Workflow

Note: you can find documentation for specific workflow rubygem versions at http://rubygems.org/gems/workflow : select a version (optional, default is latest release), click "Documentation" link. When reading on github.com, the README refers to the upcoming release.

Table of Contents

What is workflow?
Getting started
State persistence
Advanced usage
Documenting with diagrams
Support, Participation

What is workflow?

Workflow is a finite-state-machine-inspired API for modeling and interacting with what we tend to refer to as 'workflow'.

A lot of business modeling tends to involve workflow-like concepts, and the aim of this library is to make the expression of these concepts as clear as possible, using similar terminology as found in state machine theory.

So, a workflow has a state. It can only be in one state at a time. When a workflow changes state, we call that a transition. Transitions occur on an event, so events cause transitions to occur. Additionally, when an event fires, other arbitrary code can be executed, we call those actions. So any given state has a bunch of events, any event in a state causes a transition to another state and potentially causes code to be executed (an action). We can hook into states when they are entered, and exited from, and we can cause transitions to fail (guards), and we can hook in to every transition that occurs ever for whatever reason we can come up with.

Now, all that’s a mouthful, but we’ll demonstrate the API bit by bit with a real-ish world example.

Let’s say we’re modeling article submission from journalists. An article is written, then submitted. When it’s submitted, it’s awaiting review. Someone reviews the article, and then either accepts or rejects it. Here is the expression of this workflow using the API:

class Article
  include Workflow
  workflow do
    state :new do
      event :submit, :transitions_to => :awaiting_review
    end
    state :awaiting_review do
      event :review, :transitions_to => :being_reviewed
    end
    state :being_reviewed do
      event :accept, :transitions_to => :accepted
      event :reject, :transitions_to => :rejected
    end
    state :accepted
    state :rejected
  end
end

Nice, isn’t it!

Note: the first state in the definition (:new in the example, but you can name it as you wish) is used as the initial state - newly created objects start their life cycle in that state.

Let’s create an article instance and check in which state it is:

article = Article.new
article.accepted? # => false
article.new? # => true

You can also access the whole current_state object including the list of possible events and other meta information:

article.current_state
=> #<Workflow::State:0x7f1e3d6731f0 @events={
  :submit=>#<Workflow::Event:0x7f1e3d6730d8 @action=nil,
    @transitions_to=:awaiting_review, @name=:submit, @meta={}>},
  name:new, meta{}

You can also check, whether a state comes before or after another state (by the order they were defined):

article.current_state # => being_reviewed
article.current_state < :accepted # => true
article.current_state >= :accepted # => false
article.current_state.between? :awaiting_review, :rejected # => true

Now we can call the submit event, which transitions to the :awaiting_review state:

article.submit!
article.awaiting_review? # => true

Events are actually instance methods on a workflow, and depending on the state you’re in, you’ll have a different set of events used to transition to other states.

It is also easy to check, if a certain transition is possible from the current state . article.can_submit? checks if there is a :submit event (transition) defined for the current state.

Getting started

Installation

gem install workflow

Important: If you’re interested in graphing your workflow state machine, you will also need to install the activesupport and ruby-graphviz gems.

Versions up to and including 1.0.0 are also available as a single file download - lib/workflow.rb file.

Examples

After installation or downloading the library you can easily try out all the example code from this README in irb.

$ irb
require 'rubygems'
require 'workflow'

Now just copy and paste the source code from the beginning of this README file snippet by snippet and observe the output.

Transition event handler

The best way is to use convention over configuration and to define a method with the same name as the event. Then it is automatically invoked when event is raised. For the Article workflow defined earlier it would be:

class Article
  def reject
    puts 'sending email to the author explaining the reason...'
  end
end

article.review!; article.reject! will cause state transition to being_reviewed state, persist the new state (if integrated with ActiveRecord), invoke this user defined reject method and finally persist the rejected state.

Note: on successful transition from one state to another the workflow gem immediately persists the new workflow state with update_column(), bypassing any ActiveRecord callbacks including updated_at update. This way it is possible to deal with the validation and to save the pending changes to a record at some later point instead of the moment when transition occurs.

