Design a notification system in HLD - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to identify the main component responsible for sending notifications.

HLD

The component that sends notifications to users is called the [1].

Drag options to blanks, or click blank then click option'

ALoad Balancer

BUser Interface

CDatabase

DNotification Dispatcher

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing the database with the sender component.

The Notification Dispatcher is the main component that sends notifications to users.

2fill in blank

medium

Complete the code to specify the type of database best suited for storing user notification preferences.

HLD

For storing user notification preferences, a [1] database is preferred for fast key-value lookups.

Drag options to blanks, or click blank then click option'

AGraph

BRelational

CKey-Value

DTime-Series

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing relational database which is less efficient here.

A Key-Value database is ideal for quick access to user preferences by user ID.

3fill in blank

hard

Fix the error in the notification delivery flow by selecting the correct component that handles retries.

HLD

If a notification fails to deliver, the [1] component retries sending it after a delay.

Drag options to blanks, or click blank then click option'

ARetry Service

BLoad Balancer

CUser Interface

DDatabase

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing load balancer with retry logic.

The Retry Service handles resending notifications that failed initially.

4fill in blank

hard

Fill both blanks to complete the notification system's architecture components.

HLD

The [1] queues incoming notifications, while the [2] sends them to users.

Drag options to blanks, or click blank then click option'

AMessage Broker

BUser Interface

CNotification Dispatcher

DDatabase

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up UI with sending component.

The Message Broker queues notifications, and the Notification Dispatcher sends them.

5fill in blank

hard

Fill all three blanks to complete the notification filtering logic.

HLD

Notifications are filtered by [1] before sending. The system checks [2] to respect user choices and uses [3] to avoid overload.

Drag options to blanks, or click blank then click option'

Apriority

Buser preferences

Crate limiting

Ddatabase

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing database with filtering logic.

Priority filters notifications, user preferences control delivery, and rate limiting prevents overload.

Practice

1. Which component in a notification system is responsible for deciding when to send a message to a user?

easy

A. Event processor

B. Notification sender

C. User preference manager

D. Message storage

Solution

1. Step 1: Understand the role of event processor

   The event processor detects events that trigger notifications, deciding when a message should be sent.

2. Step 2: Differentiate from other components

   The notification sender delivers messages, user preference manager stores user choices, and message storage keeps records.

3. Final Answer:

   Event processor -> Option A

4. Quick Check:

   Event detection = Event processor [OK]

Hint: Event timing is handled by the event processor [OK]

Common Mistakes:

• Confusing sender with event detector
• Thinking user preferences trigger events
• Assuming storage decides timing

2. Which data structure is best suited to store user notification preferences for quick lookup?

easy

A. Linked list

B. Hash map

C. Queue

D. Stack

Solution

1. Step 1: Identify quick lookup needs

   User preferences require fast access by user ID or key, so a data structure with O(1) average lookup is ideal.

2. Step 2: Match data structures to lookup speed

   Hash maps provide constant time lookup, unlike linked lists, queues, or stacks which are slower for direct access.

3. Final Answer:

   Hash map -> Option B

4. Quick Check:

   Fast key-value access = Hash map [OK]

Hint: Use hash map for fast user preference lookup [OK]

Common Mistakes:

• Choosing linked list which is slow for lookup
• Confusing queue or stack with lookup structures
• Ignoring key-based access needs

3. Consider this simplified flow: An event triggers a notification, which is stored in a queue before delivery. What happens if the queue is full?

medium

A. Queue automatically expands without limit

B. System crashes due to overflow

C. Notifications are sent immediately bypassing the queue

D. New notifications are dropped or delayed

Solution

1. Step 1: Understand queue capacity limits

   Queues have fixed or limited size; when full, they cannot accept new items immediately.

2. Step 2: Identify common handling of full queues

   Systems usually drop new notifications or delay them until space frees up; automatic unlimited expansion is rare to avoid resource exhaustion.

3. Final Answer:

   New notifications are dropped or delayed -> Option D

4. Quick Check:

   Full queue = drop or delay new notifications [OK]

Hint: Full queue means drop or delay notifications [OK]

Common Mistakes:

• Assuming infinite queue size
• Thinking notifications bypass queue
• Believing system crashes on full queue

4. A notification system sends duplicate messages to users. Which design mistake most likely causes this?

medium

A. Notifications sent synchronously

B. User preferences not stored

C. No deduplication in event processing

D. Using a single message queue

Solution

1. Step 1: Analyze duplicate message causes

   Duplicates often occur if the system processes the same event multiple times without checking if notification was already sent.

2. Step 2: Evaluate other options

   Missing user preferences or synchronous sending do not cause duplicates; a single queue can still handle duplicates if deduplication exists.

3. Final Answer:

   No deduplication in event processing -> Option C

4. Quick Check:

   Duplicates = missing deduplication [OK]

Hint: Duplicates mean missing deduplication step [OK]

Common Mistakes:

• Blaming user preferences for duplicates
• Confusing synchronous sending with duplication
• Assuming single queue causes duplicates

5. You need to design a notification system that supports email, SMS, and push notifications with user preferences and high scalability. Which architecture pattern best fits this requirement?

hard

A. Event-driven microservices with message queues and preference service

B. Batch processing system sending notifications once daily

C. Single database polling for notifications every minute

D. Monolithic application with direct notification calls

Solution

1. Step 1: Identify scalability and multi-channel needs

   Supporting multiple notification types and scaling requires decoupling components and asynchronous processing.

2. Step 2: Match architecture patterns

   Event-driven microservices with message queues allow independent scaling, handle user preferences, and support multiple channels efficiently.

3. Step 3: Eliminate unsuitable options

   Monolithic apps limit scalability; polling causes delays; batch processing is too slow for timely notifications.

4. Final Answer:

   Event-driven microservices with message queues and preference service -> Option A

5. Quick Check:

   Scalable multi-channel = event-driven microservices [OK]

Hint: Use event-driven microservices for scalable multi-channel notifications [OK]

Common Mistakes:

• Choosing monolithic for scalability
• Using batch processing for real-time needs
• Relying on polling causing delays