}}
]
cursor = db.inventory.watch(pipeline=pipeline)
document = next(cursor)

You can control change stream output by providing an array of one or more of the following pipeline stages when configuring the change stream:

MongoClient mongoClient = new MongoClient( new MongoClientURI("mongodb://host1:port1,host2:port2..."));

// Select the MongoDB database and collection to open the change stream against

MongoDatabase db = mongoClient.getDatabase("myTargetDatabase");

MongoCollection<Document> collection = db.getCollection("myTargetCollection");

// Create $match pipeline stage.
List<Bson> pipeline = singletonList(Aggregates.match(Filters.or(
    Document.parse("{'fullDocument.username': 'alice'}"),
    Filters.in("operationType", asList("delete")))));

// Create the change stream cursor, passing the pipeline to the
// collection.watch() method

MongoCursor<Document> cursor = collection.watch(pipeline).iterator();

The pipeline list includes a single $match stage that filters any operations where the username is alice, or operations where the operationType is delete.

Passing the pipeline to the watch() method directs the change stream to return notifications after passing them through the specified pipeline.

You can control change stream output by providing an array of one or more of the following pipeline stages when configuring the change stream:

const pipeline = [
  { $match: { 'fullDocument.username': 'alice' } },
  { $addFields: { newField: 'this is an added field!' } }
];
const collection = db.collection('inventory');
const changeStream = collection.watch(pipeline);
const next = await changeStream.next();

You can control change stream output by providing an array of one or more of the following pipeline stages when configuring the change stream:

You can control change stream output by providing an array of one or more of the following pipeline stages when configuring the change stream:

You can control change stream output by providing an array of one or more of the following pipeline stages when configuring the change stream:

You can control change stream output by providing an array of one or more of the following pipeline stages when configuring the change stream:

You can control change stream output by providing an array of one or more of the following pipeline stages when configuring the change stream:

See Change Events for more information on the change stream response document format.

Lookup Full Document for Update Operations

By default, change streams only return the delta of fields during the update operation. However, you can configure the change stream to return the most current majority-committed version of the updated document.

To return the most current majority-committed version of the updated document, pass full_document='updateLookup' to the db.collection.watch() method.

In the example below, all update operations notifications include a full_document field that represents the current version of the document affected by the update operation.

cursor = db.inventory.watch(full_document='updateLookup')
document = next(cursor)

To return the most current majority-committed version of the updated document, pass FullDocument.UPDATE_LOOKUP to the db.collection.watch.fullDocument() method.

In the example below, all update operations notifications include a FullDocument field that represents the current version of the document affected by the update operation.

cursor = inventory.watch().fullDocument(FullDocument.UPDATE_LOOKUP).iterator();
next = cursor.next();

To return the most current majority-committed version of the updated document, pass { fullDocument: 'updateLookup' } to the collection.watch() method.

In the example below, all update operations notifications include a fullDocument field that represents the current version of the document affected by the update operation.

const collection = db.collection('inventory');
const changeStream = collection.watch({ fullDocument: 'updateLookup' });
const next = await changeStream.next();

To return the most current majority-committed version of the updated document, pass "fullDocument' => \MongoDB\Operation\ChangeStreamCommand::FULL_DOCUMENT_UPDATE_LOOKUP" to the watch() method.

In the example below, all update operations notifications include a fullDocument field that represents the current version of the document affected by the update operation.

$changeStream = $db->inventory->watch([], ['fullDocument' => \MongoDB\Operation\Watch::FULL_DOCUMENT_UPDATE_LOOKUP]);
$changeStream->rewind();

$firstChange = $changeStream->current();

$changeStream->next();

$nextChange = $changeStream->current();

To return the most current majority-committed version of the updated document, pass full_document='updateLookup' to the db.collection.watch() method.

In the example below, all update operations notifications include a `full_document field that represents the current version of the document affected by the update operation.

cursor = db.inventory.watch(full_document='updateLookup')
document = await cursor.next()

To return the most current majority-committed version of the updated document, pass the "fullDocument" option with the "updateLookup" value to the mongoc_collection_watch method.

