Notification system in LLD - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Architecture

intermediate

Design components for a scalable notification system

You need to design a notification system that can send alerts via email, SMS, and push notifications. Which component is essential to handle different notification types efficiently?

A A client-side script that sends notifications directly to users

B A single database table storing all notification logs without separation

C A unified notification dispatcher that routes messages to specific channels

D A monolithic service that sends all notifications synchronously

💡 Hint

Think about how to separate concerns for different notification channels.

A unified dispatcher allows the system to route notifications to the correct channel efficiently, supporting scalability and maintainability.

❓ scaling

intermediate

Estimating capacity for peak notification loads

Your notification system must handle 10,000 notifications per second during peak hours. What is the best approach to estimate the required throughput capacity?

A Ignore peak loads and design for average daily notifications

B Use the total number of users multiplied by 10 without considering delivery time

C Estimate capacity by the number of developers working on the system

D Calculate based on average notification size, delivery latency, and concurrency limits of each channel

💡 Hint

Consider factors that affect how many notifications can be sent per second.

Estimating throughput requires understanding message size, delivery latency, and concurrency limits to ensure the system can handle peak loads without delays.

❓ tradeoff

advanced

Choosing between push and pull notification models

Which tradeoff is true when choosing a push-based notification system over a pull-based one?

A Pull delivers notifications instantly without server overhead

B Push reduces client polling but requires maintaining persistent connections, increasing server resource use

C Push requires clients to poll frequently, increasing network traffic

D Pull requires servers to push messages directly to clients

💡 Hint

Think about how push and pull models handle message delivery and resource usage.

Push notifications reduce client polling but need persistent connections, which use more server resources. Pull models rely on client polling, increasing network traffic.

🧠 Conceptual

advanced

Ensuring message delivery guarantees in notification systems

Which mechanism best ensures 'at least once' delivery guarantee for notifications?

A Use message queues with acknowledgments and retries on failure

B Send notifications without tracking delivery status

C Deliver notifications only once without retries

D Use fire-and-forget messaging without persistence

💡 Hint

Consider how to handle failures and retries to avoid lost messages.

Message queues with acknowledgments and retries ensure messages are delivered at least once, even if failures occur during transmission.

❓ component

expert

Designing a notification throttling mechanism

You want to prevent users from receiving more than 5 notifications per minute. Which component design best enforces this throttling?

A A rate limiter that tracks notification counts per user and blocks excess sends

B A database trigger that deletes notifications after 5 per minute

C A client-side script that ignores notifications after 5 per minute

D A logging service that records notifications without blocking

💡 Hint

Think about where and how to enforce limits to control notification flow.

A rate limiter component tracks how many notifications each user receives and blocks sending more than the allowed limit, ensuring throttling is enforced server-side.

Practice

1. Which component in a notification system is responsible for generating events that trigger notifications?

easy

A. Delivery Channel

B. Notification Service

C. User Preferences Store

D. Event Producer

Solution

1. Step 1: Understand the role of event producers

   Event producers create or detect events that require notifying users, such as a new message or alert.

2. Step 2: Differentiate from other components

   Notification service processes events, delivery channels send notifications, and user preferences store user settings.

3. Final Answer:

   Event Producer -> Option D

4. Quick Check:

   Event source = Event Producer [OK]

Hint: Event creators are called producers in notification systems [OK]

Common Mistakes:

• Confusing notification service with event producer
• Thinking delivery channel generates events
• Assuming user preferences create events

2. Which of the following is the correct sequence of components for sending a notification after an event occurs?

easy

A. Delivery Channel -> Notification Service -> Event Producer

B. Event Producer -> Notification Service -> Delivery Channel

C. Notification Service -> Event Producer -> Delivery Channel

D. User Preferences Store -> Event Producer -> Delivery Channel

Solution

1. Step 1: Identify the logical flow of notification

   First, an event is generated by the event producer, then processed by the notification service, and finally sent via the delivery channel.

2. Step 2: Eliminate incorrect sequences

   Delivery channel cannot start the process; user preferences store is not part of the sending sequence.

3. Final Answer:

   Event Producer -> Notification Service -> Delivery Channel -> Option B

4. Quick Check:

   Event -> Process -> Send = A [OK]

Hint: Notifications flow from event to service to delivery [OK]

Common Mistakes:

• Reversing the order of components
• Including user preferences in the sending chain
• Confusing delivery channel as event source

3. Consider a notification system where users can choose email or SMS as delivery channels. If a user prefers both, what is the expected behavior when an event triggers a notification?

medium

A. Send notification via both email and SMS

B. Send notification via email only

C. Send notification via SMS only

D. Do not send any notification

Solution

1. Step 1: Understand user preference handling

   If a user selects multiple delivery channels, the system should send notifications through all preferred channels to ensure delivery.

2. Step 2: Confirm expected multi-channel delivery

   Sending via both email and SMS respects user choice and increases notification reach.

3. Final Answer:

   Send notification via both email and SMS -> Option A

4. Quick Check:

   Multiple preferences = multiple channels [OK]

Hint: Send notifications on all user-selected channels [OK]

Common Mistakes:

• Sending notification on only one channel
• Ignoring user preferences
• Not sending notification at all

4. A notification system uses a queue to handle event processing but notifications are delayed significantly. Which is the most likely cause?

medium

A. Queue is overloaded with too many events

B. User preferences are not stored

C. Delivery channel is sending notifications instantly

D. Event producer is generating too few events

Solution

1. Step 1: Analyze queue role in notification system

   Queue buffers events to handle load. If overloaded, it causes delays in processing notifications.

2. Step 2: Evaluate other options

   Missing user preferences or instant delivery does not cause delay; too few events would reduce load, not increase delay.

3. Final Answer:

   Queue is overloaded with too many events -> Option A

4. Quick Check:

   Queue overload = delay [OK]

Hint: Delays often mean queue overload, not missing data [OK]

Common Mistakes:

• Blaming delivery channel for delays
• Assuming missing preferences cause delay
• Thinking fewer events cause delays

5. You need to design a notification system that supports millions of users with personalized preferences and multiple delivery channels. Which design choice best ensures scalability and user customization?

hard

A. Use a centralized notification service with a single queue and fixed delivery channels

B. Store all user preferences in a local file on the notification server

C. Implement distributed notification services with sharded queues and dynamic delivery channel selection per user

D. Send notifications synchronously from event producers directly to users

Solution

1. Step 1: Consider scalability requirements

   Millions of users require distributed services and sharded queues to handle load without bottlenecks.

2. Step 2: Address user customization needs

   Dynamic delivery channel selection per user allows personalized notifications respecting preferences.

3. Step 3: Evaluate other options

   Centralized service and single queue create bottlenecks; synchronous sending blocks processing; local files do not scale or support dynamic preferences.

4. Final Answer:

   Implement distributed notification services with sharded queues and dynamic delivery channel selection per user -> Option C

5. Quick Check:

   Distributed + dynamic preferences = scalable & customizable [OK]

Hint: Distribute services and shard queues for scale and flexibility [OK]

Common Mistakes:

• Choosing centralized design causing bottlenecks
• Using synchronous sending blocking system
• Storing preferences in non-scalable local files