Watch method for real-time updates in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What does this change stream watch output?

Consider a MongoDB collection named orders. You run this watch method to listen for insertions:

const changeStream = db.orders.watch([{ $match: { operationType: 'insert' } }]);
changeStream.on('change', change => console.log(change));

If a new document { _id: 1, item: 'book', qty: 3 } is inserted, what will the change object contain?

MongoDB

const changeStream = db.orders.watch([{ $match: { operationType: 'insert' } }]);
changeStream.on('change', change => console.log(change));

A An object with <code>operationType</code> as 'delete' and <code>documentKey</code> containing the _id of the deleted document.

B An object with <code>operationType</code> as 'update' and <code>fullDocument</code> containing the inserted document.

C An object with <code>operationType</code> as 'insert' and <code>fullDocument</code> containing the inserted document.

D No output because watch only listens for updates, not inserts.

💡 Hint

Remember that the $match stage filters events by operation type.

The watch method listens for changes matching the filter. Since the filter is for 'insert', the change event will have operationType 'insert' and include the inserted document in fullDocument.

🧠 Conceptual

intermediate

What is the purpose of the resumeToken in MongoDB watch streams?

When using the watch method for real-time updates, MongoDB provides a resumeToken with each change event. What is its main purpose?

A To uniquely identify the change event so the stream can resume from this point if interrupted.

B To encrypt the data sent over the change stream for security.

C To filter change events by document fields.

D To limit the number of change events returned in a batch.

💡 Hint

Think about what happens if the connection drops and you want to continue listening without missing changes.

The resumeToken marks the last seen event so the watch stream can restart from that point, avoiding missed or duplicated events.

📝 Syntax

advanced

Which watch method syntax correctly listens for updates and deletes only?

You want to listen to a MongoDB collection for only update and delete operations using the watch method. Which of the following code snippets is correct?

A db.collection.watch([{ $match: { operationType: { $or: ['update', 'delete'] } } }])

B db.collection.watch([{ $match: { operationType: { $in: ['update', 'delete'] } } }])

C db.collection.watch([{ $match: { operationType: ['update', 'delete'] } }])

D db.collection.watch({ operationType: ['update', 'delete'] })

💡 Hint

Use MongoDB query operators to match multiple values in an array.

The $in operator matches any value in the specified array. The other options use invalid syntax for matching multiple values.

❓ optimization

advanced

How to reduce resource usage when watching a large collection?

You have a large MongoDB collection with frequent changes. Watching all changes causes high resource use. Which approach best reduces resource usage while still capturing relevant changes?

A Use the watch method without a pipeline and rely on polling the collection periodically.

B Watch the entire collection without any filters and discard unwanted events in your application code.

C Increase the batch size of the watch cursor to receive more events at once.

D Use a <code>$match</code> stage in the watch pipeline to filter only needed operation types or fields.

💡 Hint

Filtering early reduces the amount of data sent over the stream.

Applying a $match filter in the watch pipeline limits events sent to the client, reducing network and processing load.

🔧 Debug

expert

Why does this watch stream never receive events?

You run this code to watch a MongoDB collection for inserts:

const changeStream = db.collection.watch([{ $match: { operationType: 'insert' } }]);
changeStream.on('change', change => console.log(change));

But no events are logged even after inserting documents. What is the most likely cause?

A The MongoDB deployment is a standalone server, which does not support change streams.

B The <code>$match</code> stage is incorrectly filtering for 'insert' instead of 'insertOne'.

C The change stream requires a <code>fullDocument</code> option to receive events.

D The collection is a capped collection, which does not support change streams.

💡 Hint

Change streams require a specific type of MongoDB deployment.

Change streams only work on replica sets or sharded clusters. Standalone servers do not support them, so no events are received.