Before coming to the workshop, it would be advised to have Erlang installed. You can do it either through installers:
- Install Erlang version 18.3 and activate it
OR
- Install the latest version 19.1. The difference is with the finite state machine gen_fsm was moved to gen_statem. In this tutorial we'll be using gen_fsm.
If you try to run the Erlang shell with simply erl should be enough. Alternatively, you can query the OTP version with:
$ erl -eval 'erlang:display(erlang:system_info(otp_release)), halt().' -noshell
"18"To run our code and see the results, we're going to use rebar. Rebar is an Erlang build tool that makes it easy to compile and test Erlang applications, port drivers and releases.
- Compile code
./rebar compile
After that there's a shell script that runs the erlang node for you:
- Run code with
./start.sh
This will boot up the Erlang VM with a shell where you can call your code as well as execute some prepared tests for the following task:
We will try to simulate a scenario where we have an airport and a bunch of planes trying to land to said airport. The airport has a control tower that accepts communication from planes. The planes are by default airborne and approaching the airport.
First they will ask for permission to land. If they get a positive response, they should be allowed to commence the landing sequence, then go to the hangar as they finished up their journey. If they get a negative response back from the control tower, they stay in air and try again later.
In our case, the control tower is a gen_server, listening and reacting to events. It also keeps a state data structure of itself to keep track of the landing strip. The server accepts the following messages / calls:
- open_landing_strip - Create a landing strip and register it in the server's state
- close_landing_strip - Close down the landing strip
- close_airport - Close down the airport
- permission_to_land - Response with a state / landing strip id whether the plane which made the request can land
- land_plane - Land the plane and free up the runway
Planes are instances of a gen_fsm. This finite state machine has the following states:
- in_air - The plane is airborne, waiting to land. Possible actions / transitions: permission_to_land
- prepare_for_landing - Got permission to land and attempts landing to the given landing strip. Possible actions / transitions: land
- landed - Plane has touched ground and ready to finish its journey. Possible actions / transitions: terminate
After starting the node, you need to call control_tower:test_tower_setup/0:
====NODE STARTED=====
Eshell V6.4 (abort with ^G)
1> control_tower:test_tower_setup().
[TOWER] Opening new landing strip {landing_strip,443585,true}
Control Tower: <0.34.0> | Landing Strip: {landing_strip,443585,true}[TOWER] Landing Strip {landing_strip,443585,true} was freed up.
ok
2>First thing first is to make sure we can open and close the tower. Now, we need to take a look at the planes. First all we need is to make sure we can create a Plane process with the correct initial state. Once we have that we can call control_tower:test_single_plane/0 the same way as above.
So we have a basic tower, and a basic plane. Now all we need is the two of them working together in order to complete a landing sequence. To complete that, we need to do the following:
- On the Plane implement the missing part in the transition from in_air x permission_to_land as well as the corresponding part on the Control Tower.
- Make the synchronous call from the Plane FSM to the Control tower.
- Handle the call in the Control tower by checking the available landing strip(s)
- Reserve the landing strip for the plane
- Interpret the result on the Plane side and make a state transition accordingly
- On the Plane side, implement the transition prepare_for_landing x land
- Call the Control tower to make the landing
- Transition to state in the end.
- On the Control Tower implement the land_plane and make_landing calls
- land_plane is a synchronous call, we should return with an acknoledgement and call the async make_landing
- Add some sleep time, as landing take some effort to complete
- Add a log message about the plane landed
- Free up the landing strip
- On the Plane side, close our process with the terminate call where we send our planes to the hangar for some rest.
You can test out your progress by calling control_tower:test_single_plane_landing/0 from the Erlang shell.
Now, we have completed a single plane case, let's see if our control tower can handle multiple planes approaching the same time! Call control_tower:test_planes/0 and see how your solution works!