Circuit breaker pattern in HLD - Architecture Diagram

System Overview - Circuit breaker pattern

The Circuit Breaker pattern helps protect a system from repeated failures when calling external services. It detects failures and stops requests temporarily to avoid overloading the failing service, improving system stability and user experience.

Architecture Diagram

User
  |
  v
Load Balancer
  |
  v
API Gateway
  |
  v
Circuit Breaker
  |
  +-------------------+
  |                   |
  v                   v
Service A          Fallback Service
  |
  v
Database
  |
  v
Cache

Components

User

client

Initiates requests to the system

Load Balancer

load_balancer

Distributes incoming requests evenly across API Gateway instances

API Gateway

api_gateway

Handles client requests and routes them to services

Circuit Breaker

circuit_breaker

Monitors service health and stops requests to failing services temporarily

Service A

service

Processes business logic and queries database/cache

Fallback Service

service

Provides default or cached responses when Service A is unavailable

Database

database

Stores persistent data for Service A

Cache

cache

Stores frequently accessed data to reduce database load

Request Flow - 13 Hops

User → Load Balancer

Load Balancer → API Gateway

API Gateway → Circuit Breaker

Circuit Breaker → Service A

Service A → Cache

Cache → Service A

Service A → Database

Database → Service A

Service A → Cache

Service A → Circuit Breaker

Circuit Breaker → API Gateway

API Gateway → Load Balancer

Load Balancer → User

Failure Scenario

Component Fails:Service A

Impact:Circuit Breaker detects failures and opens circuit, stopping requests to Service A. Requests are routed to Fallback Service instead. Database and cache remain unaffected but new data cannot be processed.

Mitigation:Circuit Breaker opens to prevent overload. Fallback Service provides default or cached responses. Circuit Breaker periodically tests Service A to close circuit when healthy again.

Architecture Quiz - 3 Questions

What component decides to stop sending requests to a failing service?

A Circuit Breaker

B Load Balancer

C API Gateway

D Cache

Design Principle

The Circuit Breaker pattern protects the system from cascading failures by detecting service health and temporarily stopping requests to failing components. This improves system resilience and user experience by routing requests to fallback services and avoiding overload.

Practice

1. What is the primary purpose of the circuit breaker pattern in system design?

easy

A. To prevent repeated calls to a failing service and improve system stability

B. To increase the number of requests sent to a service

C. To store user session data efficiently

D. To encrypt data during transmission

Solution

1. Step 1: Understand the circuit breaker pattern role

   The circuit breaker pattern is designed to stop sending requests to a service that is failing repeatedly to avoid wasting resources and cascading failures.

2. Step 2: Identify the main benefit

   By preventing repeated calls to a failing service, it helps maintain overall system stability and improves user experience by failing fast.

3. Final Answer:

   To prevent repeated calls to a failing service and improve system stability -> Option A

4. Quick Check:

   Circuit breaker purpose = prevent repeated failing calls [OK]

Hint: Circuit breaker stops calls to failing services fast [OK]

Common Mistakes:

• Confusing circuit breaker with caching
• Thinking it increases request volume
• Mixing it up with encryption or session management

2. Which of the following correctly describes the open state in a circuit breaker?

easy

A. The circuit breaker allows all requests to pass through

B. The circuit breaker resets all counters to zero

C. The circuit breaker tests a limited number of requests

D. The circuit breaker blocks all requests to the failing service

Solution

1. Step 1: Recall the circuit breaker states

   The circuit breaker has three states: closed (normal operation), open (blocking requests), and half-open (testing requests).

2. Step 2: Define the open state behavior

   In the open state, the circuit breaker blocks all requests to the failing service to prevent further failures.

3. Final Answer:

   The circuit breaker blocks all requests to the failing service -> Option D

4. Quick Check:

   Open state = block requests [OK]

Hint: Open state means block all requests [OK]

Common Mistakes:

• Confusing open with closed or half-open states
• Thinking open state allows requests
• Assuming counters reset in open state

3. Consider this simplified pseudocode for a circuit breaker:

if failure_count > threshold:
    state = 'open'
if state == 'open':
    return 'fail fast'
else:
    call_service()

What will happen if failure_count exceeds the threshold?

medium

A. The service call will continue normally

B. The circuit breaker will enter half-open state

C. The circuit breaker will return 'fail fast' without calling the service

D. The failure count will reset automatically

Solution

1. Step 1: Analyze the condition for failure count

   If failure_count is greater than threshold, the state is set to 'open'.

2. Step 2: Check behavior when state is 'open'

   When state is 'open', the code returns 'fail fast' and does not call the service.

3. Final Answer:

   The circuit breaker will return 'fail fast' without calling the service -> Option C

4. Quick Check:

   Failure count > threshold = fail fast [OK]

Hint: Open state returns fail fast, no service call [OK]

Common Mistakes:

• Assuming service call still happens
• Confusing open with half-open state
• Thinking failure count resets automatically

4. A circuit breaker is stuck in the open state and never transitions to half-open. What is the most likely cause?

medium

A. The timeout to reset the circuit breaker is missing or too long

B. The service is always healthy

C. The failure count threshold is set too low

D. The circuit breaker is not counting failures

Solution

1. Step 1: Understand state transitions in circuit breaker

   The circuit breaker moves from open to half-open after a timeout period to test if the service has recovered.

2. Step 2: Identify cause of stuck open state

   If the timeout is missing or set too long, the circuit breaker will never try half-open state and remain open indefinitely.

3. Final Answer:

   The timeout to reset the circuit breaker is missing or too long -> Option A

4. Quick Check:

   Missing timeout causes stuck open state [OK]

Hint: Timeout missing or too long keeps breaker open [OK]

Common Mistakes:

• Confusing failure threshold with timeout
• Assuming service health affects state directly
• Ignoring the role of failure counting

5. You design a system using the circuit breaker pattern to call a payment service. The service fails intermittently. How should you configure the circuit breaker to balance availability and fault tolerance?

hard

A. Set a high failure threshold and long timeout to avoid blocking the service too soon

B. Set a low failure threshold and short timeout to quickly block and retry the service

C. Disable the circuit breaker to avoid blocking any requests

D. Set failure threshold to zero to block all requests immediately

Solution

1. Step 1: Understand intermittent failure impact

   Intermittent failures mean the service sometimes works and sometimes fails, so quick detection and retry is important.

2. Step 2: Choose configuration for balance

   A low failure threshold and short timeout allow the circuit breaker to quickly block failing calls and retry soon, improving fault tolerance and availability.

3. Final Answer:

   Set a low failure threshold and short timeout to quickly block and retry the service -> Option B

4. Quick Check:

   Low threshold + short timeout balances availability and fault tolerance [OK]

Hint: Low threshold and short timeout balance retries and blocking [OK]

Common Mistakes:

• Setting threshold too high delays failure detection
• Disabling circuit breaker risks cascading failures
• Setting threshold zero blocks all requests unnecessarily