Design a notification system in HLD - Architecture Diagram

System Overview - Design a notification system

This notification system sends messages to users via multiple channels like email, SMS, and push notifications. It must handle high volumes of notifications reliably and deliver them quickly.

Architecture Diagram

User
  |
  v
Load Balancer
  |
  v
API Gateway
  |
  v
Notification Service
  |          |           |
  v          v           v
Message Queue  Cache   Database
  |          |           |
  v          v           v
Email Service  SMS Service  Push Notification Service
  |
  v
User Devices

Components

User

client

Initiates notification requests

Load Balancer

load_balancer

Distributes incoming requests evenly to API Gateway instances

API Gateway

api_gateway

Handles client requests, routes them to Notification Service

Notification Service

service

Processes notification requests and pushes messages to queue

Message Queue

queue

Buffers notification messages for asynchronous processing

Cache

cache

Stores recent notification statuses for quick access

Database

database

Stores user preferences and notification history

Email Service

service

Sends email notifications to users

SMS Service

service

Sends SMS notifications to users

Push Notification Service

service

Sends push notifications to user devices

User Devices

client

Receives push notifications

Request Flow - 10 Hops

User → Load Balancer

Load Balancer → API Gateway

API Gateway → Notification Service

Notification Service → Database

Notification Service → Message Queue

Message Queue → Email Service

Message Queue → SMS Service

Message Queue → Push Notification Service

Push Notification Service → User Devices

Notification Service → Cache

Failure Scenario

Component Fails:Message Queue

Impact:Notifications cannot be queued for delivery, causing delays or loss of messages

Mitigation:Use replicated queues and fallback retry mechanisms; alert system operators

Architecture Quiz - 3 Questions

Which component handles distributing incoming user requests evenly?

A Notification Service

B API Gateway

C Load Balancer

D Message Queue

Design Principle

This design uses asynchronous message queues to decouple notification processing from delivery, improving scalability and reliability. Caching recent statuses reduces database load. Load balancers and API gateways ensure even traffic distribution and secure routing.

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