Stoplight

Stoplight is traffic control for code. It's an implementation of the circuit breaker pattern in Ruby.

⚠️️ You're currently browsing the documentation for Stoplight 4.x. If you're looking for the documentation of the previous version 3.x, you can find it here.

Does your code use unreliable systems, like a flaky database or a spotty web service? Wrap calls to those up in stoplights to prevent them from affecting the rest of your application.

Check out stoplight-admin for controlling your stoplights.

• Installation
• Basic Usage
  • Custom Errors
  • Custom Fallback
  • Custom Threshold
  • Custom Window Size
  • Custom Cool Off Time
  • Rails
• Setup
  • Data Store
    • Redis
  • Notifiers
    • IO
    • Logger
    • Community-supported Notifiers
    • How to Implement Your Own Notifier?
  • Rails
• Advanced Usage
  • Locking
  • Testing
• Maintenance Policy
• Credits

Add it to your Gemfile:

gem 'stoplight'

Or install it manually:

$ gem install stoplight

Stoplight uses Semantic Versioning. Check out the change log for a detailed list of changes.

To get started, create a stoplight:

light = Stoplight('example-pi')

Then you can run it with a block of code and it will return the result of calling the block. This is the green state. (The green state corresponds to the closed state for circuit breakers.)

light.run { 22.0 / 7 }
# => 3.142857142857143
light.color
# => "green"

If everything goes well, you shouldn't even be able to tell that you're using a stoplight. That's not very interesting though, so let's make stoplight fail.

When you run it, the error will be recorded and passed through. After running it a few times, the stoplight will stop trying and fail fast. This is the red state. (The red state corresponds to the open state for circuit breakers.)

light = Stoplight('example-zero')
# => #<Stoplight::CircuitBreaker:...>
light.run { 1 / 0 }
# ZeroDivisionError: divided by 0
light.run { 1 / 0 }
# ZeroDivisionError: divided by 0
light.run { 1 / 0 }
# Switching example-zero from green to red because ZeroDivisionError divided by 0
# ZeroDivisionError: divided by 0
light.run { 1 / 0 }
# Stoplight::Error::RedLight: example-zero
light.color
# => "red"

When the Stoplight changes from green to red, it will notify every configured notifier. See the notifiers section to learn more about notifiers.

The stoplight will move into the yellow state after being in the red state for a while. (The yellow state corresponds to the half open state for circuit breakers.) To configure how long it takes to switch into the yellow state, check out the cool off time section When stoplights are yellow, they will try to run their code. If it fails, they'll switch back to red. If it succeeds, they'll switch to green.

Some errors shouldn't cause your stoplight to move into the red state. Usually these are handled elsewhere in your stack and don't represent real failures. A good example is ActiveRecord::RecordNotFound.

To prevent some errors from changing the state of your stoplight, you can provide a custom block that will be called with the error and a handler Proc. It can do one of three things:

1. Re-raise the error. This causes Stoplight to ignore the error. Do this for errors like ActiveRecord::RecordNotFound that don't represent real failures.

2. Call the handler with the error. This is the default behavior. Stoplight will only ignore the error if it shouldn't have been caught in the first place. See Stoplight::Error::AVOID_RESCUING for a list of errors that will be ignored.

3. Do nothing. This is not recommended. Doing nothing causes Stoplight to never ignore the error. That means a NoMemoryError could change the color of your stoplights.

light = Stoplight('example-not-found')
  .with_error_handler do |error, handle|
    if error.is_a?(ActiveRecord::RecordNotFound)
      raise error
    else      
      handle.call(error)
    end
  end
# => #<Stoplight::CircuitBreaker:...>
light.run { User.find(123) }
# ActiveRecord::RecordNotFound: Couldn't find User with ID=123
light.run { User.find(123) }
# ActiveRecord::RecordNotFound: Couldn't find User with ID=123
light.run { User.find(123) }
# ActiveRecord::RecordNotFound: Couldn't find User with ID=123
light.color
# => "green"

By default, stoplights will re-raise errors when they're green. When they're red, they'll raise a Stoplight::Error::RedLight error. You can provide a fallback that will be called in both of these cases. It will be passed the error if the light was green.

