CircuitBreaker

Circuit Breaker Pattern Guide

The Circuit Breaker prevents cascading failures by temporarily blocking calls to failing services.

When to Use

• Calling external services that may fail
• Preventing resource waste on failing operations
• Fast-fail scenarios (don't wait for timeout)
• Protecting downstream services from overload

When NOT to Use

• Operations that must always attempt execution
• Single-attempt operations (use Retry instead)
• Operations with no downstream dependencies

Quick Start

uses Murphy.Policy.CircuitBreaker;

var
  Policy: ICircuitBreakerPolicy;
begin
  Policy := TCircuitBreakerBuilder
    .Handle(Exception)
    .Fail(3)
    .Within(TTimeSpan.FromSeconds(30))
    .Build;

  Policy.Execute(procedure begin CallExternalService; end);
end;

Common Scenarios

Protecting External API Calls

type
  TAPIClient = class
  private
    FCircuitBreaker: ICircuitBreakerPolicy;
  public
    constructor Create;
    function GetData: string;
  end;

constructor TAPIClient.Create;
begin
  FCircuitBreaker := TCircuitBreakerBuilder
    .Handle([EIdHTTPProtocolException, EIdSocketError])
    .Fail(5)
    .Within(TTimeSpan.FromMinutes(1))
    .Build;
end;

function TAPIClient.GetData: string;
begin
  try
    FCircuitBreaker.Execute(
      procedure
      begin
        Result := HTTP.Get('https://external-api.com/data');
      end);
  except
    on E: EBrokenCircuitException do
      raise Exception.Create('Service temporarily unavailable');
  end;
end;

Manual Circuit Control

var Policy: ICircuitBreakerPolicy;
begin
  Policy := TCircuitBreakerBuilder.Handle(Exception).Fail(3).Build;

  // Manual isolation for maintenance
  Policy.Isolate;

  // Later...
  Policy.Reset;
end;

Monitoring Circuit State

procedure MonitorService;
var
  Policy: ICircuitBreakerPolicy;
begin
  Policy := TCircuitBreakerBuilder.Handle(Exception).Fail(2).Build;

  while True do
  begin
    case Policy.CircuitState of
      TCircuitState.Closed:   WriteLn('Service: Healthy');
      TCircuitState.Open:     WriteLn('Service: DOWN');
      TCircuitState.HalfOpen: WriteLn('Service: Testing recovery');
      TCircuitState.Isolated: WriteLn('Service: Maintenance mode');
    end;

    Sleep(5000);
  end;
end;

Best Practices

1. Configure appropriate thresholds - Balance between sensitivity and tolerance
2. Set reasonable recovery times - Allow enough time for service recovery
3. Log circuit state changes - For monitoring and alerting
4. Handle broken circuit exceptions - Provide fallback behavior
5. Monitor circuit health - Track open/close events

State Machine

Closed → (failures >= threshold) → Open
Open → (duration elapsed) → HalfOpen
HalfOpen → (success) → Closed
HalfOpen → (failure) → Open

See Also

• Circuit Breaker API Reference
• Combining Patterns

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