cpp-implementations/subscriber-notifs/problem.md

System Design Problem: Stock Availability Notification Service

Problem Statement

Design a Low-Level Design (LLD) for a "Notify Me When Available" feature on an e-commerce platform. When a high-demand product (like a PS5 or iPhone) is out of stock, customers can subscribe to receive an alert when it becomes available again.

1. Functional Requirements (FR)

• Product Availability Monitoring: The system must track the stock quantity of products.
• User Subscription:
  • Users can subscribe to a specific product to receive availability alerts.
  • Users can choose their preferred communication channel (Email, SMS, or Push Notification) at the time of subscription.
  • Users should be able to unsubscribe if they lose interest.
• Notification Trigger:
  • When a product's stock status changes from "Out of Stock" (0) to "In Stock" (>0), the system must trigger alerts to all subscribed users.
• Extensibility:
  • The system must support adding new notification channels (e.g., WhatsApp, Discord webhook) in the future with minimal code changes to the core product logic.

2. Non-Functional Requirements (NFR)

• Decoupling: The inventory management system (Product logic) should not be tightly coupled with the notification logic (3rd party Email/SMS services).
• Real-time/Near Real-time: Notifications should be sent reasonably quickly after stock is updated.
• Scalability: A single popular product might have 100,000+ subscribers. The notification process should not block the inventory update transaction (e.g., the warehouse manager updating stock shouldn't wait for 100k emails to send before seeing a "Success" message).

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Scratch

• clearly observer pattern ( satisifies decoupling requirement )
• nfr 2 and 3 need async processing so we can use message queues or threads but let me implement a basic version first

interface Subscribable  (observer)
    + subscribe(Subscriber)
    + unsubscribe(Subscriber)
    + notifySubscribers()

interface Subscriber
    + onNotify(Product)

Product (subject) implements Subscribable
    - List<Subscriber> subscribers
    + subscribe(Subscriber)
    + unsubscribe(Subscriber)
    - notifySubscribers()
    + setStock(int)
        - if stock changes from 0 to >0
            - notifySubscribers()

Approach

• made observer pattern with string messages and global notif ( no filer for specific ids )
• changed to Event class with runtime polymorphism using base pointer trick
• used generics to do compile type polymorphism
• make product id a generic too and change to use map of id to list of subscribers
• move off the actual notifictions to a thread pool to handle async processing