StandardCircuit

Circuit breaker primitives for Rails apps, built on stoplight.

Wraps the upstream stoplight gem with:

• Opinionated default error taxonomy (network errors track; caller/config errors do not)
• SDK-specific adapter error bundles (Stripe, AWS, Faraday, SMTP)
• Rails event emission (standard_circuit.circuit.{opened,closed,degraded,fallback_invoked,registered}) with built-in Logger, Sentry, and Sentry::Metrics subscribers
• ActiveStorage S3 adapter with per-bucket circuit keying
• Generic ActionMailer delivery-method wrapper (supports both instance and symbol underlying: forms)
• Controller concern for standardized 503 responses on Stoplight::Error::RedLight
• Test helpers (force_open, force_closed, reset_force!) with RSpec auto-cleanup

# Gemfile
gem "standard_circuit", git: "https://github.com/rarebit-one/standard_circuit", ref: "<sha>"

Then run the install generator to drop a commented-out initializer into config/initializers/standard_circuit.rb:

bundle add standard_circuit
rails g standard_circuit:install

Pass --with-health-endpoint to also generate config/initializers/standard_circuit_health.rb (which requires the opt-in health controller); the generator prints the matching route line for you to add to config/routes.rb.

The generator is idempotent — re-running skips an existing initializer unless you pass --force.

# config/initializers/standard_circuit.rb
StandardCircuit.configure do |c|
  c.sentry_enabled = true
  c.metric_prefix = "external"

  c.register(:stripe,
    threshold: 5,
    cool_off_time: 30,
    tracked_errors: StandardCircuit::ErrorTaxonomies::Stripe.tracked,
    skipped_errors: StandardCircuit::AdapterErrors::Stripe.caller_errors)
end

# anywhere in app code
StandardCircuit.run(:stripe) do
  Stripe::PaymentIntent.create(amount:, currency:)
end

Circuit state (failure counts, colors, locks) lives in a Stoplight data store. StandardCircuit defaults to Stoplight::DataStore::Memory.new, which is per-process: each Puma worker, Sidekiq/SolidQueue worker, and console gets its own independent view of every circuit.

That is the deliberate default for a Redis-free deployment, and it is usually the right one — a circuit exists to stop this process from hammering a dead upstream, and per-process thresholds mean one unlucky worker can't trip the breaker for everyone. But be explicit about what it implies:

• Thresholds are counted per process, so an app with 4 web workers tolerates roughly 4× the configured threshold in aggregate before every worker has tripped.
• /health reports the circuit colors of the process that served the request, so two consecutive probes can legitimately disagree while a circuit is tripping.
• force_open / force_closed and reset! affect only the calling process — they are test and console tools, not an operational kill switch.

Point data_store at a shared store if you want cross-process state instead:

StandardCircuit.configure do |c|
  # Default — per-process, no external dependency.
  c.data_store = Stoplight::DataStore::Memory.new

  # Shared across processes and hosts (requires the redis gem + a Redis server).
  # c.data_store = Stoplight::DataStore::Redis.new(Redis.new(url: ENV["REDIS_URL"]))
end

The built-in Sentry subscriber is on by default (c.sentry_enabled = true) and reports every circuit-open transition at a flat :warning, with the circuit name, colors, and error in extra.

Set sentry_criticality_levels to derive the level from the circuit's registered criticality instead. That also adds circuit / circuit_criticality tags and a stable ["circuit-open", <circuit>] fingerprint, so Sentry alert rules can route on criticality (e.g. page on circuit_criticality:critical) and group per circuit:

StandardCircuit.configure do |c|
  # { critical: :error, standard: :warning, optional: :info }
  c.sentry_criticality_levels = true

  # Or override part of that map — unlisted criticalities keep the default.
  # c.sentry_criticality_levels = { optional: :debug }
end

This is opt-in, not the default. Both the level and the fingerprint feed Sentry's alerting and issue grouping, so turning it on for existing apps at gem-upgrade time would silently change what pages and re-group open issues. Leaving sentry_criticality_levels unset keeps the flat :warning shape byte-for-byte.

If you want something else entirely, set c.sentry_enabled = false and subscribe to standard_circuit.circuit.opened yourself — the payload carries criticality.

