When Booleans Are Not Enough... State Machines?

Usually, we tend to represent objects status with boolean attributes. At first, this seems right and simple enough, but as the code base evolves and gets bigger, status become complex; so do the transitions between them.

Transition between status can be seen as behaviors or actions. Therefore, we find ourselves implementing rules to enforce behaviors and validating transitions; or even worse, not enforcing or validating anything at all.

Considering using a state machine to represent the status of an object may be ideal. As each state on the machine can represent an object status, the transitions between states can represent well defined actions or behaviors that can be performed between status.

This talk focuses on how to identify when booleans are not the right type to represent status, and how using a state machine may lead you to a better and cleaner design that can enforce conditions between status by simply relying on the definition of a state machine.

Transcript

1. WHEN BOOLEANS ARE NOT ENOUGH... STATE MACHINES?
2. TWITTER / GITHUB: @harph SAN JOSE, CA 2 HARRINGTON JOSEPH SR. SOFTWARE ENGINEER
3. EXPECTATIONS WHY, HOW AND WHEN TO USE STATE MACHINES
4. ANALOGY
5. • is_active • active • is_enabled • enabled • is_disabled • disabled • has_expired • expired • is_published • published • is_deleted • deleted • is_archived • archived • is_visible • visible • is_hidden • hidden • is_ready • ready • is_valid • valid • is_blocked • blocked • failed • succeeded • is_positive • ...
6. THE PROBLEM USING BOOLEANS TO REPRESENT STATES AND ENFORCE BUSINESS RULES
7. CODE
8. class Video: def __init__(self, source): self.source = source self.is_playing = False def pause(self): # Make the call to pause the video self.is_playing = False def play(self): # Make the call to play the video self.is_playing = True def stop(self): # Make the call to stop the video self.is_playing = False
9. video = Video('s3://video/storage/demo.mov' ) video.play() video.pause() video.stop() video.is_playing
10. video.is_playing WHAT DOES IT REALLY MEAN?
11. video.is_playing True Video is playing
12. video.is_playing False Is it paused? Is it stopped?
13. class Video: def __init__(self, source): self.source = source self.is_playing = False self.is_paused = False def pause(self): # Make the call to pause the video self.is_playing = False self.is_paused = True def play(self): # Make the call to play the video self.is_playing = True self.is_paused = False def stop(self): # Make the call to stop the video self.is_playing = False self.is_paused = False
14. Is the video playing? video.is_playing Is the video paused? video.is_paused Is the video stopped? not video.is_playing and not video.is_paused
15. BUSINESS RULES 1. A video can only be played when is paused or stopped. 2. A video can only be paused when is playing. 3. A video can only be stopped when is playing or paused.
16. class Video: ... def play(self): if not self.is_playing or self.is_paused: # Make the call to play the video self.is_playing = True self.is_paused = False else: raise Exception( 'Cannot play a video that is ' 'already playing.' ) 1. A video can only be played when is paused or stopped.
17. class Video: ... def pause(self): if self.is_playing: # Make the call to pause the video self.is_playing = False self.is_paused = True else: raise Exception( 'Cannot pause a video that is ' 'not playing.' ) 2. A video can only be paused when is playing.
18. class Video: ... def stop(self): if self.is_playing or self.is_paused: # Make the call to stop the video self.is_playing = False self.is_paused = False else: raise Exception( 'Cannot stop a video that is ' 'not playing or paused.' ) 3. A video can only be stopped when is playing or paused.
19. THE CODE IS RAPIDLY BECOMING • Complex. • Bloated. • Repetitive. • Hard to test.
20. class Video: # States PLAYING = 'playing' PAUSED = 'paused' STOPPED = 'stopped' def __init__(self, source): self.source = source self.state = self.STOPPED
21. class Video: ... def play(self): if self.state != self.PLAYING: # Make the call to play the video self.state = self.PLAYING else: raise Exception( 'Cannot play a video that is ' 'already playing.' ) 1. A video can only be played when is paused or stopped.
22. class Video: ... def pause(self): if self.state == self.PLAYING: # Make the call to pause the video self.state = self.PAUSED else: raise Exception( 'Cannot pause a video that is ' 'not playing.' ) 2. A video can only be paused when is playing.
23. class Video: ... def stop(self): if self.state != self.STOP: # Make the call to stop the video self.state = self.STOPPED else: raise Exception( 'Cannot stop a video that is ' 'not playing or paused.' ) 3. A video can only be stopped when is playing or paused.
24. STATE MACHINES
25. WHAT'S A STATE MACHINE? • Finite number of states. Mathematical model of computation. • Transitions between states. • Machine. Can only be in one state at a given time.
