Notification on state change in LLD - Architecture Diagram

System Overview - Notification on state change

This system sends notifications to users when the state of an entity changes. It ensures timely updates by detecting state changes, processing them, and delivering notifications via multiple channels like email or SMS.

Architecture Diagram

User
  |
  v
Load Balancer
  |
  v
API Gateway
  |
  v
State Change Service ---> Message Queue ---> Notification Service ---> Notification Channels
                                      |                              |
                                      v                              v
                                  Database                       Cache Layer

Components

User

client

Initiates requests and receives notifications

Load Balancer

load_balancer

Distributes incoming requests evenly to API Gateway instances

API Gateway

api_gateway

Handles client requests, routes them to appropriate services

State Change Service

service

Detects and processes state changes, writes events to message queue

Message Queue

message_queue

Buffers state change events for asynchronous processing

Notification Service

service

Consumes events from queue and sends notifications

Notification Channels

external_service

Delivers notifications via email, SMS, or push

Database

database

Stores state data and notification preferences

Cache Layer

cache

Speeds up access to frequently used data like user preferences

Request Flow - 9 Hops

User → Load Balancer

Load Balancer → API Gateway

API Gateway → State Change Service

State Change Service → Database

State Change Service → Message Queue

Notification Service → Message Queue

Notification Service → Cache Layer

Notification Service → Notification Channels

Notification Channels → User

Failure Scenario

Component Fails:Message Queue

Impact:Notifications are delayed or lost because events cannot be queued for processing

Mitigation:Use a replicated, durable message queue with retry mechanisms and alerting to restore service quickly

Architecture Quiz - 3 Questions

Which component ensures that user requests are evenly distributed to prevent overload?

A API Gateway

B Message Queue

C Load Balancer

D Notification Service

Design Principle

This architecture uses asynchronous messaging to decouple state changes from notification delivery, improving scalability and reliability. Caching user preferences reduces latency, and load balancing ensures even request distribution.

Practice

1. What is the main purpose of a notification system on state change in software design?

easy

A. To inform interested components immediately when data changes

B. To store data permanently in a database

C. To increase the size of the application

D. To delay updates until the user refreshes manually

Solution

1. Step 1: Understand the role of notifications

   Notifications alert parts of a system or users when something important changes.

2. Step 2: Identify the purpose of state change notifications

   They ensure components get updates immediately without waiting or manual refresh.

3. Final Answer:

   To inform interested components immediately when data changes -> Option A

4. Quick Check:

   Notification = Immediate update [OK]

Hint: Notifications alert on change, not store or delay [OK]

Common Mistakes:

• Confusing notification with data storage
• Thinking notifications delay updates
• Assuming notifications increase app size

2. Which of the following is the correct method name to notify observers in a typical observer pattern implementation?

easy

A. unsubscribe()

B. updateState()

C. subscribe()

D. notifyObservers()

Solution

1. Step 1: Recall observer pattern methods

   Common methods include subscribe, unsubscribe, and notifyObservers.

2. Step 2: Identify the method that sends updates

   notifyObservers() is used to alert all subscribed observers about changes.

3. Final Answer:

   notifyObservers() -> Option D

4. Quick Check:

   Notify method = notifyObservers() [OK]

Hint: Notify method usually named notifyObservers() [OK]

Common Mistakes:

• Confusing subscribe with notify
• Using updateState() which changes state, not notify
• Mixing unsubscribe with notification

3. Consider this simplified code snippet for a notification system:

class Subject:
    def __init__(self):
        self.observers = []
    def subscribe(self, observer):
        self.observers.append(observer)
    def notify(self, message):
        for obs in self.observers:
            obs.update(message)

class Observer:
    def update(self, message):
        print(f"Received: {message}")

subject = Subject()
obs1 = Observer()
subject.subscribe(obs1)
subject.notify("State changed")

What will be the output when subject.notify("State changed") is called?

medium

A. Error: update method missing

B. No output

C. Received: State changed

D. Received: None

Solution

1. Step 1: Trace subscription and notification

   Observer obs1 is subscribed to subject, so it is in the observers list.

2. Step 2: Check notify method behavior

   notify calls update on each observer with the message "State changed".

3. Final Answer:

   Received: State changed -> Option C

4. Quick Check:

   Observer prints message on notify [OK]

Hint: Subscribed observers receive and print messages [OK]

Common Mistakes:

• Assuming notify does nothing without explicit call
• Thinking update method is missing
• Expecting no output if observers list is empty

4. In the following code, what is the main issue that prevents observers from receiving notifications?

class Subject:
    def __init__(self):
        self.observers = set()
    def subscribe(self, observer):
        self.observers.add(observer)
    def notify(self, message):
        for obs in self.observers:
            obs.receive(message)

class Observer:
    def update(self, message):
        print(f"Got: {message}")

subject = Subject()
obs1 = Observer()
subject.subscribe(obs1)
subject.notify("Update")

medium

A. Observers are stored in a set instead of a list

B. Method notify calls obs.receive but Observer has update method

C. subscribe method uses add instead of append

D. Observer class is missing

Solution

1. Step 1: Check method called in notify

   notify() calls obs.receive(message).

2. Step 2: Check Observer class methods

   Observer defines update(message), but no receive() method.

3. Final Answer:

   Method notify calls obs.receive but Observer has update method -> Option B

4. Quick Check:

   Method name mismatch causes AttributeError [OK]

Hint: Check method names called vs defined in observers [OK]

Common Mistakes:

• Confusing set vs list for storing observers
• Ignoring method name mismatches
• Assuming missing class when it exists

5. You are designing a scalable notification system for state changes in a distributed application. Which design choice best supports efficient notifications to thousands of subscribers without blocking the main process?

hard

A. Use asynchronous message queues to dispatch notifications

B. Notify all subscribers synchronously in a loop

C. Store notifications in a database and poll subscribers

D. Send notifications only when the system restarts

Solution

1. Step 1: Understand scalability needs

   Thousands of subscribers require non-blocking, efficient notification delivery.

2. Step 2: Evaluate design options

   Synchronous loops block main process; polling adds delay; restart notifications are impractical.

3. Step 3: Identify best practice

   Asynchronous message queues decouple notification sending, allowing scalable, fast delivery.

4. Final Answer:

   Use asynchronous message queues to dispatch notifications -> Option A

5. Quick Check:

   Async queues = scalable notifications [OK]

Hint: Async queues handle many subscribers efficiently [OK]

Common Mistakes:

• Using synchronous loops causing delays
• Relying on polling which is slow
• Sending notifications only on restart