The Complete Guide to Firebase Cloud Messaging (FCM) Interview Questions

Getting ready for a technical interview involving Firebase Cloud Messaging (FCM)? This comprehensive guide has everything you need to thoroughly prepare and gain the confidence to ace those FCM interview questions

As a blogger who frequently writes programming tutorials, I often get asked about the types of FCM interview questions that come up and how to approach answering them. Over the years, I’ve compiled a list of the most common and critical FCM questions that interviewers like to ask. I’m sharing these today along with detailed sample responses to help you knock your next FCM interview out of the park!

What Exactly is FCM?

Let’s start with the basics. Firebase Cloud Messaging (FCM) is a cross-platform cloud solution by Google that enables sending notifications and messages to client apps. The key advantage of FCM is that it handles all the complexity of queuing messages and delivering them reliably on your behalf.

With FCM, you can notify a client app that new email or other data is available to sync. You can send notifications to drive user re-engagement and retention You can also target specific devices based on various criteria.

The best part? FCM works across platforms – you can use it for both Android and iOS apps The FCM SDKs make client implementation seamless,

Why Do Interviewers Ask FCM Interview Questions?

FCM is a very popular messaging platform used by mobile and web developers worldwide. It powers notifications and messaging for many top apps. Hence, interviewers like to ask FCM questions for a couple of reasons:

• To check your real-world hands-on experience with FCM during development and debugging.
• To assess your understanding of core FCM concepts like upstream messaging, token handling, message types etc.
• To gauge your knowledge of specific FCM APIs and parameters.
• To evaluate your approach to architecting and scaling an FCM implementation.

The more thoroughly you can answer FCM questions, the better impression you make on interviewers regarding your development skills.

25 Common FCM Interview Questions and Answers

Here are some of the most frequently asked Firebase Cloud Messaging interview questions that you should prepare for:

Q1. What is the difference between FCM and GCM?

GCM or Google Cloud Messaging was the legacy cloud messaging service by Google. FCM is the new and improved version of GCM. The key differences are:

• Simpler APIs – FCM provides a streamlined HTTP v1 API and supports the latest Android SDKs. GCM had more complex HTTP and XMPP interfaces.

• Better Analytics – FCM offers more powerful analytics and insights compared to GCM.

• Topics Messaging – FCM allows topic-based messaging which was unavailable in GCM.

• Integration – FCM has better integration with tools like Firebase Console and other Firebase services.

Q2. How does FCM handle reliability and message delivery?

FCM maintains multiple servers worldwide to route messages via the fastest available path. This architecture ensures high reliability and resiliency against server failures.

FCM attempts to deliver messages immediately. However, sometimes delays may occur due to network connectivity issues.

For reliability, FCM implements a store-and-forward mechanism. Any messages not delivered immediately are stored or “queued” in FCM while it waits for the device to come online. This ensures no messages are lost.

Q3. How do you handle FCM token generation and token refresh?

The FCM registration token is the unique identifier that allows sending messages to a specific app instance on a device.

When a user first launches the app, we need to call the FCM SDK method to get an FCM token. This token should be persisted on the app server against that user ID.

If the FCM token changes due to uninstall/reinstall or data wipe, our app needs to call the token refresh method and update the new token value on the server.

Handling token refresh is critical to ensure existing users continue receiving messages after getting a new token.

Q4. What are the key use cases for FCM Upstream Messaging?

Upstream messaging allows apps to send data to the server using FCM’s communication channel. The main use cases are:

• User-generated content – Apps can instantly upload photos, videos etc. to server

• Analytics – Apps can send live usage and engagement analytics to server

• Multiplayer gaming – Games can transmit player actions in real-time during gameplay

• Chat/messaging – Instant messaging apps can send new messages using upstream

Q5. How do you handle FCM messages when the app is in foreground/background?

If the app is in the foreground, the FCM SDK calls the onMessageReceived callback as soon as a message arrives. We can handle the message however we want.

If the app is in the background, FCM automatically displays the message in the system notification tray. For data messages, FCM delivers the payload in the intent extras when the user taps the notifications.

Q6. How can you send an FCM notification payload and additional data payload?

FCM provides two types of payloads – notification and data. To send both, we need to use the ‘notification’ and ‘data’ parameters when sending a message from the server.

For example:

{  "notification" : {    "title": "New email",    "body" : "You have 2 unread emails"  },  "data" : {    "from" : "[email protected]",    "email_id" : "12345"  }}

This allows us to include notification details along with additional custom key-value data.

Q7. How does FCM handle delivering messages to iOS devices?

For iOS apps, FCM communicates with APNs or Apple Push Notification service to deliver messages.

When an iOS app registers for remote notifications, APNs issues a unique device token. This token is sent to the app server which must forward it to FCM.

When a message is sent to FCM targeting this device token, FCM routes it to APNs for final delivery to the iOS device.

Q8. How do you implement priority messaging with FCM?

FCM provides two priority levels for messages – normal and high priority.

To send a high priority message, we need to set the “priority” field to “high” when sending the message payload from the server.

For example:

{   "notification" : {     "title": "Weather Alert",     "body" : "Severe thunderstorm warning"    },   "priority" : "high"}

High priority FCM messages trigger immediate delivery and wake sleeping devices, which consumes extra battery but ensures urgent messages are received instantly.

Q9. How does FCM handle offline or unreachable devices?

For devices that are offline or unreachable, FCM stores and retries the messages for up to 4 weeks until the device becomes available again. This ensures eventual message delivery.

If multiple messages are sent while offline, only the last message is delivered upon reconnection while the earlier ones are discarded. This avoids overloading users with stale information.

Q10. What are the key limits of FCM for real-time use cases?

There are a few limits:

• Latency – End-to-end latency can vary from few hundred milliseconds to several seconds based on device state, network etc. Real-time apps need consistent low latency.

• Message Order – FCM does not guarantee in-order message delivery. For real-time data sync, order matters.

• Payload Size – Max payload size is 4KB which may be inadequate for real-time apps sending lots of data.

• Platform Differences – FCM capabilities differ across Android, iOS and Web which creates complexity.

So FCM has some inherent limits for real-time communication use cases requiring millisecond latency, ordering etc.

Q11. How can we integrate FCM with Firebase Auth for authenticated messaging?

We can leverage Firebase Auth to implement authenticated FCM messaging as follows:

1. When a user signs in, generate an ID token using Firebase Auth SDK

2. On app server, verify ID token signature to authenticate the user

3. Once authenticated, register this user’s FCM token against their user ID

4. When sending FCM messages, lookup user’s FCM token via user ID and populate in registration_ids field

This ensures only authenticated users receive messages and each user only gets messages intended for them.

Q12. How can we send multicast messages with FCM?

FCM provides a multicast message API that allows sending