State Machines Are Everywhere - Why don't you use them?

Ruby developers don't tend to use state machines. State machines are a great tool build a mental model of the dynamic behaviour of objects and systems. They help to write code that is so simple that there are no obvious deficiencies. Why don't you use them?

Transcript

1. STATE MACHINES ARE EVERYWHERE LET'S FACE IT
2. KLAAS-JAN WIERENGA "KLASS" / KJ CHURCH CATCHUP SERVICE RUBY / RAILS SINCE 1.0 (2007) AMAZON AWS / CHEF @kjw github.com/kjwierenga
3. CEILIDH WHISKY
4. STATE MACHINES ARE EVERYWHERE
5. WHY DON'T YOU USE THEM?
6. STATE IS EVERYWHERE
7. QUANTUM PHYSICS ▸ quantum objects ▸ quantum states ▸ quantum leaps (transitions) ▸ interaction via intermediate artifacts (bosons)
8. STATE OF MATTER
9. TCP PROTOCOL
10. EVERY WEBSITE IS A STATE MACHINE Every URL is an application state. Hyperlinks are the transitions between states.
11. FAITH DEVELOPMENT SEVEN STAGES ACROSS LIFE SPAN PROF. JAMES FOWLER
12. RELIGIOUS DISAFFILIATION ONLY FOUR STAGES :-)
13. EVERYONE'S FAVOURITE RADIO STATION IS WII-FM
14. WHAT IS A STATE MACHINE? EXAMPLE WHY PROGRAM A STATE MACHINE? WHEN TO USE? RUBY GEMS
15. IROBOT WILL SMITH
17. HOLOGRAPHIC PROJECTOR = STATE MACHINE
18. MY RESPONSES ARE LIMITED
19. YOU MUST ASK THE RIGHT QUESTIONS
20. PROGRAM TERMINATED
21. WHAT IS A STATE MACHINE?
22. COMPUTER SCIENCE DETERMINISTIC FINITE AUTOMATA Accepts "ac", "abc", "abbc", "abbbbc"
23. ABSTRACT MODELS OF COMPUTATION TOM STUART
24. OBJECTS AND SYSTEMS
25. WHAT IS A STATE MACHINE? "Sequence of states in response to events and the responses to those events"
26. WHAT IS STATE? Situation where an object (or matter) ▸ satisfies some condition ▸ performs some activity ▸ waits for some event
27. WHAT IS AN EVENT? ▸ message ▸ timeout ▸ condition occuring that can trigger a state transition
28. WHAT IS A TRANSITION "Change of current state in reaction to an event, given that certain conditions are satisfied."
29. STATE MACHINE = THE SIMPLEST COMPUTER
30. WHERE AM I? WHAT HAPPENED? WHAT DO I DO NEXT?
31. EXAMPLE
32. UML DIAGRAMS? I find that Rubyists don't draw too many UML diagrams. So this may look like higher glyphics to you, I'm not sure. - JIM WEIRICH
33. ACTIVERECORD LIFECYCLE
35. WHY WOULD YOU PROGRAM A STATE MACHINE?
36. PROGRAMMING = BUG INJECTION
37. INCREASED COMPLEXITY LEADS TO MORE BUGS
38. I conclude that there are two ways of constructing a software design: One way is to make it so simple that there are obviously no deficiencies and the other way is to make it so complicated that there are no obvious deficiencies. The first method is far more difficult. C.A.R. (TONY) HOARE
39. MENTAL MODEL DIVERGES FROM ACTUAL BEHAVIOUR
40. MATCH MENTAL MODEL TO ACTUAL BEHAVIOUR
41. STATE MACHINE = MATCH
42. REDUCE COMPLEXITY OF FLOW
43. SELF- DOCUMENTING (WHY AND HOW)
44. MONITOR BEHAVIOUR
45. WHY DON'T YOU USE THEM?
46. ERICSSON 1999 IPAD
47. CALLING MOM
49. FSM # Initial state On Hook phone = MicroMachine.new(:on_hook) # On Hook phone.when(:digit, :on_hook => :dialing) phone.when(:incoming_call, :on_hook => :ringing) # Dialing phone.when(:digit, :dialing => :dialing) phone.when(:off_hook, :dialing => :alerting) phone.when(:on_hook, :dialing => :on_hook) # Alerting phone.when(:connected, :alerting => :connected) phone.when(:busy, :alerting => :busy) phone.when(:on_hook, :alerting => :on_hook) # Connected phone.when(:on_hook, :connected => :on_hook) # Ringing phone.when(:off_hook, :ringing => :connected) phone.when(:on_hook, :ringing => :on_hook)
50. BEHAVIOUR i, number = 0, [9,1,1] digits = [] phone.on(:dialing) { digits << number[i]; i += 1 } phone.on(:alerting) { puts "Dial number #{digits.join}" } phone.on(:connected) { puts "Say: Happy mothersday!" }
