Push notification integration in HLD - Scalability & System Analysis

Scalability Analysis - Push notification integration

Growth Table: Push Notification Integration

Users	Push Requests/Second	Message Volume	Infrastructure Changes
100 users	~10 req/s	Low volume, simple queue	Single server, basic push service
10,000 users	~1,000 req/s	Moderate volume, queue grows	Load balancer, multiple push workers, caching
1,000,000 users	~100,000 req/s	High volume, large queues	Distributed push services, sharded queues, CDN for payloads
100,000,000 users	~10,000,000 req/s	Very high volume, massive queues	Multi-region clusters, advanced sharding, edge caching, auto-scaling

First Bottleneck

The first bottleneck is the message queue and push service throughput. As user count grows, the system struggles to enqueue and deliver notifications fast enough. Single servers and simple queues cannot handle high concurrent push requests, causing delays and dropped messages.

Scaling Solutions

• Horizontal scaling: Add more push worker servers behind a load balancer to distribute load.
• Message queue sharding: Split queues by user segments or notification types to reduce contention.
• Caching: Cache notification payloads or user tokens to reduce repeated database lookups.
• Use CDN: For large payloads like images, use CDN to offload delivery from push servers.
• Auto-scaling: Automatically add/remove push workers based on traffic spikes.
• Multi-region deployment: Deploy push services closer to users to reduce latency and network load.

Cost Analysis

At 1 million users sending 1 notification per second, expect ~1 million push requests per second. Each request is small (~1 KB), so bandwidth is about 1 GB/s. Storage for logs and retries can grow to terabytes daily. Infrastructure costs include multiple servers, message queues, and CDN usage. Efficient batching and filtering reduce costs.

Interview Tip

Start by explaining the push flow simply: app server sends notification to queue, workers deliver to devices. Discuss bottlenecks like queue throughput and network limits. Then propose scaling steps: horizontal scaling, sharding, caching, CDN. Always justify why each step solves the bottleneck.

Self Check

Your database handles 1000 QPS. Traffic grows 10x to 10,000 QPS. What do you do first?

Answer: Add read replicas and implement caching to reduce load on the main database before scaling application servers.

Key Result

Push notification systems first hit bottlenecks at message queue throughput and delivery speed; horizontal scaling, sharding, and caching are key to handle millions of users efficiently.

Practice

1. What is the main purpose of integrating push notifications in a mobile app?

easy

A. To keep users updated even when the app is closed

B. To increase app size significantly

C. To slow down the app performance

D. To disable user interaction

Solution

1. Step 1: Understand push notification purpose

   Push notifications are messages sent to users to keep them informed or engaged.

2. Step 2: Identify correct purpose in options

   Only To keep users updated even when the app is closed correctly states that notifications keep users updated even when app is closed.

3. Final Answer:

   To keep users updated even when the app is closed -> Option A

4. Quick Check:

   Push notifications = keep users updated [OK]

Hint: Push notifications inform users outside the app [OK]

Common Mistakes:

• Thinking notifications increase app size
• Believing notifications slow app down
• Assuming notifications disable interaction

2. Which code snippet correctly requests user permission for push notifications in a mobile app?

easy

A. requestNotificationPermission();

B. enablePushNotifications();

C. askUserForNotificationPermission();

D. requestPermissionForNotifications();

Solution

1. Step 1: Identify standard permission request method

   Common method names include 'requestPermissionForNotifications' to ask user consent.

2. Step 2: Compare options for correct syntax

   requestPermissionForNotifications(); uses a clear, standard naming pattern for requesting permission.

3. Final Answer:

   requestPermissionForNotifications(); -> Option D

4. Quick Check:

   Permission request method = requestPermissionForNotifications() [OK]

Hint: Look for method names starting with 'requestPermission' [OK]

Common Mistakes:

• Using method names that don't request permission
• Confusing enabling notifications with requesting permission
• Using non-standard or undefined method names

3. Given this code snippet handling a push notification:

onNotificationReceived(notification) {
  if (notification.type === 'message') {
    showAlert(notification.title);
  }
}

What happens when a notification with type 'message' arrives?

medium

A. An alert with the notification title is shown

B. The notification is ignored

C. The app crashes due to missing handler

D. A log is printed but no alert

Solution

1. Step 1: Analyze the notification type check

   The code checks if notification.type equals 'message'.

2. Step 2: Understand the action on matching type

   If true, it calls showAlert with notification.title, displaying an alert.

3. Final Answer:

   An alert with the notification title is shown -> Option A

4. Quick Check:

   Type 'message' triggers alert display [OK]

Hint: Check condition and action inside notification handler [OK]

Common Mistakes:

• Assuming notification is ignored
• Thinking app crashes without error
• Confusing logging with alert display

4. Identify the error in this push notification handler code:

function handleNotification(notification) {
  if notification.isRead = false {
    displayNotification(notification);
  }
}

medium

A. Missing semicolon after function declaration

B. Using assignment '=' instead of comparison '==' in if condition

C. Incorrect function name 'handleNotification'

D. Notification object missing required fields

Solution

1. Step 1: Check the if condition syntax

   The condition uses '=' which assigns value instead of comparing.

2. Step 2: Identify correct comparison operator

   It should use '==' or '===' to compare values, not '='.

3. Final Answer:

   Using assignment '=' instead of comparison '==' in if condition -> Option B

4. Quick Check:

   Use '==' for comparison, not '=' [OK]

Hint: Remember '=' assigns, '==' compares in conditions [OK]

Common Mistakes:

• Confusing assignment and comparison operators
• Ignoring syntax errors in if statements
• Assuming function name causes error

5. You want to send a push notification only if the user has granted permission and the app is in background. Which logic correctly implements this?

hard

A. if (userPermissionGranted && appState !== 'background') { sendPushNotification(); }

B. if (userPermissionGranted || appState === 'background') { sendPushNotification(); }

C. if (userPermissionGranted && appState === 'background') { sendPushNotification(); }

D. if (!userPermissionGranted && appState === 'background') { sendPushNotification(); }

Solution

1. Step 1: Understand conditions for sending notification

   Notification should send only if permission is granted AND app is in background.

2. Step 2: Evaluate logical operators in options

   if (userPermissionGranted && appState === 'background') { sendPushNotification(); } uses '&&' (AND) correctly to check both conditions.

3. Final Answer:

   if (userPermissionGranted && appState === 'background') { sendPushNotification(); } -> Option C

4. Quick Check:

   Use AND (&&) to require both conditions [OK]

Hint: Use AND (&&) to combine required conditions [OK]

Common Mistakes:

• Using OR (||) instead of AND (&&)
• Negating permission incorrectly
• Checking wrong app state