Circuit Breaker Pattern

💡 What Is the Circuit Breaker Pattern?

The Circuit Breaker is a reliability pattern used when one service calls another (e.g., your API calling a payment gateway).
It stops repeatedly calling a failing service to avoid wasting resources and causing more failures.

Think of it like an electrical circuit breaker in your house:
If there’s a problem, it “trips” and cuts the flow until things are safe.

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🏷️ Why We Need It

• Prevents cascading failures across microservices.

• Gives the failing service time to recover.

• Improves user experience (you can show a quick fallback message instead of waiting for timeouts).

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⚙️ How It Works

A circuit breaker has three states:

1. Closed (normal)

   • All requests pass through.

   • If calls keep failing beyond a threshold (e.g., 5 failures in 30 seconds), it opens.

2. Open (tripped)

   • Requests are blocked immediately (or a fallback is returned).

   • The system waits a fixed time (e.g., 1 minute).

3. Half-Open

   • After the wait, it lets a few test requests through.

   • If they succeed → back to Closed.

   • If they fail → back to Open.

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Simple Diagram

[Closed] -- too many failures --> [Open]
[Open] -- wait & try --> [Half-Open]
[Half-Open] -- success --> [Closed]
[Half-Open] -- fail --> [Open]

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🛠️ Using in .NET (with Polly)

Polly is a popular .NET library for resilience.

Example:

using Polly;
using Polly.CircuitBreaker;

var circuitBreaker = Policy
    .Handle<HttpRequestException>()
    .CircuitBreakerAsync(
        exceptionsAllowedBeforeBreaking: 3,
        durationOfBreak: TimeSpan.FromSeconds(30)
    );

await circuitBreaker.ExecuteAsync(async () =>
{
    // Your external call, e.g., HTTP to payment service
    await httpClient.GetAsync("https://payment.example.com/process");
});

Explanation:

• After 3 failures, the circuit goes Open.

• It stays open for 30 seconds before trying again.

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✅ Best Practices

• Set realistic thresholds (failure count, duration).

• Provide fallback logic (cached data, default response).

• Monitor and log state changes.

• Combine with retry (exponential backoff) for transient errors.

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🔑 Summary Table

State	What Happens
Closed	Normal, calls go through.
Open	Calls fail fast, no external calls.
Half-Open	Test a few requests to check recovery.

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🏁 Quick Recap

The Circuit Breaker pattern:

• Detects repeated failures when calling external services.

• Opens the circuit to stop calls temporarily.

• Half-opens to test if the service has recovered.

• In .NET, use Polly to implement it easily.

This protects your microservices or APIs from cascading outages and keeps your system responsive even when dependencies fail.