Real-time updates with listeners in GCP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ service_behavior

intermediate

How does a Firestore listener behave on document update?

You set up a Firestore listener on a document to get real-time updates. What happens when the document is updated?

A The listener triggers only if the document is deleted, not on updates.

B The listener triggers only once when first attached and never again.

C The listener triggers only if the document is created, not on updates.

D The listener triggers once immediately and then again every time the document changes.

💡 Hint

Think about how real-time updates work in Firestore listeners.

Firestore listeners trigger immediately with the current document data and then again every time the document changes, providing real-time updates.

❓ Architecture

intermediate

Best architecture for real-time updates with Cloud Pub/Sub

You want to build a system that sends real-time updates from a backend to multiple clients. Which architecture best supports this using Cloud Pub/Sub?

A Backend publishes messages to a Pub/Sub topic; clients subscribe directly to the topic to receive updates.

B Clients publish messages to a Pub/Sub topic; backend subscribes to process updates.

C Backend writes updates to Cloud Storage; clients poll the storage for changes.

D Backend sends HTTP requests directly to clients without Pub/Sub.

💡 Hint

Consider how Pub/Sub supports message distribution to multiple subscribers.

Cloud Pub/Sub allows the backend to publish messages to a topic, and multiple clients can subscribe to that topic to receive real-time updates efficiently.

❓ security

advanced

Securing Firestore listeners for real-time updates

You want to ensure only authorized users receive real-time updates from Firestore listeners. Which method best enforces this?

A Encrypt data on the client side before sending it to Firestore.

B Use Firestore security rules to restrict read access based on user authentication and document fields.

C Disable Firestore listeners and use polling instead.

D Use a VPN to restrict network access to Firestore.

💡 Hint

Think about how Firestore controls access to data.

Firestore security rules allow fine-grained control over who can read documents, ensuring listeners only receive updates they are authorized to see.

❓ Configuration

advanced

Configuring Cloud Functions to trigger on Firestore updates

Which Cloud Function trigger configuration correctly listens to document updates in Firestore?

A functions.firestore.document('collection/{docId}').onDelete((snap, context) => { ... })

B functions.firestore.document('collection/{docId}').onCreate((snap, context) => { ... })

C functions.firestore.document('collection/{docId}').onUpdate((change, context) => { ... })

D functions.pubsub.topic('topicName').onPublish((message) => { ... })

💡 Hint

Focus on the trigger that responds to document updates.

The onUpdate trigger listens specifically for changes to existing Firestore documents.

✅ Best Practice

expert

Optimizing Firestore listeners for cost and performance

You have many clients listening to Firestore documents for real-time updates. What is the best practice to reduce costs and improve performance?

A Use collection group queries with listeners to reduce the number of listeners needed.

B Use polling every few seconds instead of listeners to reduce Firestore reads.

C Set up a listener on each individual document separately for precise updates.

D Disable offline persistence on clients to reduce local storage usage.

💡 Hint

Think about how to minimize the number of active listeners.

Collection group queries allow a single listener to monitor multiple documents across collections, reducing the number of listeners and Firestore reads, which saves cost and improves performance.