26. USER LOGIN STATE MACHINE
27. DESIGNING A STATE MACHINE 1. Define a finite number of states. 2. Lay down the transitions between states. 3. Select the initial state.
28. 1. Define a finite number of states.
29. 2. Lay down the transitions between states.
30. 3. Select the initial state.
31. CODE
32. pip install transitions github.com/pytransitions/transitions
33. from transitions import Machine class Video: # Define the states PLAYING = 'playing' PAUSED = 'paused' STOPPED = 'stopped'
34. from transitions import Machine class Video: # Define the states PLAYING = 'playing' PAUSED = 'paused' STOPPED = 'stopped' def __init__(self, source): self.source = source # Define the transitions transitions = [ # 1. A video can only be played when is paused or stopped. {'trigger': 'play', 'source': self.PAUSED, 'dest': self.PLAYING}, {'trigger': 'play', 'source': self.STOPPED, 'dest': self.PLAYING}, # 2. A video can only be paused when is playing. {'trigger': 'pause', 'source': self.PLAYING, 'dest': self.PAUSED}, # 3. A video can only be stopped when is playing or paused. {'trigger': 'stop', 'source': self.PLAYING, 'dest': self.STOPPED}, {'trigger': 'stop', 'source': self.PAUSED, 'dest': self.STOPPED}, ]
35. from transitions import Machine class Video: ... def __init__(self, source): self.source = source # Define the transitions transitions = [ # 1. A video can only be played when is paused or stopped. {'trigger': 'play', 'source': self.PAUSED, 'dest': self.PLAYING}, {'trigger': 'play', 'source': self.STOPPED, 'dest': self.PLAYING}, # 2. A video can only be paused when is playing. {'trigger': 'pause', 'source': self.PLAYING, 'dest': self.PAUSED}, # 3. A video can only be stopped when is playing or paused. {'trigger': 'stop', 'source': self.PLAYING, 'dest': self.STOPPED}, {'trigger': 'stop', 'source': self.PAUSED, 'dest': self.STOPPED}, ] # Create the state machine self.machine = Machine( model=self, transitions=transitions, initial=self.STOPPED )
36. class Video: ... def pause(self): # Make the call to pause the video ... def play(self): # Make the call to play the video ... def stop(self): # Make the call to stop the video ...
37. video = Video('s3://video/storage/demo.mov' ) video.play() # State is 'playing' video.state # 'playing' video.pause() # State is 'paused' video.stop() # State is 'stopped' video.pause() # ??? MachineError: "Can't trigger event pause from state stopped!"
38. TESTING HOW DO WE TEST THIS? 1. We don't. 2. Machine is initialized with the expected transitions and initial state. 3. Actual functionality ( play, pause and stop ).
39. ADDING A NEW STATE
40. RULES 1. A video can only be played when is paused, stopped or rewinding. 2. A video can only be paused when is playing or rewinding. 3. A video can only be stopped when is playing, paused or rewinding. 4. A video can only be rewinded when is playing or paused.
41. from transitions import Machine class Video: ... REWINDING = 'rewinding' def __init__(self, source): ... transitions = [ # 1. A video can only be played when is paused, stopped or rewinding. {'trigger': 'play', 'source': self.PAUSED, 'dest': self.PLAYING}, {'trigger': 'play', 'source': self.STOPPED, 'dest': self.PLAYING}, {'trigger': 'play', 'source': self.REWINDING, 'dest': self.PLAYING}, # 2. A video can only be paused when is playing or rewinding. {'trigger': 'pause', 'source': self.PLAYING, 'dest': self.PAUSED}, {'trigger': 'pause', 'source': self.REWINDING, 'dest': self.PAUSED}, # 3. A video can only be stopped when is playing, paused or rewinding. {'trigger': 'stop', 'source': self.PLAYING, 'dest': self.STOPPED}, {'trigger': 'stop', 'source': self.PAUSED, 'dest': self.STOPPED}, {'trigger': 'stop', 'source': self.REWINDING, 'dest': self.STOPPED}, # 4. A video can only be rewinded when is playing or paused. {'trigger': 'rewind', 'source': self.PLAYING, 'dest': self.REWINDING}, {'trigger': 'rewind', 'source': self.PAUSED, 'dest': self.REWINDING}, ] ...
42. WHEN ARE BOOLEANS NOT ENOUGH? • When multiple booleans represent a single state. • When business rules are enforced by multiple booleans.
43. WHEN TO USE STATE MACHINES? • When states are not binary. • When you have to account for future states. • When you have to enforce a complex set of business rules.
44. transitions = [ # 1. A video can only be played when is paused, stopped or rewinding. {'trigger': 'play', 'source': self.PAUSED, 'dest': self.PLAYING}, {'trigger': 'play', 'source': self.STOPPED, 'dest': self.PLAYING}, {'trigger': 'play', 'source': self.REWINDING, 'dest': self.PLAYING}, # 2. A video can only be paused when is playing or rewinding. {'trigger': 'pause', 'source': self.PLAYING, 'dest': self.PAUSED}, {'trigger': 'pause', 'source': self.REWINDING, 'dest': self.PAUSED}, ... In summary, consider using state machines to represent states and enforce business rules www.hjoseph.com @harph