Circuitbox

Circuitbox is a Ruby circuit breaker gem. It protects your application from failures of its service dependencies. It wraps calls to external services and monitors for failures in one minute intervals. Using a circuit's defaults once more than 5 requests have been made with a 50% failure rate, Circuitbox stops sending requests to that failing service for 90 seconds. This helps your application gracefully degrade.

Resources about the circuit breaker pattern:

• http://martinfowler.com/bliki/CircuitBreaker.html

Upgrading to 2.x? See 2.0 upgrade

Usage

Circuitbox.circuit(:your_service, exceptions: [Net::ReadTimeout]) do
  Net::HTTP.get URI('http://example.com/api/messages')
end

Circuitbox will return nil for failed requests and open circuits. If your HTTP client has its own conditions for failure, you can pass an exceptions option.

class ExampleServiceClient
  def circuit
    Circuitbox.circuit(:yammer, exceptions: [Zephyr::FailedRequest])
  end

  def http_get
    circuit.run(exception: false) do
      Zephyr.new("http://example.com").get(200, 1000, "/api/messages")
    end
  end
end

Using the run method will throw an exception when the circuit is open or the underlying service fails.

  def http_get
    circuit.run do
      Zephyr.new("http://example.com").get(200, 1000, "/api/messages")
    end
  end

Global Configuration

Circuitbox defaults can be configured through Circuitbox.configure. There are two defaults that can be configured:

• default_circuit_store - Defaults to a Circuitbox::MemoryStore. This can be changed to a compatible Moneta store.
• default_notifier - Defaults to Circuitbox::Notifier::ActiveSupport if ActiveSupport::Notifications is defined, otherwise defaults to Circuitbox::Notifier::Null

After configuring circuitbox through Circuitbox.configure, the internal circuit cache of Circuitbox.circuit is cleared.

Any circuit created manually through Circuitbox::CircuitBreaker before updating the configuration will need to be recreated to pick up the new defaults.

The following is an example Circuitbox configuration:

  Circuitbox.configure do |config|
    config.default_circuit_store = Circuitbox::MemoryStore.new
    config.default_notifier = Circuitbox::Notifier::Null.new
  end

Per-Circuit Configuration

class ExampleServiceClient
  def circuit
    Circuitbox.circuit(:your_service, {
      # exceptions circuitbox tracks for counting failures (required)
      exceptions:       [YourCustomException],

      # seconds the circuit stays open once it has passed the error threshold
      sleep_window:     300,

      # length of interval (in seconds) over which it calculates the error rate
      time_window:      60,

      # number of requests within `time_window` seconds before it calculates error rates (checked on failures)
      volume_threshold: 10,

      # the store you want to use to save the circuit state so it can be
      # tracked, this needs to be Moneta compatible, and support increment
      # this overrides what is set in the global configuration
      circuit_store: Circuitbox::MemoryStore.new,

      # exceeding this rate will open the circuit (checked on failures)
      error_threshold:  50,

      # Customized notifier
      # this overrides what is set in the global configuration
      notifier: Notifier.new
    })
  end
end

You can also pass a Proc as an option value which will evaluate each time the circuit breaker is used. This lets you configure the circuit breaker without having to restart the processes.

Circuitbox.circuit(:yammer, {
  sleep_window: Proc.new { Configuration.get(:sleep_window) },
  exceptions: [Net::ReadTimeout]
})

Circuit Store

Holds all the relevant data to trip the circuit if a given number of requests fail in a specified period of time. Circuitbox also supports Moneta. As moneta is not a dependency of circuitbox it needs to be loaded prior to use. There are a lot of moneta stores to choose from but some pre-requisits need to be satisfied first:

• Needs to support increment, this is true for most but not all available stores.
• Needs to support expiry.
• Needs to support bulk read.
• Needs to support concurrent access if you share them. For example sharing a KyotoCabinet store across process fails because the store is single writer multiple readers, and all circuits sharing the store need to be able to write.

Notifications

See Circuit Notifications

Faraday

Circuitbox ships with a Faraday HTTP client middleware. The versions of faraday the middleware has been tested against is >= 0.17 through ~> 2.0. The middleware does not support parallel requests through a connections in_parallel method.

require 'faraday'
require 'circuitbox/faraday_middleware'

conn = Faraday.new(:url => "http://example.com") do |c|
  c.use Circuitbox::FaradayMiddleware
end

response = conn.get("/api")
if response.success?
  # success
else
  # failure or open circuit
end

By default the Faraday middleware returns a 503 response when the circuit is open, but this as many other things can be configured via middleware options

• default_value value to return for open circuits, defaults to 503 response wrapping the original response given by the service and stored as original_response property of the returned 503, this can be overwritten with either
  • a static value
  • a lambda which is passed the original_response and original_error. original_response will be populated if Faraday returns an error response, original_error will be populated if an error was thrown before Faraday returned a response.

c.use Circuitbox::FaradayMiddleware, default_value: lambda { |response, error| ... }

• identifier circuit id, defaults to request url

c.use Circuitbox::FaradayMiddleware, identifier: "service_name_circuit"

• circuit_breaker_options options to initialize the circuit with defaults to { exceptions: Circuitbox::FaradayMiddleware::DEFAULT_EXCEPTIONS }. Accepts same options as Circuitbox:CircuitBreaker#new

c.use Circuitbox::FaradayMiddleware, circuit_breaker_options: {}

• open_circuit lambda determining what response is considered a failure, counting towards the opening of the circuit

c.use Circuitbox::FaradayMiddleware, open_circuit: lambda { |response| response.status >= 500 }

Installation

Add this line to your application's Gemfile:

gem 'circuitbox'

And then execute:

$ bundle

Or install it yourself as:

$ gem install circuitbox

Contributing

1. Fork it
2. Create your feature branch (git checkout -b my-new-feature)
3. Commit your changes (git commit -am 'Add some feature')
4. Push to the branch (git push origin my-new-feature)
5. Create new Pull Request