Circuit Breaker Pattern in Python

Overview

The Circuit Breaker pattern is a design pattern used to improve system resilience by preventing continuous failures in a system. It acts as a safeguard by monitoring requests and stopping repeated failures from affecting performance.

How It Works

1. Closed State: The system operates normally and allows requests.
2. Open State: If failures exceed a threshold, the circuit "opens," stopping further requests for a cooldown period.
3. Half-Open State: After a cooldown, a few test requests are allowed. If they succeed, the circuit closes; otherwise, it stays open.

Why Use Circuit Breaker?

• Prevents cascading failures in distributed systems.
• Improves system reliability and recovery.
• Reduces unnecessary load on failing services.

Example Usage in Python

import uvicorn
from fastapi import FastAPI
from fastapi.exceptions import HTTPException
from circuit_breaker import CircuitBreakerMiddleware, CircuitBreakerInputDto

app = FastAPI()


@app.get("/success-endpoint")
async def success_endpoint():
    return "success"


@app.get("/failure-endpoint")
async def faulty_endpoint():
    raise HTTPException(status_code=500)


app.add_middleware(
    CircuitBreakerMiddleware,
    circuit_breaker_input=CircuitBreakerInputDto(
        exception_list=(Exception,),
        half_open_retry_count=3,
        half_open_retry_timeout_seconds=30,
    ),
)

if __name__ == "__main__":
    uvicorn.run(app, host="0.0.0.0", port=8000)