light = Stoplight('example-fallback')
  .with_fallback { |e| p e; 'default' }
# => #<Stoplight::CircuitBreaker:..>
light.run { 1 / 0 }
# #<ZeroDivisionError: divided by 0>
# => "default"
light.run { 1 / 0 }
# #<ZeroDivisionError: divided by 0>
# => "default"
light.run { 1 / 0 }
# Switching example-fallback from green to red because ZeroDivisionError divided by 0
# #<ZeroDivisionError: divided by 0>
# => "default"
light.run { 1 / 0 }
# nil
# => "default"

Some bits of code might be allowed to fail more or less frequently than others. You can configure this by setting a custom threshold.

light = Stoplight('example-threshold')
  .with_threshold(1)
# => #<Stoplight::CircuitBreaker:...>
light.run { fail }
# Switching example-threshold from green to red because RuntimeError
# RuntimeError:
light.run { fail }
# Stoplight::Error::RedLight: example-threshold

The default threshold is 3.

By default, all recorded failures, regardless of the time these happen, will count to reach the threshold (hence turning the light to red). If needed, a window size can be set, meaning you can control how many errors per period of time will count to reach the red state.

By default, every recorded failure contributes to reaching the threshold, regardless of when it occurs, causing the stoplight to turn red. By configuring a custom window size, you control how errors are counted within a specified time frame. Here's how it works:

Let's say you set the window size to 2 seconds:

window_size_in_seconds = 2

light = Stoplight('example-threshold')
 .with_window_size(window_size_in_seconds)
 .with_threshold(1) #=> #<Stoplight::CircuitBreaker:...>

light.run { 1 / 0 } #=> #<ZeroDivisionError: divided by 0>
sleep(3)
light.run { 1 / 0 }

Without the window size configuration, the second light.run { 1 / 0 } call will result in a Stoplight::Error::RedLight exception being raised, as the stoplight transitions to the red state after the first call. With a sliding window of 2 seconds, only the errors that occur within the latest 2 seconds are considered. The first error causes the stoplight to turn red, but after 3 seconds (when the second error occurs), the window has shifted, and the stoplight switches to green state causing the error to raise again. This provides a way to focus on the most recent errors.

The default window size is infinity, so all failures counts.

Stoplights will automatically attempt to recover after a certain amount of time. A light in the red state for longer than the cool off period will transition to the yellow state. This cool off time is customizable.

light = Stoplight('example-cool-off')
  .with_cool_off_time(1)
# => #<Stoplight::CircuitBreaker:...>
light.run { fail }
# RuntimeError:
light.run { fail }
# RuntimeError:
light.run { fail }
# Switching example-cool-off from green to red because RuntimeError
# RuntimeError:
sleep(1)
# => 1
light.color
# => "yellow"
light.run { fail }
# RuntimeError:

The default cool off time is 60 seconds. To disable automatic recovery, set the cool off to Float::INFINITY. To make automatic recovery instantaneous, set the cool off to 0 seconds. Note that this is not recommended, as it effectively replaces the red state with yellow.

Stoplight was designed to wrap Rails actions with minimal effort. Here's an example configuration:

class ApplicationController < ActionController::Base
  around_action :stoplight

  private

  def stoplight(&block)
    Stoplight("#{params[:controller]}##{params[:action]}")
      .with_fallback do |error|
        Rails.logger.error(error)
        render(nothing: true, status: :service_unavailable)
      end
      .run(&block)
  end
end

Stoplight uses an in-memory data store out of the box.

require 'stoplight'
# => true
Stoplight.default_data_store
# => #<Stoplight::DataStore::Memory:...>

If you want to use a persistent data store, you'll have to set it up. Currently the only supported persistent data store is Redis.

Make sure you have the Redis gem installed before configuring Stoplight.

require 'redis'
# => true
redis = Redis.new
# => #<Redis client ...>
data_store = Stoplight::DataStore::Redis.new(redis)
# => #<Stoplight::DataStore::Redis:...>
Stoplight.default_data_store = data_store
# => #<Stoplight::DataStore::Redis:...>

Stoplight sends notifications to standard error by default.

