Payment strategy pattern in LLD - System Design Exercise

Design: Payment Processing System with Strategy Pattern

Design focuses on the payment processing logic using the strategy pattern. It excludes UI design, external payment gateway implementations, and detailed security protocols.

Functional Requirements

FR1: Support multiple payment methods such as credit card, PayPal, and bank transfer

FR2: Allow adding new payment methods without changing existing code

FR3: Process payments securely and handle failures gracefully

FR4: Provide a unified interface for clients to make payments

FR5: Log payment transactions for auditing

Non-Functional Requirements

NFR1: Handle up to 1000 payment requests per second

NFR2: Response time for payment processing should be under 2 seconds

NFR3: Ensure high availability with 99.9% uptime

NFR4: Design should be extensible for future payment methods

Think Before You Design

Questions to Ask

❓ Question 1

❓ Question 2

❓ Question 3

❓ Question 4

❓ Question 5

Key Components

Payment interface defining common payment method

Concrete payment strategy classes for each payment method

Context class that uses a payment strategy

Transaction logger

Error handling and retry mechanism

Design Patterns

Strategy pattern for payment method selection

Factory pattern to create payment strategy instances

Decorator pattern for adding logging or security features

Observer pattern for notifying other systems after payment

Reference Architecture

Client
  |
  v
PaymentContext <---- PaymentStrategyFactory
  |  \
  |   \__ PaymentStrategy (interface)
  |        |       |        |
  |        |       |        |
  |    CreditCardPayment  PayPalPayment  BankTransferPayment
  |
  v
TransactionLogger

Components

PaymentStrategy

Interface / Abstract Class

Defines a common method to process payments

CreditCardPayment

Concrete Class

Implements payment processing for credit cards

PayPalPayment

Concrete Class

Implements payment processing for PayPal

BankTransferPayment

Concrete Class

Implements payment processing for bank transfers

PaymentContext

Class

Uses a PaymentStrategy to process payments without client knowing details

PaymentStrategyFactory

Factory Class

Creates appropriate PaymentStrategy instances based on input

TransactionLogger

Class

Logs payment transactions for auditing and debugging

Request Flow

1. Client requests payment with chosen method and payment details

2. PaymentContext asks PaymentStrategyFactory for the correct PaymentStrategy

3. PaymentStrategy processes the payment according to its method

4. PaymentContext receives success or failure response

5. TransactionLogger records the payment transaction details

6. Client receives confirmation or error message

Database Schema

Entities: - PaymentTransaction: id (PK), amount, currency, method, status, timestamp, details Relationships: - PaymentTransaction stores each payment attempt with method and status - No complex relationships needed for strategy pattern

Scaling Discussion

Bottlenecks

Payment processing latency due to external gateway calls

High load causing delays in strategy selection or logging

Single point of failure in PaymentContext or Logger

Difficulty adding new payment methods if tightly coupled

Solutions

Use asynchronous processing and queues for external calls

Cache strategy instances or use stateless strategies for concurrency

Deploy PaymentContext and Logger as scalable microservices with load balancing

Keep strategy implementations independent and use factory pattern for easy extension

Interview Tips

Time: Spend 10 minutes understanding requirements and clarifying questions, 20 minutes designing the strategy pattern with components and flow, 10 minutes discussing scaling and trade-offs, 5 minutes summarizing.

Explain how strategy pattern enables flexible payment method addition

Describe separation of concerns between context and strategies

Highlight how factory pattern complements strategy for object creation

Discuss error handling and logging importance

Mention scalability and extensibility considerations