Circuit Breaker Pattern: Building Resilient Microservices

The Circuit Breaker Pattern is a stability-oriented design pattern that prevents a network or service failure from cascading through your entire system.

🏗️ The Problem

In a microservices architecture, Service A calls Service B. If Service B is slow or failing, Service A might hang or fail while waiting for a response. This can lead to a “cascade failure,” where the entire system goes down because one service is slow.

🚀 The .NET Implementation (using Polly)

In .NET, Polly is the most popular library for implementing resiliency patterns like Circuit Breaker, Retry, and Timeout.

1. The States of the Circuit

Closed: The system is healthy, and requests flow normally.
Open: The system is failing. Requests are rejected immediately (Fast-Fail) to save resources.
Half-Open: A trial state. A few requests are allowed through to see if the target service has recovered.

2. Implementation with Polly

using Polly;
using Polly.CircuitBreaker;

// 1. Define the policy
var circuitBreakerPolicy = Policy
    .Handle<HttpRequestException>()
    .CircuitBreakerAsync(
        exceptionsAllowedBeforeBreaking: 3, // Break if 3 consecutive failures
        durationOfBreak: TimeSpan.FromSeconds(30) // Wait for 30s before trying again
    );

// 2. Use the policy
await circuitBreakerPolicy.ExecuteAsync(async () =>
{
    // The actual HTTP call!
    var response = await _httpClient.GetAsync("https://service-b/api/data");
    return response;
});

📊 Comparison: Retry vs. Circuit Breaker

Pattern	Purpose	When to use?
Retry	Handle “Transient” (temporary) errors.	”Glitch” in the network.
Circuit Breaker	Handle “Long-term” or “Major” failures.	Service B is down or overloaded.

💡 Why use Circuit Breaker?

Fast-Fail: Return an error quickly instead of letting the user wait for a long timeout.
Self-Healing: Allows the failing service time to recover without being hammered with new requests.
Graceful Degradation: Show a “cached” version of data or a friendly error message when the circuit is open.