Payment handling in LLD - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the primary goal of a payment handling system?

To securely process financial transactions between a payer and a payee, ensuring accuracy, reliability, and compliance with regulations.

beginner

Name two common components of a payment handling system.

1. Payment Gateway: Interfaces with banks and payment networks.
2. Transaction Processor: Validates and records transactions.

intermediate

Why is idempotency important in payment processing?

It prevents duplicate charges by ensuring that repeated requests with the same identifier are processed only once.

beginner

Explain the role of a payment gateway in simple terms.

It acts like a secure messenger that sends payment details from the customer to the bank and returns the approval or rejection.

intermediate

What are common security measures in payment handling systems?

Encryption of data, PCI DSS compliance, tokenization of card details, and secure authentication methods.

What does idempotency ensure in payment processing?

A Prevents duplicate transactions

B Speeds up payment processing

C Encrypts payment data

D Authenticates users

Idempotency ensures that repeated requests with the same identifier do not cause duplicate charges.

Which component acts as a secure messenger between customer and bank?

A Transaction Processor

B Database

C Load Balancer

D Payment Gateway

The payment gateway securely sends payment details to the bank and returns the response.

Which security standard is commonly followed in payment systems?

A PCI DSS

B ISO 9001

C HIPAA

D GDPR

PCI DSS is the Payment Card Industry Data Security Standard for protecting card data.

What is tokenization in payment handling?

A Logging transaction history

B Encrypting data with a key

C Replacing sensitive data with a non-sensitive equivalent

D Authenticating user identity

Tokenization replaces sensitive card details with tokens to reduce risk.

Which of these is NOT a typical step in payment processing?

A Settlement

B Compilation

C Authorization

D Capture

Compilation is unrelated; authorization, capture, and settlement are key steps.

Describe the flow of a payment transaction from customer to bank and back.

Think of the payment gateway as a secure messenger.

List and explain key security practices in payment handling systems.

Focus on protecting sensitive payment information.

Practice

1. What is the primary purpose of a payment handling system in software design?

easy

A. To store user profile pictures

B. To securely process and record financial transactions

C. To manage user login and authentication

D. To display product information to users

Solution

1. Step 1: Understand the role of payment handling

   Payment handling systems focus on managing money transfers safely and reliably.

2. Step 2: Identify the core function

   The core function is to process payments securely and keep records of transactions.

3. Final Answer:

   To securely process and record financial transactions -> Option B

4. Quick Check:

   Payment handling = Secure transaction processing [OK]

Hint: Payment handling means safe money transfer and record keeping [OK]

Common Mistakes:

• Confusing payment handling with user authentication
• Thinking it manages product display
• Assuming it stores user media files

2. Which of the following is the correct sequence of steps in a typical payment processing flow?

easy

A. Notify user -> Record transaction -> Process payment -> Validate payment details

B. Record transaction -> Validate payment details -> Notify user -> Process payment

C. Validate payment details -> Process payment -> Record transaction -> Notify user

D. Process payment -> Notify user -> Validate payment details -> Record transaction

Solution

1. Step 1: Identify logical payment flow order

   First, payment details must be validated to ensure correctness.

2. Step 2: Follow with processing, recording, and notifying

   After validation, payment is processed, transaction recorded, then user notified.

3. Final Answer:

   Validate payment details -> Process payment -> Record transaction -> Notify user -> Option C

4. Quick Check:

   Payment flow = Validate -> Process -> Record -> Notify [OK]

Hint: Payment flows from validation to processing, then record and notify [OK]

Common Mistakes:

• Not validating before processing
• Not recording transaction before notifying
• Mixing notification before processing

3. Consider this simplified payment processing pseudocode:

def process_payment(amount, card_info):
    if not validate_card(card_info):
        return "Invalid card"
    if amount <= 0:
        return "Invalid amount"
    if not charge_card(card_info, amount):
        return "Charge failed"
    record_transaction(card_info, amount)
    return "Payment successful"

What will be the output of process_payment(100, 'expired_card') if validate_card returns False for expired cards?

medium

A. "Invalid card"

B. "Charge failed"

C. "Payment successful"

D. "Invalid amount"

Solution

1. Step 1: Check card validation result

   Since validate_card returns False for expired cards, the first if condition triggers.

2. Step 2: Return error message immediately

   The function returns "Invalid card" without further processing.

3. Final Answer:

   "Invalid card" -> Option A

4. Quick Check:

   Expired card -> validate_card = False -> "Invalid card" [OK]

Hint: If validation fails, function returns error immediately [OK]

Common Mistakes:

• Assuming charge_card runs despite invalid card
• Confusing invalid amount with invalid card
• Expecting success despite validation failure

4. A payment system logs duplicate transactions when retrying failed payments. Which design fix will best prevent this issue?

medium

A. Use unique transaction IDs and check before recording

B. Increase payment timeout duration

C. Remove transaction logging entirely

D. Allow multiple retries without checks

Solution

1. Step 1: Identify cause of duplicate logs

   Retries cause repeated transaction records without uniqueness checks.

2. Step 2: Implement unique transaction IDs and check

   Assign unique IDs and verify before logging to avoid duplicates.

3. Final Answer:

   Use unique transaction IDs and check before recording -> Option A

4. Quick Check:

   Unique IDs prevent duplicate transaction logs [OK]

Hint: Unique IDs stop duplicate transaction records [OK]

Common Mistakes:

• Ignoring duplicate checks on retries
• Removing logging which loses audit trail
• Increasing timeout doesn't fix duplicates

5. You are designing a payment system that must handle 10,000 transactions per second with minimal latency and high reliability. Which architectural approach best supports this requirement?

hard

A. Store all payment requests in a single database table and process sequentially

B. Process all payments synchronously on a single server to ensure order

C. Use client-side scripts to process payments directly without server validation

D. Use a distributed message queue to process payments asynchronously with multiple worker nodes

Solution

1. Step 1: Analyze scalability and latency needs

   Handling 10,000 TPS requires distributing load and minimizing blocking.

2. Step 2: Choose asynchronous distributed processing

   Using a message queue with multiple workers allows parallel processing and reliability.

3. Step 3: Eliminate options causing bottlenecks or insecurity

   Single server or sequential DB processing causes bottlenecks; client-side processing lacks security.

4. Final Answer:

   Use a distributed message queue to process payments asynchronously with multiple worker nodes -> Option D

5. Quick Check:

   High TPS + low latency = distributed async processing [OK]

Hint: Distribute load with async workers for high TPS [OK]

Common Mistakes:

• Using single server causing bottlenecks
• Sequential DB processing slowing throughput
• Relying on client-side payment processing