Workflow Orchestrator

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A ruby DSL for modeling business logic as Finite State Machines.

The aim of this library is to make the expression of these concepts as clear as possible, utilizing the expressiveness of ruby language, and using similar terminology as found in state machine theory.

Concepts

  • State: A workflow is in exactly one state at a time. State may optionally be persisted using ActiveRecord.
  • State transition: Change of state can be observed and intercepted
  • Events: Events cause state transitions to occur
  • Actions: Actions constitute of parts of our business logic which are executed in response to state transitions.

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.

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:0x007fa1ab36f750
#  @events={:submit=>#<Workflow::Event:0x007fa1ab36f638 @action=nil, @meta={}, @name=:submit, @transitions_to=:awaiting_review>},
#  @meta={},
#  @name=:new

On Ruby 1.9 and above, you can check whether a state comes before or after another state (by the order they were defined):

article.current_state.name
# => 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.

Installation

`gem install workflow`

`include Workflow` in your model.

If you're using ActiveRecord, Workflow will by default use a "workflow_state" column on your model.

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

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

The old, deprecated way

The old way, using a block is still supported but deprecated:

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.

Integration with ActiveRecord

Workflow library can handle the state persistence fully automatically. You only need to define a string field on the table called workflow_state and include the workflow mixin in your model class as usual:

class Order < ActiveRecord::Base
  include Workflow
  workflow do
    # list states and transitions here
  end
end

On a database record loading all the state check methods e.g. article.state, article.awaiting_review? are immediately available. For new records or if the workflow_state field is not set the state defaults to the first state declared in the workflow specification. In our example it is :new, so Article.new.new? returns true and Article.new.approved? returns false.

At the end of a successful state transition like article.approve! the new state is immediately saved in the database.

You can change this behaviour by overriding persist_workflow_state method.

Scopes

Workflow library also adds automatically generated scopes with names based on states names:

class Order < ActiveRecord::Base
  include Workflow
  workflow do
    state :approved
    state :pending
  end
end

# returns all orders with `approved` state
Order.with_approved_state

# returns all orders except for those having `approved` state
Order.without_approved_state

# returns all orders except for those having `pending` state
Order.without_pending_state

Custom workflow database column

meuble contributed a solution for using custom persistence column easily, e.g. for a legacy database schema:

class LegacyOrder < ActiveRecord::Base
  include Workflow

  workflow_column :foo_bar # use this legacy database column for
                           # persistence
end

You can also set the column name inline into the workflow block:

class LegacyOrder < ActiveRecord::Base
  include Workflow

  workflow :foo_bar do
    state :approved
    state :pending
  end
end

Single table inheritance

Single table inheritance is also supported. Descendant classes can either inherit the workflow definition from the parent or override with its own definition.

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.

Custom Versions of Existing Adapters

Other adapters (such as a custom ActiveRecord plugin) can be selected by adding a workflow_adapter class method, eg.

class Example < ActiveRecord::Base
  def self.workflow_adapter
    MyCustomAdapter
  end
  include Workflow

  # ...
end

(The above will include MyCustomAdapter instead of Workflow::Adapter::ActiveRecord.)

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.

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 do
      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.

Enum values or other custom values

If you don't want to store your state as a string column, you can specify the value associated with each state. Yu can use an int (like an enum) or a shorter string, or whatever you want.

Just pass the "value" for the state as the second parameter to the "state" method.

Class Foo < ActiveRecord::Base
  include Workflow

  workflow do
    state :one, 1 do
      event :increment, :transitions_to => :two
    end
    state :two, 2
    on_transition do |from, to, triggering_event, *event_args|
      Log.info "#{from} -> #{to}"
    end
  end
end

Your database column will store the values 1, 2, etc. But you'll still use the state symbols for querying.

foo = Foo.create
foo.current_state # => :one
foo.workflow_state # => 1 #You really shouldn't use this column directly...
foo.increment!
foo.two? # => true
foo.workflow_state # => true

Hopefully obvious, but if you ever change the value of a state, you'll need to do a migration/address existing records in your data store. However you are free to change the "name" of a state, willy-nilly.

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

Please also have a look at the advanced end to end example.

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.

Note: this is not triggered by Workflow::NoTransitionAllowed exceptions.

on_unavailable_transition

If you want to do custom handling when an unavailable transition is called, you can define an 'on_unavailable_transition' hook in your workflow. For example

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

  on_unavailable_transition do |from, to_name, *args|
    Log.warn "Workflow: #{from} does not have #{to_name} available to it"
  end
end

If backward! is called when in the first state, 'on_unavailable_transition' is invoked and workflow stays in a 'first' state. This example surpresses the Workflow::NoTransitionAllowed exception from being raised, if you still want it to be raised you can simply call it yourself or return false.

This is particularly useful when you don't want a processes to be aborted due to the workflow being in an unexpected state.

Guards

If you want to halt the transition conditionally, you can just raise an exception in your 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
  • on_entry
  • after_transition

Multiple Workflows

I am frequently asked if it's possible to represent multiple "workflows" in an ActiveRecord class.

The solution depends on your business logic and how you want to structure your implementation.

Use Single Table Inheritance

One solution can be to do it on the class level and use a class hierarchy. You can use single table inheritance so there is only single orders table in the database. Read more in the chapter "Single Table Inheritance" of the ActiveRecord documentation. Then you define your different classes:

class Order < ActiveRecord::Base
  include Workflow
end

class SmallOrder < Order
  workflow do
    # workflow definition for small orders goes here
  end
end

class BigOrder < Order
  workflow do
    # workflow for big orders, probably with a longer approval chain
  end
end

Individual workflows for objects

Another solution would be to connect different workflows to object instances via metaclass, e.g.

# Load an object from the database
booking = Booking.find(1234)

# Now define a workflow - exclusively for this object,
# probably depending on some condition or database field
if # some condition
  class << booking
    include Workflow
    workflow do
      state :state1
      state :state2
    end
  end
# if some other condition, use a different workflow

You can also encapsulate this in a class method or even put in some ActiveRecord callback. Please also have a look at the full working 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

About

Workflow Orchestrator is maintained by Lorefnon along with many contributors.

This project was derived (forked) from the gem geekq/workflow by Vladimir Dobriakov, which was forked from the original repo authored by Ryan Allen. Both appear to be unmaintained as of 2016.

While it is largely compatible with geekq/workflow but breaking API changes will be introduced in coming versions. In addition, the intent is to extract the persistence and rails dependent features in different gems, leaving only the FSM management features in the core.

History

Copyright (c) 2016 Lorefnon

Copyright (c) 2010-2014 Vladimir Dobriakov

Copyright (c) 2008-2009 Vodafone

Copyright (c) 2007-2008 Ryan Allen, FlashDen Pty Ltd

Based on the work of Ryan Allen and Scott Barron

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