Circuit Breaker

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NORA includes a per-registry circuit breaker that tracks upstream failures and short-circuits requests when a registry is known to be down. This avoids slow timeouts cascading into your build pipelines.

The circuit breaker is disabled by default and must be explicitly enabled.

Environment Variables

Variable	Default	Description
`NORA_CB_ENABLED`	`false`	Enable upstream circuit breaker
`NORA_CB_THRESHOLD`	`5`	Consecutive failures before opening the circuit
`NORA_CB_RESET_TIMEOUT`	`30`	Seconds to wait before probing a failed upstream

How It Works

The circuit breaker operates as a state machine with three states per upstream registry:

          success
  ┌─────────────────────┐
  │                     │
  ▼     N failures      │
Closed ──────────► Open ──(reset_timeout)──► HalfOpen
  ▲                  │                          │
  │                  │  reject (503 +           │ probe
  │                  │  Retry-After)            │ succeeds
  │                  ▼                          │
  │              caller gets 503               │
  └────────────────────────────────────────────┘
           probe fails → back to Open

Closed — Normal operation. All requests pass through to the upstream. Each failure increments a counter.

Open — The upstream has failed failure_threshold consecutive times. All proxy requests immediately receive 503 Service Unavailable with a Retry-After header. No upstream connections are attempted.

HalfOpen — After reset_timeout seconds, a single probe request is allowed through. If it succeeds, the breaker closes. If it fails, the breaker re-opens.

config.toml

[circuit_breaker]
enabled = true
failure_threshold = 5
reset_timeout = 30

Per-Registry Overrides

Override the threshold or timeout for specific registries. The key format is "registry:url":

[circuit_breaker]
enabled = true
failure_threshold = 5
reset_timeout = 30

[circuit_breaker.overrides."docker:https://registry-1.docker.io"]
failure_threshold = 10
reset_timeout = 120

[circuit_breaker.overrides."npm:https://registry.npmjs.org"]
failure_threshold = 3
reset_timeout = 60

This lets you be more tolerant of occasional Docker Hub failures while cutting off flaky npm upstreams faster.

Prometheus Metrics

Metric	Type	Labels	Description
`nora_circuit_breaker_state`	Gauge	`registry`	Current state: 0 = Closed, 1 = Open, 2 = HalfOpen
`nora_circuit_breaker_rejections_total`	Counter	`registry`	Total requests rejected by an open breaker

Example alert (Prometheus):

- alert: NoraCircuitBreakerOpen
  expr: nora_circuit_breaker_state == 1
  for: 5m
  labels:
    severity: warning
  annotations:
    summary: "NORA circuit breaker open for {{ $labels.registry }}"

Example: Docker Compose

services:
  nora:
    image: getnora/nora:latest
    environment:
      NORA_CB_ENABLED: "true"
      NORA_CB_THRESHOLD: "5"
      NORA_CB_RESET_TIMEOUT: "30"
    ports:
      - "4000:4000"