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Watch for Changes

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  • Overview
  • Sample Data
  • Open a Change Stream
  • Example
  • Modify the Change Stream Output
  • Example
  • Modify the Behavior of Watch()
  • Example
  • Additional Information
  • API Documentation

In this guide, you can learn how to monitor document changes with a change stream.

A change stream outputs new change events, providing access to real-time data changes. You can open a change stream on a collection, database, or client object.

To run the examples in this guide, load these documents into the db.courses collection with the following snippet:

coll := client.Database("db").Collection("courses")
docs := []interface{}{
Course{Title: "World Fiction", Enrollment: 35},
Course{Title: "Abstract Algebra", Enrollment: 60},
Course{Title: "Modern Poetry", Enrollment: 12},
Course{Title: "Plate Tectonics", Enrollment: 35},
}
result, err := coll.InsertMany(context.TODO(), docs)

Tip

Non-existent Databases and Collections

If the necessary database and collection don't exist when you perform a write operation, the server implicitly creates them.

Each document contains a description of a university course that includes the course title and maximum enrollment, corresponding to the title and enrollment fields in each document.

Note

Each example output shows truncated _data, clusterTime, and ObjectID values because the driver generates them uniquely.

To open a change stream, use the Watch() method. The Watch() method requires a context parameter and a pipeline parameter. To return all changes, pass in an empty Pipeline object.

The following example opens a change stream on the db.courses collection and outputs all changes:

coll := client.Database("db").Collection("courses")
// open a change stream with an empty pipeline parameter
changeStream, err := coll.Watch(context.TODO(), mongo.Pipeline{})
if err != nil {
panic(err)
}
defer changeStream.Close(context.TODO())
// iterate over the cursor to print the change-stream events
for changeStream.Next(context.TODO()) {
fmt.Println(changeStream.Current)
}

If you modify the db.courses collection in a separate program or shell, this code will print your changes as they occur. Inserting a document with a title value of "Advanced Screenwriting" and an enrollment value of 20 results in the following change-stream event:

map[_id:map[_data:...] clusterTime: {...} documentKey:map[_id:ObjectID("...")]
fullDocument:map[_id:ObjectID("...") enrollment:20 title:Advanced Screenwriting] ns:
map[coll:courses db:db] operationType:insert]

Use the pipeline parameter to modify the change stream output. This parameter allows you to only watch for certain change events. Format the pipeline parameter as an array of documents, with each document representing an aggregation stage.

You can use the following pipeline stages in this parameter:

  • $addFields

  • $match

  • $project

  • $replaceRoot

  • $replaceWith

  • $redact

  • $set

  • $unset

The following example opens a change stream on the db database, but only watches for new delete operations:

db := client.Database("db")
pipeline := bson.D{{"$match", bson.D{{"operationType", "delete"}}}}
changeStream, err := db.Watch(context.TODO(), mongo.Pipeline{pipeline})
if err != nil {
panic(err)
}
defer changeStream.Close(context.TODO())
for changeStream.Next(context.TODO()) {
fmt.Println(changeStream.Current)
}

Deleting the document with the title value of "Advanced Screenwriting" in a separate program or shell results in the following change-stream event:

{"_id": {"_data": "..."},"operationType": "delete","clusterTime":
{"$timestamp":{"t":"...","i":"..."}},"ns": {"db": "db","coll": "courses"},
"documentKey": {"_id": {"$oid":"..."}}}

Note

The Watch() method was called on the db database, so the code outputs new delete operations in any collection within this database.

Use an opts parameter to modify the behavior of the Watch() method.

You can specify the following options in the opts parameter:

  • ResumeAfter

  • StartAfter

  • FullDocument

  • BatchSize

  • MaxAwaitTime

  • Collation

  • StartAtOperationTime

For more information on these fields, visit the MongoDB manual.

The following example calls the Watch() method on the db.courses collection. It specifies the FullDocument options parameter to output a copy of the entire modified document:

coll := client.Database("db").Collection("courses")
opts := options.ChangeStream().SetFullDocument(options.UpdateLookup)
changeStream, err := coll.Watch(context.TODO(), mongo.Pipeline{}, opts)
if err != nil {
panic(err)
}
defer changeStream.Close(context.TODO())
for changeStream.Next(context.TODO()) {
fmt.Println(changeStream.Current)
}

Updating the enrollment value of the document with the title of "World Fiction" from 35 to 30 results in the following change-stream event:

{"_id": {"_data": "..."},"operationType": "update","clusterTime": {"$timestamp":
{"t":"...","i":"..."}},"fullDocument": {"_id":
{"$oid":"..."},"title": "World Fiction","enrollment":
{"$numberInt":"30"}}, "ns": {"db": "db","coll": "courses"},"documentKey": {"_id":
{"$oid":"..."}}, "updateDescription": {"updatedFields": {"enrollment": {"$numberInt":"30"}},
"removedFields": [],"truncatedArrays": []}}

Without specifying the FullDocument option, the same update operation no longer outputs the "fullDocument" value.

For a runnable example of a change stream, see Watch for Changes.

For more information on change streams, see Change Streams.

To learn more about the Watch() method, visit the following API documentation links:

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