In the example below, all update operations notifications include a fullDocument field that represents the current version of the document affected by the update operation.

BSON_APPEND_UTF8 (&opts, "fullDocument", "updateLookup");
stream = mongoc_collection_watch (collection, pipeline, &opts);
mongoc_change_stream_next (stream, &change);
if (mongoc_change_stream_error_document (stream, &error, NULL)) {
   MONGOC_ERROR ("%s\n", error.message);
}

mongoc_change_stream_destroy (stream);

To return the most current majority-committed version of the updated document, pass "FullDocument = ChangeStreamFullDocumentOption.UpdateLookup" to the collection.Watch() method.

In the example below, all update operations notifications include a FullDocument field that represents the current version of the document affected by the update operation.

var options = new ChangeStreamOptions { FullDocument = ChangeStreamFullDocumentOption.UpdateLookup };
var enumerator = inventory.Watch(options).ToEnumerable().GetEnumerator();
enumerator.MoveNext();
var next = enumerator.Current;
enumerator.Dispose();

To return the most current majority-committed version of the updated document, pass full_document: 'updateLookup' to the watch() method.

In the example below, all update operations notifications include a full_document field that represents the current version of the document affected by the update operation.

cursor = inventory.watch([], full_document: 'updateLookup').to_enum
next_change = cursor.next

Note

If there are one or more majority-committed operations that modified the updated document after the update operation but before the lookup, the full document returned may differ significantly from the document at the time of the update operation.

However, the deltas included in the change stream document always correctly describe the watched collection changes that applied to that change stream event.

See Change Events for more information on the change stream response document format.

Resume a Change Stream

Change streams are resumable by specifying a resumeAfter token when opening the cursor. For the resumeAfter token, use the _id value of the change stream event document. Passing the _id value to the change stream attempts to resume notifications starting after the specified operation.

Important

  • The oplog must have enough history to locate the operation associated with the token or the timestamp, if the timestamp is in the past.
  • You cannot resume a change stream after an invalidate event (for example, a collection drop or rename) closes the stream.

In the example below, resume_token contains the change stream notification id. The resume_after modifier takes a parameter that must resolve to a resume token. Passing the resume_token to the resume_after modifier directs the change stream to attempt to resume notifications starting after the operation specified in the resume token.

resume_token = document.get("_id")
cursor = db.inventory.watch(resume_after=resume_token)
document = next(cursor)

In the example below, the resumeToken contains the change stream notification id. The resumeAfter() method takes a parameter that must resolve to a resume token. Passing the resumeToken to the resumeAfter() method directs the change stream to attempt to resume notifications starting after the operation specified in the resume token.

BsonDocument resumeToken = next.getResumeToken();
cursor = inventory.watch().resumeAfter(resumeToken).iterator();
next = cursor.next();

In the example below, resumeToken contains the change stream notification id. The resumeAfter takes a parameter that must resolve to a resume token. Passing the resumeToken to the resumeAfter modifier directs the change stream to attempt to resume notifications starting after the operation specified.

const collection = db.collection('inventory');
const changeStream = collection.watch();
const change1 = await changeStream.next();

const resumeAfter = change1._id;
changeStream.close();

const newChangeStream = collection.watch({ resumeAfter });
const change2 = await newChangeStream.next();

In the example below, $resumeToken contains the change stream notification id. The resumeAfter option takes a value that must resolve to a resume token. Passing the $resumeToken to the resumeAfter option directs the change stream to attempt to resume notifications starting after the operation specified in the resume token.

