A parent process that traps the exit signal and spawns and links to a worker process will be sent an exit message when the worker process terminates, even if the worker terminates due to an unexpected error. The parent process can now restart the worker process if the worker terminates. The parent process is said to be supervising the worker process, the parent is a process supervisor.
To study process supervision you will construct an Erlang system with one supervisor process and one worker process called bang.
Once every second, the bang process
decrements a counter and sends the counter value in a message to the supervisor.
When the counter reaches zero the bang process terminates with reason bang. To
simulate that the bang process may fail unexpectedly, once every second, the bang process will
gamble with death and with probability 30 % terminate with exit reason random_death.
The supervisor process must trap the exit signal. To start and link to the bang process the
supervisor use spawn_link/1. If the bang process terminates, the supervisor
will receive an exit message including the PID and exit reason of the terminated process.
When receiving an exit message with reason
random_death the supervisor
should restart the bang process with the last received counter value. When receiving an exit message with reason
bang the supervisor
should print >>BANG<< and terminate normally.
In the below example, the supervisor with PID <0.58.0> starts a bang process
with PID <0.98.0> counting down from 5 to 0. After sending counter value 4
to the supervisor, the bang process randomly dies and is restarted by the
supervisor. When the bang process reaches counter value zero it terminates with
reason bang without sending the counter value to the supervisor. When the bang
process terminates with reason bang the supervisor prints >>BANG<< and
terminates normally.
erlang> bang:start().
Supervisor with PID <0.58.0> started
bang(5) with PID <0.98.0> started
5 tick
4 tock
bang(3) with PID <0.98.0> died
bang(3) with PID <0.99.0> started
3 tick
2 tock
1 tick
0 tock
>>BANG<<
ok
erlang>
Another example where the bang process dies and is restarted by the supervisor multiple times.
erlang> bang:start().
Supervisor with PID <0.58.0> started
bang(5) with PID <0.106.0> started
bang(5) with PID <0.106.0> died
bang(5) with PID <0.107.0> started
5 tick
4 tock
3 tick
bang(2) with PID <0.107.0> died
bang(2) with PID <0.108.0> started
bang(2) with PID <0.108.0> died
bang(2) with PID <0.109.0> started
2 tock
bang(1) with PID <0.109.0> died
bang(1) with PID <0.110.0> started
1 tick
0 tock
>>BANG<<
ok
erlang>
In the module-8/mandatory/src/bang.erl module you find a skeleton of the system.
Compile the bang.erl module. Don’t forget to end the expression with a trailing
. (dot).
erlang> c(bang).
On success you now should see the following in the Erlang shell.
{ok, bang}
erlang>
Start the system from the Erlang shell.
erlang> bang:start().
The system now starts and generated the following output.
Supervisor with PID <0.133.0> started
bang(5) with PID <0.144.0> started
5 tick
4 tock
3 tick
2 tock
1 tick
0 tock
** exception exit: bang
In start/0, make the supervisor trap the exit signal.
In bang/2, uncomment the call to death:gamble/1.
In supervisor_loop/1, make the supervisor handle the exit messages correctly.