Knifeswitch

Yet another circuit breaker gem. This one strives to be as small and simple as possible. In the effort to remain simple, it currently only supports Rails with MySQL.

# Instantiate circuit
circuit = Knifeswitch::Circuit.new(
  namespace:       'whatever',
  exceptions:      [TimeoutExceptionToCatch, Timeout::Error],
  error_threshold: 5, # Open circuit after 5 consecutive errors
  error_timeout:   60 # Stay open for 60 seconds
)

response = circuit.run { client.request(...) }
# 'run' will raise Knifeswitch::CircuitOpen if its error_threshold has
# been exceeded: after a watched exception has been raised 5 times.
# 
# The error threshold counter is shared among all Knifeswitch::Circuit
# instances with the same namespace. The counters are stored in the db,
# so the state is fully distributed among all your workers/webservers.
#
# After the circuit opens, it will close back down after 60 seconds of
# rejecting requests (by raising Knifeswitch::CircuitOpen).
#
# When closed, it will just run the block like normal and return the result.

To "disable" knifeswitch globally, set the environment variable KNIFESWITCH=OFF. This makes all calls to Knifeswitch::Circuit#run yield unconditionally.

Add this line to your application's Gemfile:

gem 'knifeswitch'

And then execute:

$ bundle

Finally, generate migrations:

$ rake knifeswitch:create_migrations

$ bin/dockertest

You'll need to set up the test Rails app's database. Edit test/dummy/config/database.yml as you see fit, and run:

$ cd test/dummy && rake knifeswitch:create_migrations db:create db:migrate

After that you can run:

$ bin/test

in the project root with no problem. If you end up changing the migration generation rake task, you'll have to manually remove the old migration from the dummy app.

To keep the gem simple, Knifeswitch depends on Rails. Technically, it should be pretty simple to make Knifeswitch work without the Rails dependency, but for us since we use Rails it's easier to just keep it as is.

Knifeswitch softly depends on MySQL because it uses MySQL's ON DUPLICATE KEY UPDATE syntax.