Discover finite state machine with gen_statem

This repo is related to the Erlang/ Elixir meetup on 05/05/2020.

🙏 Organized by https://erlang-connection.eu | twitter: @ErlangConnect

📺 You can find the slides here 👇.

Code, tests, and boilerplate

This repo contains the code, the tests and the Liveview demo app shown during the meetup.

There is also some reusable boilerplate (code & tests) to start playing with gen_statem.

The boilerplates, lives in lib/fsm/door with their related tests in test/fsm/door.

Demos

To start your Phoenix server:

• Setup the project with mix setup
• Start Phoenix endpoint with mix phx.server

Now you can visit localhost:4000 from your browser.

Demo 1: Light

code: lib/fsm/light/light_fsm.ex | tests: test/fsm/light/light_fsm_test.exs

Rules

• Light is turned off by default
• When the "on" button is pressed, light is turned on
• When the "off" button is pressed, light is turned off
• After a given time, the light turns off itself

You will learn:

• how to write your first finite state machine using the simple state_function syntax
• the state_timeout feature: after a given time in state X, do something
• the state_enter feature: do something anytime the current state changes
• how to write tests for your FSM

Demo 2: Payphone

code: lib/fsm/payphone/payphone_fsm.ex | tests: test/fsm/payphone/payphone_fsm_test.exs

Sequence:

• pick up the phone
• add some credit
• dial a number
• speak
• hang up the phone

Rules

• User can add credit when both dialing and calling.
• User can hang up at any time.
• When user runs out of credit, the phone hangs up itself.
• When the credit is low, the user gets a voice notification

You will learn:

• the other syntax: handle_event_function which can be more flexible
• the generic_timeout feature
• how to use absolute time for timeouts

Demo 3: Traffic lights

code: lib/fsm/traffic_light/traffic_light_manager_fsm.ex | tests: test/fsm/traffic_light/traffic_light_manager_fsm_test.exs

Architecture

• PeopleFsm: the FSM for the people's traffic light
• CarFsm: the FSM for the car's traffic light
• ManagerFsm: the FSM that orchestrate the two others in order to achieve your goal

TrafficLight.PeopleFsm:

TrafficLight.CarFsm:

TrafficLight.ManagerFsm (the one that manage the two others):

Goal:

Crossing the road.

Sequence:

• By default, the traffic lights let the cars pass.
• Occasionally, people ask to cross the road using a button.
• Traffic light color changes to let the people pass.
• After a given time, traffic lights let the cars pass again

You will learn:

• how to decompose your domain in several FSMs
• how to coordinate those FSMs with ... another FSMs :-)
• the postpone feature
• the internal_event feature