$resumeToken = ($lastChange !== null) ? $lastChange->_id : null;

if ($resumeToken === null) {
    throw new \Exception('resumeToken was not found');
}

$changeStream = $db->inventory->watch([], ['resumeAfter' => $resumeToken]);
$changeStream->rewind();

$nextChange = $changeStream->current();

In the example below, resume_token contains the change stream notification id. The resume_after modifier takes a parameter that must resolve to a resume token. Passing the resume_token to the resume_after modifier directs the change stream to attempt to resume notifications starting after the operation specified in the resume token.

resume_token = document.get("_id")
cursor = db.inventory.watch(resume_after=resume_token)
document = await cursor.next()

In the example below, the resumeAfter option is appended to the stream options to recreate the stream after it has been destroyed. Passing the _id to the change stream attempts to resume notifications starting after the operation specified.

stream = mongoc_collection_watch (collection, pipeline, NULL);
if (mongoc_change_stream_next (stream, &change)) {
   bson_iter_init_find (&iter, change, "_id");
   BSON_APPEND_VALUE (&opts, "resumeAfter", bson_iter_value (&iter));

   mongoc_change_stream_destroy (stream);
   stream = mongoc_collection_watch (collection, pipeline, &opts);
   mongoc_change_stream_next (stream, &change);
   mongoc_change_stream_destroy (stream);
} else {
   if (mongoc_change_stream_error_document (stream, &error, NULL)) {
      MONGOC_ERROR ("%s\n", error.message);
   }

   mongoc_change_stream_destroy (stream);
}

In the example below, the resumeToken is retrieved from the last change stream document and passed to the Watch() method as an option. Passing the resumeToken to the Watch() method directs the change stream to attempt to resume notifications starting after the operation specified in the resume token.

  var resumeToken = lastChangeStreamDocument.ResumeToken;
  var options = new ChangeStreamOptions { ResumeAfter = resumeToken };
  var enumerator = inventory.Watch(options).ToEnumerable().GetEnumerator();
  enumerator.MoveNext();
  var next = enumerator.Current;
  enumerator.Dispose();

In the example below, resume_token contains the change stream notification id. The resume_after modifier takes a parameter that must resolve to a resume token. Passing the resume_token to the resume_after modifier directs the change stream to attempt to resume notifications starting after the operation specified in the resume token.

resume_token = next_change['_id']
cursor = inventory.watch([], resume_after: resume_token).to_enum
resumed_change = cursor.next

Resume Token and featureCompatibilityVersion

If the featureCompatibilityVersion (fcv) is set to "4.0" or greater, newly opened change streams return a hex-encoded string for the resume token data, i.e. the _id._data value. This change allows for the ability to compare and sort the resume tokens. If the fcv is 3.6, newly opened change streams return a BinData for the resume token data.

Important

The fcv value at the time of the cursor’s opening determine the resume token data type. That is, the modification of the fcv does not affect the resume tokens for change streams already opened before the fcv change.

Regardless of the fcv value, a 4.0 replica set or a sharded cluster can resume a change stream using either the BinData or string resume token.

As such, a 4.0 deployment can use a resume token from a change stream opened on a collection from a 3.6 deployment.

Use Cases

Change streams can benefit architectures with reliant business systems, informing downstream systems once data changes are durable. For example, change streams can save time for developers when implementing Extract, Transform, and Load (ETL) services, cross-platform synchronization, collaboration functionality, and notification services.

Access Control

For deployments enforcing Authentication and authorization:

  • To open a change stream against specific collection, applications must have privileges that grant changeStream and find actions on the corresponding collection.

    { resource: { db: <dbname>, collection: <collection> }, actions: [ "find", "changeStream" ] }
    
  • To open a change stream on a single databases, applications must have privileges that grant changeStream and find actions on all non-system collections in a database.

    { resource: { db: <dbname>, collection: "" }, actions: [ "find", "changeStream" ] }
    
  • To open a change stream on an entire deployment, applications must have privileges that grant changeStream and find actions on all non-system collections for all databases in the deployment.

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

Event Notification

Change streams only notify on data changes that have persisted to a majority of data-bearing members in the replica set. This ensures that notifications are triggered only by majority-committed changes that are durable in failure scenarios.

For example, consider a 3-member replica set with a change stream cursor opened against the primary. If a client issues an insert operation, the change stream only notifies the application of the data change once that insert has persisted to a majority of data-bearing members.

If an operation is associated with a multi-document transactions, the change event document includes the txnNumber and the lsid.