Idempotency for safe retries in HLD - Architecture Diagram

System Overview - Idempotency for safe retries

This system ensures that repeated requests with the same intent do not cause duplicate effects. It supports safe retries by making operations idempotent, so clients can resend requests without risk of unintended side effects.

Key requirements include tracking request uniqueness, preventing duplicate processing, and providing consistent responses for repeated requests.

Architecture Diagram

User
  |
  v
Load Balancer
  |
  v
API Gateway
  |
  v
Idempotency Service
  |           |
  v           v
Cache       Backend Service
  |           |
  v           v
Database    Database

Components

User

client

Sends requests to the system, may retry on failure

Load Balancer

load_balancer

Distributes incoming requests evenly to API Gateway instances

API Gateway

api_gateway

Receives requests, performs authentication, routing, and forwards to Idempotency Service

Idempotency Service

service

Checks if request with same idempotency key was processed; returns cached response or forwards request

Cache

cache

Stores recent request results keyed by idempotency key for fast lookup

Backend Service

service

Processes the actual business logic for the request

Database

database

Stores persistent data and idempotency records for durability

Request Flow - 13 Hops

User → Load Balancer

Load Balancer → API Gateway

API Gateway → Idempotency Service

Idempotency Service → Cache

Cache → Idempotency Service

Idempotency Service → Backend Service

Backend Service → Database

Database → Backend Service

Backend Service → Idempotency Service

Idempotency Service → Cache

Idempotency Service → API Gateway

API Gateway → Load Balancer

Load Balancer → User

Failure Scenario

Component Fails:Cache

Impact:Idempotency Service cannot quickly find cached responses, causing repeated requests to hit Backend Service and Database, increasing load and latency.

Mitigation:System falls back to checking idempotency records in Database to avoid duplicate processing, though with higher latency.

Architecture Quiz - 3 Questions

Which component ensures that repeated requests with the same idempotency key do not cause duplicate processing?

A Backend Service

B Load Balancer

C Idempotency Service

D API Gateway

Design Principle

This architecture demonstrates the principle of idempotency by using a dedicated service and cache to detect and handle repeated requests safely. It prevents duplicate side effects by storing request results keyed by unique idempotency keys, enabling safe retries and improving user experience.

Practice

1. What is the main purpose of idempotency in system design for safe retries?

easy

A. To increase the number of retries allowed

B. To ensure repeated requests produce the same result without side effects

C. To speed up the processing of new requests

D. To encrypt data during transmission

Solution

1. Step 1: Understand idempotency concept

   Idempotency means that making the same request multiple times results in the same outcome without causing additional changes.

2. Step 2: Identify the purpose in retries

   In safe retries, idempotency prevents duplicate processing and side effects by returning the same result for repeated requests.

3. Final Answer:

   To ensure repeated requests produce the same result without side effects -> Option B

4. Quick Check:

   Idempotency = Same result, no side effects [OK]

Hint: Idempotency means retrying won't change the outcome [OK]

Common Mistakes:

• Confusing idempotency with performance optimization
• Thinking idempotency increases retry limits
• Assuming idempotency encrypts data

2. Which of the following is the correct way to implement idempotency in an API request?

easy

A. Retry the request without any tracking

B. Encrypt the request payload for security

C. Ignore request IDs and process every request as new

D. Use a unique request ID to detect duplicates and avoid reprocessing

Solution

1. Step 1: Identify idempotency implementation method

   Idempotency requires tracking requests to detect duplicates, typically using a unique request ID.

2. Step 2: Evaluate options

   Only using a unique request ID allows the system to recognize repeated requests and avoid reprocessing.

3. Final Answer:

   Use a unique request ID to detect duplicates and avoid reprocessing -> Option D

4. Quick Check:

   Unique request ID = Idempotency key [OK]

Hint: Unique request ID prevents duplicate processing [OK]

Common Mistakes:

• Retrying blindly without tracking
• Ignoring request IDs
• Confusing encryption with idempotency

3. Consider a system that stores results of processed requests keyed by a unique request ID. If a client sends the same request twice with the same ID, what will the system return on the second request?

medium

A. A new result generated by reprocessing

B. An error indicating duplicate request

C. The original result from the first request

D. A timeout response

Solution

1. Step 1: Understand stored result usage

   The system caches the result of the first request using the unique request ID.

2. Step 2: Determine response for repeated request

   When the same request ID is received again, the system returns the cached original result to maintain idempotency.

3. Final Answer:

   The original result from the first request -> Option C

4. Quick Check:

   Repeated request returns cached result [OK]

Hint: Same ID returns cached result, not reprocessed output [OK]

Common Mistakes:

• Expecting an error on duplicate requests
• Assuming reprocessing happens every time
• Confusing timeout with duplicate handling

4. A developer implemented idempotency by storing request IDs but forgot to store the response results. What problem will this cause on retries?

medium

A. The system will reprocess the request causing side effects

B. The system will return the original result correctly

C. The system will reject all requests

D. The system will crash due to missing data

Solution

1. Step 1: Analyze missing response storage impact

   Without storing the response, the system cannot return the original result on retries.

2. Step 2: Identify retry behavior

   Since the system lacks the cached result, it reprocesses the request, causing potential side effects and breaking idempotency.

3. Final Answer:

   The system will reprocess the request causing side effects -> Option A

4. Quick Check:

   Missing response storage = Reprocessing on retry [OK]

Hint: Store both request ID and response to avoid reprocessing [OK]

Common Mistakes:

• Assuming storing request ID alone is enough
• Expecting system to reject retries automatically
• Thinking system crashes without response data

5. You design a payment system that must be idempotent to handle retries safely. Which combination of techniques best ensures idempotency and prevents duplicate charges?

hard

A. Use unique request IDs, store payment results, and check before processing

B. Allow retries without tracking, rely on client to avoid duplicates

C. Encrypt payment data and retry immediately on failure

D. Process every request independently to ensure payment completion

Solution

1. Step 1: Identify idempotency requirements for payments

   Payments must not be charged multiple times on retries, so tracking requests and results is essential.

2. Step 2: Evaluate techniques for safe retries

   Using unique request IDs to detect duplicates, storing results to return on retries, and checking before processing prevents duplicate charges.

3. Final Answer:

   Use unique request IDs, store payment results, and check before processing -> Option A

4. Quick Check:

   Unique ID + stored result = Safe payment retries [OK]

Hint: Combine unique IDs and result storage for safe retries [OK]

Common Mistakes:

• Relying on client to avoid duplicates
• Ignoring result storage
• Processing every request without checks