Circuit Breaker Invoker

The circuit breaker prevents cascade failures by monitoring handler health per handler.

Configuration

invoker.NewCircuitBreaker(invoker.CircuitBreakerConfig{
    FailureThreshold: 5,
    SuccessThreshold: 2,
    OpenTimeout:      30 * time.Second,
}, metrics)

Field	Type	Description
FailureThreshold	int	Consecutive failures before opening
SuccessThreshold	int	Successes in half-open to close
OpenTimeout	time.Duration	Time before transitioning to half-open

States

     ┌──────────┐   failures >= threshold   ┌──────────┐
     │  Closed  │ ────────────────────────→ │   Open   │
     └──────────┘                           └──────────┘
          ↑                                      │
          │ successes >= threshold     timeout    │
          │                                      ↓
     ┌──────────────┐                     ┌──────────────┐
     │              │ ←────────────────── │  Half-Open   │
     └──────────────┘                     └──────────────┘
                          (on failure → Open again)

• Closed: Normal operation. Failures increment counter; successes reset it.
• Open: All requests immediately return ErrCircuitOpen. After OpenTimeout, transitions to half-open.
• Half-Open: Trial requests are allowed. Successes close the circuit; any failure reopens it.

Per-Handler Isolation

Each handler gets its own circuit breaker instance. A failing “send-email” handler won’t affect a healthy “update-db” handler.

Terminal Error Handling

Errors marked with ErrSendToDLQ (from the retry invoker) do not count as failures for the circuit breaker. This prevents business-level terminal errors from tripping the circuit.

Metrics

Metric	Type	Description
eventbus_circuit_state	Gauge	Current state (0=closed, 1=open, 2=half-open)
eventbus_circuit_open_total	Counter	Times circuit opened
eventbus_circuit_half_open_total	Counter	Times circuit went half-open
eventbus_circuit_blocked_total	Counter	Requests blocked by open circuit