Stoplight.default_notifiers
# => [#<Stoplight::Notifier::IO:...>]

If you want to send notifications elsewhere, you'll have to set them up.

Stoplight can notify not only into STDOUT, but into any IO object. You can configure the Stoplight::Notifier::IO notifier for that.

require 'stringio'

io = StringIO.new
# => #<StringIO:...>
notifier = Stoplight::Notifier::IO.new(io)
# => #<Stoplight::Notifier::IO:...>
Stoplight.default_notifiers += [notifier]
# => [#<Stoplight::Notifier::IO:...>, #<Stoplight::Notifier::IO:...>]

Stoplight can be configured to use the Logger class from the standard library.

require 'logger'
# => true
logger = Logger.new(STDERR)
# => #<Logger:...>
notifier = Stoplight::Notifier::Logger.new(logger)
# => #<Stoplight::Notifier::Logger:...>
Stoplight.default_notifiers += [notifier]
# => [#<Stoplight::Notifier::IO:...>, #<Stoplight::Notifier::Logger:...>]

• stoplight-sentry

You you want to implement your own notifier, the following section contains all the required information.

Pull requests to update this section are welcome.

A notifier has to implement the Stoplight::Notifier::Base interface:

def notify(light, from_color, to_color, error)
  raise NotImplementedError
end

For convenience, you can use the Stoplight::Notifier::Generic module. It takes care of the message formatting, and you have to implement only the put method, which takes message sting as an argument:

class IO < Stoplight::Notifier::Base
  include Generic
   
  private
    
  def put(message)
    @object.puts(message)
  end
end

Stoplight is designed to work seamlessly with Rails. If you want to use the in-memory data store, you don't need to do anything special. If you want to use a persistent data store, you'll need to configure it. Create an initializer for Stoplight:

# config/initializers/stoplight.rb
require 'stoplight'
Stoplight.default_data_store = Stoplight::DataStore::Redis.new(...)
Stoplight.default_notifiers += [Stoplight::Notifier::Logger.new(Rails.logger)]

Although stoplights can operate on their own, occasionally you may want to override the default behavior. You can lock a light using #lock(color) method. Color should be either Stoplight::Color::GREEN or Stoplight::Color::RED.

light = Stoplight('example-locked')
# => #<Stoplight::CircuitBreaker:..>
light.run { true }
# => true
light.lock(Stoplight::Color::RED)
# => #<Stoplight::CircuitBreaker:..>
light.run { true } 
# Stoplight::Error::RedLight: example-locked

Code in locked red lights may still run under certain conditions! If you have configured a custom data store and that data store fails, Stoplight will switch over to using a blank in-memory data store. That means you will lose the locked state of any stoplights.

You can go back to using the default behavior by unlocking the stoplight using #unlock.

light.unlock
# => #<Stoplight::CircuitBreaker:..>

Stoplights typically work as expected without modification in test suites. However there are a few things you can do to make them behave better. If your stoplights are spewing messages into your test output, you can silence them with a couple configuration changes.

Stoplight.default_error_notifier = -> _ {}
Stoplight.default_notifiers = []

If your tests mysteriously fail because stoplights are the wrong color, you can try resetting the data store before each test case. For example, this would give each test case a fresh data store with RSpec.

before(:each) do
  Stoplight.default_data_store = Stoplight::DataStore::Memory.new
end

Sometimes you may want to test stoplights directly. You can avoid resetting the data store by giving each stoplight a unique name.

stoplight = Stoplight("test-#{rand}")

Stoplight supports the latest three minor versions of Ruby, which currently are: 3.0.x, 3.1.x, and 3.2.x. Changing the minimum supported Ruby version is not considered a breaking change. We support the current stable Redis version (7.2) and the latest release of the previous major version (6.2.9)

Stoplight is brought to you by @camdez and @tfausak from @OrgSync. @bolshakov is the current maintainer of the gem. A complete list of contributors is available on GitHub. We were inspired by Martin Fowler's CircuitBreaker article.

Stoplight is licensed under the MIT License.