Like acts as state machine (aasm), but _way_ better (it's in Ruby too!)
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What is apollo?

In Greek and Roman mythology, Apollo (in Greek, Ἀπόλλων—Apóllōn or Ἀπέλλων—Apellōn), is one of the most important and diverse of the Olympian deities. The ideal of the kouros (a beardless youth), Apollo has been variously recognized as a god of light and the sun; truth and prophecy; archery; medicine, healing and plague; music, poetry, and the arts; and more. Apollo is the son of Zeus and Leto, and has a twin sister, the chaste huntress Artemis. Wikipedia: Dionysus (2010/04/23)

Apollo is an fork of Workflow.


Features & Issues

This is a brand new project, so if you find bugs or use cases that would helpful to satisfy, post an Issue on Github.

This project is intended to be a very eclectic mix of helpful tools, so please feel free to send pull requests. That said, make absolutely sure your modifications are well tested. The hodge-podge nature of this project may make it prone to issues, so no code will be pulled-in that is not fully_tested.

Note on Patches/Pull Requests

  • Fork the project.
  • Make your feature addition or bug fix.
  • Add tests for it. This is important so I don't break it in a future version unintentionally.
  • Commit, do not mess with rakefile, version, or history. (if you want to have your own version, that is fine but bump version in a commit by itself I can ignore when I pull)
  • Send me a pull request. Bonus points for topic branches.

Documentation Warning

Beware that the below documentation has NOT been validated for the changes from Workflow to Apollo.

What is workflow?

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

A lot of business modeling tends to involve apollo-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 apollo using the API:

class Article
  include Apollo
  apollo do
    state :new do
      event :submit, :to => :awaiting_review
    state :awaiting_review do
      event :review, :to => :being_reviewed
    state :being_reviewed do
      event :accept, :to => :accepted
      event :reject, :to => :rejected
    state :accepted
    state :rejected

Nice, isn't it!

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

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

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

=> #<Apollo::State:0x7f1e3d6731f0 @events={
  :submit=>#<Apollo::Event:0x7f1e3d6730d8 @action=nil, 
    @to=:awaiting_review, @name=:submit, @meta={}>}, 
  name:new, meta{}

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

article.awaiting_review? # => true

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


gem install apollo

Alternatively you can just download the lib/apollo.rb and put it in the lib folder of your Rails or Ruby application.


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

$ irb
require 'rubygems'
require 'apollo'

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 apollo defined earlier it would be:

class Article
  def reject
    puts 'sending email to the author explaining the reason...'
end!; article.reject! will cause a state transition, persist the new state (if integrated with ActiveRecord) and invoke this user defined reject method.

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

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

article2 =
article2.submit!!('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, :to => :being_reviewed do |reviewer|
  # store the reviewer

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

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

class Order < ActiveRecord::Base
  include Apollo
  apollo do
    # list states and transitions here

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 apollo_state field is not set the state defaults to the first state declared in the apollo specification. In our example it is :new, so returns true and 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_apollo_state method.

Custom apollo database column

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

class LegacyOrder < ActiveRecord::Base
  include Apollo
  apollo_column :foo_bar # use this legacy database column for
                           # persistence

Single table inheritance

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

Custom apollo state persistence

If you do not use a relational database and ActiveRecord, you can still integrate the apollo very easily. To implement persistence you just need to override load_apollo_state and persist_apollo_state(new_value) methods. Lets see an example for using CouchDB, a document oriented database.

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 'apollo'

class User < CouchTiny::Document
  include Apollo
  apollo do
    state :submitted do
      event :activate_via_link, :to => :proved_email
    state :proved_email

  def load_apollo_state

  def persist_apollo_state(new_value)
    self[:apollo_state] = new_value

Please also have a look at the full source code.

Accessing your apollo specification

You can easily reflect on apollo specification programmatically - for the whole class or for the current object. Examples: # lists possible events from here[:reject].to # => :rejected

#=> [:rejected, :awaiting_review, :being_reviewed, :accepted, :new]

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

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

class MyProcess
  include Apollo
  apollo do
    state :main, :meta => {:importance => 8}
    state :supplemental, :meta => {:importance => 1}
puts MyProcess.apollo_spec.states[:supplemental].meta[:importance] # => 1

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

Advanced transition hooks


We already had a look at the declaring callbacks for particular apollo 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 apollo 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.


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

apollo do
  state :one do
    event :increment, :to => :two
  state :two
  on_transition do |from, to, triggering_event, *event_args| "#{from} -> #{to}"


If you want to halt the transition conditionally, you can just raise an exception. There is a helper called halt!, which raises the Apollo::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

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

Hook order

The whole event sequence is as follows:

* event specific action
* on_transition (if action did not halt)
* on_exit
* PERSIST WORKFLOW STATE, i.e. transition
* on_entry

Documenting with diagrams

You can generate a graphical representation of your apollo for documentation purposes. S. Apollo::create_apollo_diagram.


Copyright 2010 Travis D. Warlick, Jr.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
See the License for the specific language governing permissions and
limitations under the License.

The original work from Workflow

Author: 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.

Earlier versions

The workflow library was originally written by Ryan Allen.

The version 0.3 was almost completely (including ActiveRecord integration, API for accessing apollo specification, method_missing free implementation) rewritten by Vladimir Dobriakov keeping the original apollo DSL spirit.