You can also define event handler accepting/requiring additional arguments:

class Article
  def review(reviewer = '')
    puts "[#{reviewer}] is now reviewing the article"
  end
end

article2 = Article.new
article2.submit!
article2.review!('Homer Simpson') # => [Homer Simpson] is now reviewing the article

Alternative way is to use a block (only recommended for short event implementation without further code nesting):

event :review, :transitions_to => :being_reviewed do |reviewer|
  # store the reviewer
end

We’ve noticed, that mixing the list of events and states with the blocks invoked for particular transitions leads to a bumpy and poorly readable code due to a deep nesting. We tried (and dismissed) lambdas for this. Eventually we decided to invoke an optional user defined callback method with the same name as the event (convention over configuration) as explained before.

State persistence

ActiveRecord

Note: Workflow 2.0 is a major refactoring for the worklow library. If your application suddenly breaks after the workflow 2.0 release, you’ve probably got your Gemfile wrong ;-). workflow uses semantic versioning. For highest compatibility please reference the desired major+minor version.

Note on ActiveRecord/Rails 4.*, 5.\* Support:

Since integration with ActiveRecord makes over 90% of the issues and maintenance effort, and also to allow for an independent (faster) release cycle for Rails support, starting with workflow version 2.0 in January 2019 the support for ActiveRecord (4.*, 5.\* and newer) has been extracted into a separate gem. Read at workflow-activerecord, how to include the right gem.

To use legacy built-in ActiveRecord 2.3 - 4.* support, reference Workflow 1.2 in your Gemfile:

gem 'workflow', '~> 1.2'

Custom workflow state persistence

If you do not use a relational database and ActiveRecord, you can still integrate the workflow very easily. To implement persistence you just need to override load_workflow_state and persist_workflow_state(new_value) methods. Next section contains an example for using CouchDB, a document oriented database.

Tim Lossen implemented support for remodel / redis key-value store.

Integration with CouchDB

We are using the compact couchtiny library here. But the implementation would look similar for the popular couchrest library.

require 'couchtiny'
require 'couchtiny/document'
require 'workflow'

class User < CouchTiny::Document
  include Workflow
  workflow do
    state :submitted do
      event :activate_via_link, :transitions_to => :proved_email
    end
    state :proved_email
  end

  def load_workflow_state
    self[:workflow_state]
  end

  def persist_workflow_state(new_value)
    self[:workflow_state] = new_value
    save!
  end
end

Please also have a look at the full source code.

Adapters to support other databases

I get a lot of requests to integrate persistence support for different databases, object-relational adapters, column stores, document databases.

To enable highest possible quality, avoid too many dependencies and to avoid unneeded maintenance burden on the workflow core it is best to implement such support as a separate gem.

Only support for the ActiveRecord will remain for the foreseeable future. So Rails beginners can expect workflow to work with Rails out of the box. Other already included adapters stay for a while but should be extracted to separate gems.

If you want to implement support for your favorite ORM mapper or your favorite NoSQL database, you just need to implement a module which overrides the persistence methods load_workflow_state and persist_workflow_state. Example:

module Workflow
  module SuperCoolDb
    module InstanceMethods
      def load_workflow_state
        # Load and return the workflow_state from some storage.
        # You can use self.class.workflow_column configuration.
      end

      def persist_workflow_state(new_value)
        # save the new_value workflow state
      end
    end

    module ClassMethods
      # class methods of your adapter go here
    end

    def self.included(klass)
      klass.send :include, InstanceMethods
      klass.extend ClassMethods
    end
  end
end

The user of the adapter can use it then as:

class Article
  include Workflow
  include Workflow:SuperCoolDb
  workflow do
    state :submitted
    # ...
  end
end

I can then link to your implementation from this README. Please let me also know, if you need any interface beyond load_workflow_state and persist_workflow_state methods to implement an adapter for your favorite database.

Advanced usage

Conditional event transitions

Conditions can be a "method name symbol" with a corresponding instance method, a proc or lambda which are added to events, like so:

state :off
  event :turn_on, :transition_to => :on,
                  :if => :sufficient_battery_level?

  event :turn_on, :transition_to => :low_battery,
                  :if => proc { |device| device.battery_level > 0 }
end

# corresponding instance method
def sufficient_battery_level?
  battery_level > 10
end

When calling a device.can_<fire_event>? check, or attempting a device.<event>!, each event is checked in turn:

With no :if check, proceed as usual.
If an :if check is present, proceed if it evaluates to true, or drop to the next event.
If you’ve run out of events to check (eg. battery_level == 0), then the transition isn’t possible.

