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

Practice

1. What is the main benefit of using the Payment Strategy Pattern in a payment system?

easy

A. It allows switching between different payment methods without changing the main code.

B. It forces all payment methods to use the same currency.

C. It stores all payment data in a single database table.

D. It encrypts payment information automatically.

Solution

1. Step 1: Understand the purpose of the Payment Strategy Pattern

   The pattern is designed to let the system switch payment methods easily without modifying the main logic.

2. Step 2: Analyze the options

   Only It allows switching between different payment methods without changing the main code. describes this benefit correctly. Other options describe unrelated features.

3. Final Answer:

   It allows switching between different payment methods without changing the main code. -> Option A

4. Quick Check:

   Payment Strategy Pattern = Switch payment methods easily [OK]

Hint: Focus on flexibility to switch payment methods without code changes [OK]

Common Mistakes:

• Confusing strategy pattern with data storage or encryption
• Thinking it enforces currency or database rules
• Assuming it handles security automatically

2. Which of the following is the correct way to define a payment strategy interface in a typical object-oriented language?

easy

A. interface PaymentStrategy { void pay(double amount); }

B. class PaymentStrategy { void pay(amount); }

C. function PaymentStrategy(amount) { return pay; }

D. var PaymentStrategy = pay => amount;

Solution

1. Step 1: Identify the correct syntax for an interface

   In object-oriented languages, interfaces declare method signatures without implementation. interface PaymentStrategy { void pay(double amount); } uses 'interface' and a method signature correctly.

2. Step 2: Check other options

   class PaymentStrategy { void pay(amount); } is a class, not an interface. Options C and D use function syntax, not interface definitions.

3. Final Answer:

   interface PaymentStrategy { void pay(double amount); } -> Option A

4. Quick Check:

   Interface syntax = interface PaymentStrategy { void pay(double amount); } [OK]

Hint: Look for 'interface' keyword and method signature format [OK]

Common Mistakes:

• Using class instead of interface for strategy definition
• Confusing function syntax with interface
• Missing method parameter types

3. Given the following code snippet implementing the Payment Strategy Pattern, what will be the output?

class PaymentStrategy {
  pay(amount) { throw 'Not implemented'; }
}

class CreditCardPayment extends PaymentStrategy {
  pay(amount) { return `Paid ${amount} with Credit Card`; }
}

class PayPalPayment extends PaymentStrategy {
  pay(amount) { return `Paid ${amount} with PayPal`; }
}

class PaymentContext {
  constructor(strategy) { this.strategy = strategy; }
  executePayment(amount) { return this.strategy.pay(amount); }
}

const context = new PaymentContext(new PayPalPayment());
console.log(context.executePayment(100));

medium

A. Paid 100 with Credit Card

B. Not implemented

C. Paid 100 with PayPal

D. Error: strategy.pay is not a function

Solution

1. Step 1: Trace the object creation and method calls

   The PaymentContext is created with a PayPalPayment strategy. Calling executePayment(100) calls PayPalPayment's pay method.

2. Step 2: Understand the pay method output

   PayPalPayment's pay returns 'Paid 100 with PayPal'. This string is printed.

3. Final Answer:

   Paid 100 with PayPal -> Option C

4. Quick Check:

   Context uses PayPalPayment = Output with PayPal [OK]

Hint: Check which strategy instance is passed to context [OK]

Common Mistakes:

• Assuming default or CreditCardPayment is used
• Expecting an error from base class
• Confusing method override behavior

4. Identify the error in the following Payment Strategy Pattern implementation:

class PaymentStrategy {
  pay(amount) { console.log('Paying ' + amount); }
}

class BitcoinPayment extends PaymentStrategy {
  pay() { console.log('Paying with Bitcoin'); }
}

const payment = new BitcoinPayment();
payment.pay(50);

medium

A. PaymentStrategy should not have a pay method implementation.

B. BitcoinPayment's pay method does not accept the amount parameter.

C. BitcoinPayment should not extend PaymentStrategy.

D. Calling pay with 50 causes a syntax error.

Solution

1. Step 1: Compare method signatures in base and subclass

   PaymentStrategy's pay expects an amount parameter, but BitcoinPayment's pay method does not accept any parameters.

2. Step 2: Understand the impact of signature mismatch

   Calling payment.pay(50) passes an argument, but BitcoinPayment's pay ignores it, causing unexpected behavior or errors.

3. Final Answer:

   BitcoinPayment's pay method does not accept the amount parameter. -> Option B

4. Quick Check:

   Method signature mismatch = BitcoinPayment's pay method does not accept the amount parameter. [OK]

Hint: Check if subclass methods match base method parameters [OK]

Common Mistakes:

• Thinking base class should not implement pay
• Assuming inheritance is wrong
• Confusing runtime error with syntax error

5. You are designing a payment system that must support credit cards, PayPal, and a new cryptocurrency payment method. Using the Payment Strategy Pattern, which design approach best supports adding the new method with minimal changes?

hard

A. Use a global variable to switch payment methods inside the main payment function.

B. Modify the existing CreditCardPayment class to handle cryptocurrency payments.

C. Add cryptocurrency payment logic inside the PaymentContext class directly.

D. Create a new class implementing the PaymentStrategy interface for cryptocurrency and pass it to the payment context.

Solution

1. Step 1: Understand the open/closed principle in design

   The system should be open for extension but closed for modification. Adding a new payment method should not require changing existing classes.

2. Step 2: Evaluate each option

   Create a new class implementing the PaymentStrategy interface for cryptocurrency and pass it to the payment context. creates a new class implementing the interface, fitting the pattern and minimizing changes. Options B and C modify existing classes, violating the principle. Use a global variable to switch payment methods inside the main payment function. uses a global variable, which is poor design.

3. Final Answer:

   Create a new class implementing the PaymentStrategy interface for cryptocurrency and pass it to the payment context. -> Option D

4. Quick Check:

   New class for new method = Create a new class implementing the PaymentStrategy interface for cryptocurrency and pass it to the payment context. [OK]

Hint: Add new payment as new class, avoid changing existing code [OK]

Common Mistakes:

• Modifying existing payment classes
• Adding logic inside context class
• Using global variables for strategy switching