State Machine Checker

Gem Version

Verify (as in prove) properties of your state machines.

Given a definition of a state machine and a CTL formula this library performs model checking to verify whether the formula is satisfied by the state machine.

For example, let's say we have implemented a very simple state machine to represent credit card payments:

class Payment
  state_machine initial: :checkout do
    event :started_processing do
      transition from: [:checkout], to: :processing

    event :finished_processing do
      transition from: [:processing], to: :completed

    event :processing_failed do
      transition from: [:processing], to: :failed

We might want to verify that every Payment eventually completes. This property can be represented as the CTL Formula AF completed — for all paths (A) eventually (F) completed is true. state_machine_checker includes a DSL for writing CTL properties and a rspec matcher, so we can write a spec to verify this property:

it "eventually completes" do
  formula = AF(:completed?)
  expect { }.to satisfy(formula)

This spec will fail and give the following message:

1) Payment eventually completes
     Failure/Error: expect { }.to satisfy(formula)

       Expected state machine for Payment#state to satisfy "AF(completed?)" but it does not.
       Counterexample: [:started_processing, :processing_failed]

A counterexample is given as a series of events: if started_processing is followed by processing_failed then a Payment will be in the failed state and will never reach completed.

For more examples see payment_spec.rb.

For a more in depth discussion and implementation details see the paper.


  • The state machine must be static — once the definition is parsed no states or transitions should be added or removed.
  • Atoms should only depend on the current state.
  • Only the state_machines gem is currently supported, but adapters for other state machine gems could be added.


Add this line to your application's Gemfile:

gem 'state_machine_checker'

And then execute:

$ bundle

Or install it yourself as:

$ gem install state_machine_checker


After checking out the repo, run bin/setup to install dependencies. Then, run rake spec to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and tags, and push the .gem file to


Bug reports and pull requests are welcome on GitHub at This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the Contributor Covenant code of conduct.


The gem is available as open source under the terms of the MIT License.

Code of Conduct

Everyone interacting in the StateMachineChecker project’s codebases, issue trackers, chat rooms and mailing lists is expected to follow the code of conduct.