Slides from the Caribbean Developer Conference. 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.

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. TAKEAWAYS
● Using booleans to represent states can get messy quite
fast.
● Enforcing business rules by inferring the state from a
set of booleans leads to unreadable, unmaintainable
and hard to test code.
● State machines are not a silver bullet.
● State machines remove the complexity of testing
business rules.

45. THANK YOU
@harph