Every circuit lifecycle moment is emitted as a Rails event. On Rails 8.1+ the canonical bus is Rails.event; on older Rails versions the gem transparently falls back to ActiveSupport::Notifications. Detection happens per-emit, so subscribers do not need to care which backend is live.

Note on standard_circuit.run.completed: the per-call event for cost / latency / success-rate dashboards. Fires on every Runner#execute invocation and on force_open runs; not emitted for force_closed runs (which intentionally bypass the runner). All payload keys are always present — error_class and error_message are nil on :success. Payload duration uses duration_ms (numeric), not event.duration, so subscribers work identically on the Rails.event and ActiveSupport::Notifications backends.

Note on standard_circuit.circuit.registered: subscribers are wired up after the StandardCircuit.configure block yields, so any c.register calls inside that block fire before any subscriber can hear them. This event is reliable only for post-boot, dynamic register / register_prefix calls — do not rely on it for a boot-time circuit inventory.

Built-in subscribers (Logger / Sentry / Metrics) are registered automatically by the gem's Railtie. Host apps can subscribe to the namespace however they like:

# Rails 8.1+
class MyAuditSubscriber
  def emit(event)
    return unless event[:name].start_with?("standard_circuit.")
    Rails.logger.info("circuit event: #{event[:name]} #{event[:payload].inspect}")
  end
end
Rails.event.subscribe(MyAuditSubscriber.new)

# Older Rails
ActiveSupport::Notifications.subscribe(/\Astandard_circuit\./) do |name, _start, _finish, _id, payload|
  Rails.logger.info("circuit event: #{name} #{payload.inspect}")
end

# Quick host-supplied callable (auto-wired at boot via the Railtie)
StandardCircuit.configure do |c|
  c.add_notifier(->(name, payload) { MyAlerting.notify(name, payload) })
end

ControllerSupport.circuit_open_fallback only works for non-streaming responses — once a Live controller has flushed any output, Rails can't render an error template over the wire. For a streaming controller, catch Stoplight::Error::RedLight inside the streaming proc and write a degraded payload before the stream closes:

class Api::MessagesController < ApplicationController
  include ActionController::Live

  def stream
    response.headers["Content-Type"] = "application/x-ndjson"

    StandardCircuit.run(:openai) do
      llm.stream do |chunk|
        response.stream.write({ delta: chunk }.to_json + "\n")
      end
    end
  rescue Stoplight::Error::RedLight
    # Only reachable when the circuit was already open at call time —
    # Stoplight raises RedLight before executing the block, not mid-stream.
    # Errors raised mid-stream propagate as their original class through the
    # `ensure` below; add a broader rescue if you also need to write a
    # terminal NDJSON line for those.
    response.stream.write({ error: "service_unavailable" }.to_json + "\n")
  ensure
    response.stream.close
  end
end

Same pattern applies in background jobs (where circuit_open_fallback doesn't help): wrap the work in StandardCircuit.run and rescue Stoplight::Error::RedLight to either discard_on (avoid thundering retries) or retry_on with backoff (defer until cool-off), depending on whether eventual delivery is required.

StandardCircuit ships an opt-in controller that renders StandardCircuit.health_report as JSON. It returns 503 when the rolled-up status is :critical (so orchestrators pull the instance out of rotation) and 200 otherwise.

It's opt-in — not auto-required — so apps that don't want a health route don't pay for it.

# config/routes.rb
require "standard_circuit/health_controller"

Rails.application.routes.draw do
  get "/health", to: "standard_circuit/health#show"
end

The controller inherits from ActionController::API to sidestep app-level filters (authentication, bootstrap redirects, etc.) so probes can call it anonymously.

If your app also mounts StandardHealth::Engine at /health, draw the aggregate route first:

get "/health", to: "standard_circuit/health#show"        # aggregate — FIRST
mount StandardHealth::Engine => "/health", as: :standard_health

StandardHealth::Engine registers sub-paths only (/alive, /ready, /diagnostics/env) — it never serves the aggregate tier itself. An app that mounts the engine and assumes /health is covered silently has no aggregate tier at all, with no boot error and no failing route spec to reveal it. The ordering is load-bearing; draw the aggregate route explicitly, first.

MIT