Code-Memo

✅ STEP 5: High-Level Architecture — Amazon (E-Commerce Platform)

We’ll design for:

Product listings
Cart & checkout
Payments
Order management
Search
Recommendations
Reviews
High availability and global scale

🔧 1. Key Architectural Principles

Principle	Purpose
Microservices	Separate concerns: search, cart, orders, etc.
Event-Driven	Decouple checkout, payments, notifications
CQRS	Use separate read/write models for scalability
Geo-Sharded	Data and traffic split by region
Eventually Consistent	Allow eventual sync where strict ACID is not needed
High Availability	Fault-tolerant, replicated services and storage

🧩 2. System Overview

Major domains:

Client Layer
API Gateway
Core Backend Services
Product Catalog & Inventory
Search & Recommendations
Order Processing Pipeline
Payment & Billing
Notifications & Reviews
Infra & Storage

📦 3. Component Breakdown

✅ 3.1. Client Layer

Platform	Function
Web, iOS, Android	Browse, search, add to cart, checkout
Admin Panels	Seller portals, inventory mgmt, order tracking

✅ 3.2. API Gateway

Auth (OAuth2/JWT)
Request routing
Rate limiting
A/B testing and feature flags

✅ 3.3. Core Backend Services

Service	Description
User Service	User profiles, addresses, preferences
Product Service	Product metadata, categories, variations
Inventory Service	Tracks real-time stock availability
Cart Service	Add/update/remove items
Checkout Service	Validates cart, reserves stock, creates order
Order Service	Tracks order state (placed → shipped → delivered)
Payment Service	Handles card/wallet/EMI integrations
Shipment Service	Assigns carriers, tracks deliveries
Review Service	Stores reviews, verifies purchases
Seller Service	Merchant data, pricing, offers, compliance
Promotion Service	Coupons, lightning deals, Prime offers

✅ 3.4. Product Catalog & Inventory

Component	Description
Catalog DB	Stores product metadata (title, description, price, images)
Inventory DB	Real-time stock per SKU/warehouse
Image CDN	Optimized image delivery via CloudFront/S3
Versioned Index	Supports rollback during updates

✅ 3.5. Search & Recommendation Systems

Subsystem	Technology
Search Engine	Elasticsearch / OpenSearch
Auto-suggest Engine	Trie or ML-based on history
Recommendation Engine	Collaborative filtering + ML
Trending Engine	Real-time + batch analytics
Personalization	Vector embeddings of user-product context

✅ 3.6. Order Processing Pipeline

Cart Checkout →
- Validate inventory
- Create Order
- Reserve stock
- Process payment
- Trigger fulfillment

Stage	Tool/Tech
Event Bus	Kafka (`order_placed`, `payment_confirmed`)
Fulfillment Service	Assigns warehouse, prints label
Packing & Dispatch	Tracks via carrier API
Order Lifecycle	Updates customer, triggers notifications
Return/Refund	Reverse transaction + inventory sync

✅ 3.7. Payment & Billing

Service	Description
Payment Gateway	Card, wallet, EMI, UPI, etc.
Tokenization	Secure vault for card details
Fraud Detection	Heuristics + ML models (e.g. location mismatch, bulk orders)
Ledger Service	Tracks debit/credit per transaction
Billing Service	Generates invoices, tax calculations

✅ 3.8. Notifications & Messaging

Email, SMS, push (order placed, shipped, etc.)
Kafka-backed async message triggers
Delayed retries with exponential backoff
Firebase (push), Twilio (SMS), SES (email)

✅ 3.9. Storage Systems

Data Type	Storage
Product Catalog	MySQL / Spanner
Inventory	Redis (real-time) + DB (durable)
Orders	Cassandra (append-only), or DynamoDB
Payments	SQL + secure vault
Reviews	MongoDB / Document DB
Search Index	Elasticsearch
Cache	Redis / Memcached
Image/Media	S3 + CDN (CloudFront)

🧪 4. User Journey: Browse → Buy

1. User browses → Product Service
2. Search suggestions → Search Service
3. Adds to cart → Cart Service (stored in Redis)
4. Checkout:
   a. Cart validated
   b. Inventory reserved
   c. Order created
   d. Payment processed
5. Kafka → Fulfillment Service assigns warehouse
6. Shipping scheduled, user notified
7. Kafka → Notification + Analytics + Recommendations

🌍 5. Deployment & Scale Strategy

Component	Strategy
Product Catalog	Sharded by product ID prefix
Order Services	Sharded by region & order ID
Kafka	Topic-per-domain, regional clusters
Payment	Global ledger + regional gateways
Search	Global index per region, merged for cross-border search
Inventory	Stored per-warehouse, synced globally
CDN	Edge caches for images, videos, and product pages

🔐 6. Security & Compliance

Concern	Approach
PII/PCI Security	Tokenization, encryption at rest, SOC2 compliant vault
Rate Limiting	IP/User based limits on sensitive APIs
KYC for Sellers	Required by Seller Service
Audit Logs	Append-only event store per user/session
Data Retention	Configurable per region (GDPR, CCPA)

📊 System Diagram (Highly Detailed)

+-----------------+         +-------------------+         +---------------------------+
|  Client (Web)   | <--->   |    API Gateway    | <--->   | Core Microservices       |
+-----------------+         +-------------------+         |---------------------------|
                                                           | - User Service           |
                                                           | - Product Service        |
                                                           | - Cart Service           |
                                                           | - Inventory Service      |
                                                           | - Order Service          |
                                                           | - Checkout Service       |
                                                           | - Payment Service        |
                                                           | - Review Service         |
                                                           | - Notification Service   |
                                                           +------------+-------------+
                                                                        |
                +------------------ Kafka ------------------------------+
                |                   |                  |               |
                v                   v                  v               v
    +------------------+  +------------------+   +-----------------+  +-----------------+
    | Fulfillment Svc  |  | Analytics Engine |   | Notification Svc|  | Recommendation  |
    +------------------+  +------------------+   +-----------------+  +-----------------+

                           +-----------+         +--------------------+
                           |  Elasticsearch       |  Redis (Inventory, Carts)|
                           +-----------+         +--------------------+

                           +--------------------------------------------+
                           |     S3 + CloudFront (images/media/CDN)    |
                           +--------------------------------------------+

🧠 Special Design Considerations

Feature	Note
Prime System	Different shipping rules, filters, seller eligibility
Lightning Deals	Queued inventory slots, auto-start/stop
Currency & Locale	Prices, languages, units localized
Multi-vendor Orders	Split order across multiple sellers/warehouses
Retry Logic	Payment, email failures handled via retries