Change streams on databases in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Output of a simple change stream watching a database

Consider a MongoDB change stream opened on a database watching all collections. If a document is inserted into a collection, what does the change stream event's operationType field show?

MongoDB

const changeStream = db.watch();
// Insert a document into any collection
// What is the value of operationType in the change event?

A "replace"

B "insert"

C "update"

D "delete"

💡 Hint

Think about what operation happens when a new document is added.

When a new document is added to any collection in the watched database, the change stream event's operationType is "insert".

🧠 Conceptual

intermediate

Understanding resume tokens in change streams

What is the purpose of a resume token in MongoDB change streams?

A To limit the number of events returned in a batch

B To filter change events by operation type

C To uniquely identify the last seen event so the stream can resume from there

D To encrypt the change stream data for security

💡 Hint

Think about how you can continue watching changes after a disconnection.

A resume token uniquely identifies the last event seen in a change stream, allowing the stream to resume from that point without missing events.

📝 Syntax

advanced

Correct syntax to watch changes on a database with a filter

Which of the following MongoDB commands correctly opens a change stream on a database filtering only insert operations?

MongoDB

const pipeline = [ { $match: { operationType: "insert" } } ];
const changeStream = db.watch(pipeline);

A const changeStream = db.watch([{ $match: { operationType: "insert" } }]);

B const changeStream = db.watch([{ $filter: { operationType: "insert" } }]);

C const changeStream = db.watch([{ match: { operationType: "insert" } }]);

D const changeStream = db.watch({ $match: { operationType: "insert" } });

💡 Hint

Remember the aggregation pipeline syntax uses $match inside an array.

The watch method accepts an array of aggregation pipeline stages. The $match stage filters events by operationType. Option A uses correct syntax.

🔧 Debug

advanced

Why does this change stream not receive any events?

A developer runs this code to watch changes on a database but never receives any events, even after inserting documents: const changeStream = db.watch([{ $match: { operationType: "insert" } }]); changeStream.on('change', (next) => console.log(next)); What is the most likely reason?

A The change stream is opened on the database, but the insertions happen in a different database

B The $match stage is incorrectly filtering out all events

C The changeStream.on event listener is not set up correctly

D The MongoDB server does not support change streams on databases

💡 Hint

Check if the watched database matches where inserts happen.

Change streams watch a specific database or collection. If inserts happen in another database, the stream won't see them.

❓ optimization

expert

Optimizing change stream pipeline for minimal data transfer

You want to watch changes on a database but only need the document key (_id) and operationType fields in the change events to reduce network usage. Which pipeline optimizes the change stream to return only these fields?

A [ { $project: { documentKey: 0, operationType: 1 } } ]

B [ { $match: { operationType: { $exists: true } } }, { $project: { _id: 0 } } ]

C [ { $project: { fullDocument: 0 } } ]

D [ { $project: { 'documentKey._id': 1, operationType: 1, _id: 0 } } ]

💡 Hint

Use $project to include only needed fields and exclude the default _id.

Option D uses $project to include only documentKey._id and operationType, excluding the default _id field from the change event, minimizing data sent.