Circuit Breakers for well designed applications.
Add this line to your application's Gemfile:
And then execute:
Or install it yourself as:
$ gem install breaker
Circuit Breaker Pattern
The circuit breaker pattern is described in the (wonderful) book Release It by Michael T. Nygard. A circuit breaker in programming terms is modeled after a circuit breaker in the real world. A circuit breaker protects the larger system from failures in other systems. They are especially useful for protecting an application from finnicky remote systems.
The circuit is a state machine. It has three states: open, closed, and
half-open. The circuit starts off in
closed state--normal operation.
If the operation failures N times (the failure threshold) the circuit
open. Calls in the
open state fail immediately with an
an exception. After a specified time period has passed (retry
timeout) the circuit moves into
half-open. Calls happen normally. If
a call fails the state moves to
open. If the call suceeds it moves
closed. All calls are capped with a timeout. If a timeouts count
Most interaction should go through
Breaker.circuit. This is a
factory method that creates
Breaker::Circuit objects. It requires
one argument: the name. It also takes an
options hash for
customizing the circuit. Thirdly it takes a block to run inside the
circuit. Here are some examples:
# Simplest example: protect some code with a circuit breaker Breaker.circuit 'twitter' do Tweet.post 'Oh Hai' end
Breaker.circuit is an upsert opertion. It will create or update an
existing circuit. Pass the options hash to customize the circuit's
circuit = Breaker.circuit 'twitter', timeout: 5 circuit.run do Tweet.post 'Oh Hai' end
Breaker::Circuit instances which can be
saved for later. There use depends no how persistence works.
Circuit breakers are only really useful in a large system (perhaps distributed). Some information must be shared acrosss subsystems. Say there are 5 different services in the system. Each is talking to 2 different systems protected by circuit breakers. Either of the systems go down. It's natural that the failure should propogate through the system so that each service knows the shared ones are down. This is where state and persistence come into play.
The breaker gems bundles a simple in memory repository. This is process specific. If you need to share state across multiple processes then you must write your own repository.
The repository manages fueses (in the eletrical sense). A fuse
maintains state. The repository must implement one method:
which creates or updates a fuse given by name. The repistory can
return some sort of persistent fuse where writer methods write to
lib/breaker/in_memory_repo.rb for an example. The class is
- Fork it
- Create your feature branch (
git checkout -b my-new-feature)
- Commit your changes (
git commit -am 'Add some feature')
- Push to the branch (
git push origin my-new-feature)
- Create new Pull Request