51. MESSAGE SEQUENCE # Normal scenario phone.trigger!(:digit) phone.trigger!(:digit) phone.trigger!(:digit) phone.trigger!(:off_hook) phone.trigger!(:connected) phone.trigger!(:on_hook)
53. OUTPUT $ ruby examples/micromachine/phone.rb digit [ On_hook -> Dialing ] digit [ Dialing -> Dialing ] digit [ Dialing -> Dialing ] off_hook [ Dialing -> Alerting ] Dial number 911 connected [ Alerting -> Connected ] Say: Happy Mothersday! on_hook [ Connected -> On_hook ]
54. YOU CALLING MOM CALLING YOU
55. CALL COLLISION
58. CONCURRENT ASYNCHRONOUS PROCESSES
60. WHEN TO USE?
61. INTERNAL STATE DETERMINES BEHAVIOUR
62. MULTIPLE INTERDEPENDENT BOOLEANS
63. CONCURRENT ASYNCHRONOUS PROCESSES
64. WHEN YOU CAN!
65. RUBY GEMS
66. THE RUBY TOOLBOX SEARCH 'STATE MACHINE' ▸ 1 Category (15 Projects) ▸ 146 Projects total
68. ORM Integration class Event < ActiveRecord::Base def confirm!; confirmation.trigger(:confirm);end def cancel!; confirmation.trigger(:cancel); end def reset!; confirmation.trigger(:reset); end def confirmation @confirmation ||= begin fsm = MicroMachine.new(confirmation_state || "pending") fsm.when(:confirm, :pending => :confirmed) fsm.when(:cancel, :confirmed => :cancelled) fsm.when(:reset, :confirmed => :pending, :cancelled => :pending) fsm.on(:any) { self.confirmation_state = confirmation.state } fsm end end end
69. SIMPLE 50 LINES IMPLEMENTATION class MicroMachine InvalidEvent = Class.new(NoMethodError) attr :transitions_for attr :state def initialize initial_state @state = initial_state @transitions_for = Hash.new { |hash, key| hash[key] = {} } @callbacks = Hash.new { |hash, key| hash[key] = [] } end def on key, &block @callbacks[key] << block end def when(event, transitions) transitions_for[event].merge!(transitions) end def trigger event if trigger?(event) from, @state = @state, transitions_for[event][@state] callbacks = @callbacks[@state] + @callbacks[:any] callbacks.each { |callback| callback.call(event, from) } true else false end end def trigger?(event) raise InvalidEvent unless transitions_for.has_key?(event) transitions_for[event][state] ? true : false end def events transitions_for.keys end def states events.map { |e| transitions_for[e].to_a }.flatten.uniq end def ==(some_state) state == some_state end end
70. REAL WORLD EXAMPLES
71. SPREE COMMERCE Spree::Order
73. TRAVIS-CI states :created, :started, :passed, :failed, :errored, :canceled
74. CIRCUITBREAKER PATTERN
76. THREE QUESTIONS
77. WHERE AM I?
78. WHAT JUST HAPPENED?
79. WHAT DO I DO NEXT BECAUSE OF IT?
80. FREE WILL?
81. KILL THE ROBOT?
83. DON'T LET YOUR PROGRAMS TURN INTO RANDOM SEGMENTS OF CODE THAT GROUP TOGETHER TO FORM UNEXPECTED PROTOCOLS
84. USE STATE MACHINES TO MAKE THEM SO SIMPLE THAT THERE ARE OBVIOUSLY NO DEFICIENCIES
85. PROGRAM TERMINATED
86. ?
87. REFERENCES Booch, Jacobsen, Rumbauch, The Unified Modelling Language User Guide (Addison Wesley Longman, Inc., 1999). Prof. James W. Fowler. Stages of Faith. Retrieved from http://en.wikipedia.org/wiki/Fowler's_stages_of_faith_development Martin Fowler. CircuitBreaker. Retrieved from http://martinfowler.com/bliki/CircuitBreaker.html Peter Gordon, The Incredible Rate of Diminishing Returns of Fixing Software Bugs. Retrieved from http://superwebdeveloper.com/2009/11/25/the-incredible-rate-of-diminishing-returns-of-fixing-software-bugs/ Hacker News, Why Developers Never Use State Machines. Retrieved from https://news.ycombinator.com/item?id=2949543 Miro Samek, Ph.D. Practical statecharts in C/C++ (CMP Books, 2002). Alan Skorkin, Why Developers Never Use State Machines. Retrieved from http://www.skorks.com/2011/09/why-developers-never-use-state-machines/ Dale Schumacher, Debugging Actor Systems. Retrieved from http://www.dalnefre.com/wp/2012/07/debugging-actor-systems/ Tom Stuart, Understanding Computation (O'Reilly, 2013).
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