You can also pass additional arguments, which can be evaluated by :if methods or procs. See examples in conditionals_test.rb

Advanced transition hooks

on_entry/on_exit

We already had a look at the declaring callbacks for particular workflow events. If you would like to react to all transitions to/from the same state in the same way you can use the on_entry/on_exit hooks. You can either define it with a block inside the workflow definition or through naming convention, e.g. for the state :pending just define the method on_pending_exit(new_state, event, *args) somewhere in your class.

on_transition

If you want to be informed about everything happening everywhere, e.g. for logging then you can use the universal on_transition hook:

workflow do
  state :one do
    event :increment, :transitions_to => :two
  end
  state :two
  on_transition do |from, to, triggering_event, *event_args|
    Log.info "#{from} -> #{to}"
  end
end

on_error

If you want to do custom exception handling internal to workflow, you can define an on_error hook in your workflow. For example:

workflow do
  state :first do
    event :forward, :transitions_to => :second
  end
  state :second

  on_error do |error, from, to, event, *args|
    Log.info "Exception(#{error.class}) on #{from} -> #{to}"
  end
end

If forward! results in an exception, on_error is invoked and the workflow stays in a 'first' state. This capability is particularly useful if your errors are transient and you want to queue up a job to retry in the future without affecting the existing workflow state.

Guards

If you want to halt the transition conditionally, you can just raise an exception in your [transition event handler](#transition_event_handler). There is a helper called halt!, which raises the Workflow::TransitionHalted exception. You can provide an additional halted_because parameter.

def reject(reason)
  halt! 'We do not reject articles unless the reason is important' \
    unless reason =~ /important/i
end

The traditional halt (without the exclamation mark) is still supported too. This just prevents the state change without raising an exception.

You can check halted? and halted_because values later.

Hook order

The whole event sequence is as follows:

before_transition
event specific action
on_transition (if action did not halt)
on_exit
PERSIST WORKFLOW STATE (i.e. transition) or on_error
on_entry
after_transition

Accessing your workflow specification

You can easily reflect on workflow specification programmatically - for the whole class or for the current object. Examples:

article2.current_state.events # lists possible events from here
article2.current_state.events[:reject].transitions_to # => :rejected

Article.workflow_spec.states.keys
#=> [:rejected, :awaiting_review, :being_reviewed, :accepted, :new]

Article.workflow_spec.state_names
#=> [:rejected, :awaiting_review, :being_reviewed, :accepted, :new]

# list all events for all states
Article.workflow_spec.states.values.collect &:events

You can also store and later retrieve additional meta data for every state and every event:

class MyProcess
  include Workflow
  workflow do
    state :main, :meta => {:importance => 8}
    state :supplemental, :meta => {:importance => 1}
  end
end
puts MyProcess.workflow_spec.states[:supplemental].meta[:importance] # => 1

The workflow library itself uses this feature to tweak the graphical representation of the workflow. See below.

Defining workflow dynamically from JSON

For an advance example please see workflow_from_json_test.rb.

Compose workflow definition with `include`

In case you have very extensive workflow definition or would like to reuse workflow definition for different classes, you can include parts like in the including a child workflow definition example.

Documenting with diagrams

You can generate a graphical representation of the workflow for a particular class for documentation purposes. Use Workflow::create_workflow_diagram(class) in your rake task like:

namespace :doc do
  desc "Generate a workflow graph for a model passed e.g. as 'MODEL=Order'."
  task :workflow => :environment do
    require 'workflow/draw'
    Workflow::Draw::workflow_diagram(ENV['MODEL'].constantize)
  end
end

Support, Participation

Reporting bugs

http://github.com/geekq/workflow/issues

Development Setup

sudo apt-get install graphviz # Linux
brew install graphviz # Mac OS
cd workflow
gem install bundler
bundle install
# run all the tests
bundle exec rake test

Check list for you pull request

❏ unit tests for the new behavior provided: new tests fail without you change, all tests succeed with your change
❏ documentation update included

Other 3rd party libraries

ActiveAdmin-Workflow - is an integration with ActiveAdmin.

About

Author: Vladimir Dobriakov, https://infrastructure-as-code.de

Based on the work of Ryan Allen and Scott Barron

Licensed under MIT license, see the MIT-LICENSE file.

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