This document discusses state machines, Petri nets, and the Symfony workflow component. It describes finite state machines as a mathematical model for modeling computation and transitions between states. Petri nets are introduced as a generalization of state machines using a bipartite graph structure. The Symfony workflow component provides an implementation of workflows and state machines using concepts from Petri nets. Examples of configuring and using workflows in Symfony are presented, along with code smells that may indicate an opportunity to introduce a state machine pattern.

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state machines
and
symfony workflow
gytis semėnas
gytis@helis.lt || gytis@semenas.lt

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state machines?

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symfony workflow?

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What to expect?
1. State machines
2. Petri nets
3. Symfony workflow
4. Code smells

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finite state machines
(FSM)

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old

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mathematical model of computation

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all among us

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(Q, Σ, δ, q0, F)

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finite state machine
● list of states
● initial state
● conditions for each transition
● can be in one state at a time

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moore
output on state

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moore oven
State: OFF
output: no heat
State Heating
output: heat
State Idling
output: no
heat
BAKE
pressed
OFF
pressed
too hot
too cold
OFF
pressed

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mealy
output on transition (based on state and input)

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mealy oven
Off
Baking
{heat false}
Bake
pressed
{heat on}
OFF
Pressed
{heat off}
too hot
{heat off}
too cold
{heat on}

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mealy vs moore
the end result vs the process

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why use it
● helps with the design process
● easy to read
● easy to test
● easy to maintain
● easy to onboard
● easy to track

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represents real world

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catch22

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petri nets

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bipartite graph
place
transition
place
placeplace

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bipartite graph + tokens
place
transition
place
placeplace

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state is in the marking

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state is in the marking
place
transition
place
placeplace
place
transition

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state is in the marking
place
transition
place
placeplace
place
transition

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state machine?
transition
place
place
place
transition

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workflow net
● subset of petri net.
● a single input (source) place with no
previous transitions
● output place (sink) with no following
transitions.

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advanced petri nets
mathematical properties:
- boundedness
- safeness
- deadlock free.
- reachability.
parallelization, buffering, resource sharing
optimization, verification
coloured petri nets, weights...

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how are the
workflow net, petri net and state machine
related?

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how are they related?
petri net
workflow net
FSM

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available implementations
state pattern.
custom solution ;>
https://github.com/yohang/Finite
https://github.com/winzou/StateMachineBundle
https://github.com/florianv/petrinet
https://github.com/alterway/component-workflow

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symfony workflow

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workflow

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workflow

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configuration
Example by @lyrixx

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configuration
Example by @lyrixx

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state machine validation
A transition in StateMachine can only have one output.
A transition in StateMachine can only have one input.
A transition from a place/state must have an unique name.

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usage

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usage in twig

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events workflow.leave
workflow.[exact_workflow].leave
workflow.[exact_workflow].leave.[exact_place]
workflow.transition (can be used as mealy)
workflow.enter
workflow.entered (can be used as moore)
workflow.announce
workflow.guard

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demo

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code smells

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looks familiar?

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looks familiar?

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looks familiar?

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what to look for?
● an attribute named state or
status e.g. $chair->getState()

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what to look for?
● an attribute named state or status e.g. $reservation->getStatus()

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what to look for?
● methods that return
boolean values

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what to look for?
● methods that return boolean values

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what to look for?
● records that are only valid for a certain period of time.
e.g. subscription expiry date
● or records with NULLABLE datetime fields paid_at, deleted_at, published_at, etc.

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a way of thinking
© Tobias Nyholm

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credits
Grégoire Pineau @lyrixx
Tobias Nyholm @tobiasNyholm

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kthxbye
gytis@helis.lt || gytis@semenas.lt