Erlang monitor multiple processes

I need to monitor a bunch of worker processes. Currently I'm able to monitor 1 process through 1 monitor. How do i scale this to monitoring N worker processes. Do i need to spawn N monitors as well? If so then what happens if one of those spawned monitors failed/crashed?

Solution

Do i need to spawn N monitors as well?

No:

-module(mo).
-compile(export_all).

worker(Id) ->
    timer:sleep(1000 * rand:uniform(5)),
    io:format("Worker~w: I'm still alive~n", [Id]),
    worker(Id).

create_workers(N) ->
    Workers = [  % { {Pid, Ref}, Id }
        { spawn_monitor(?MODULE, worker, [Id]), Id }
        || Id <- lists:seq(1, N)
    ],
    monitor_workers(Workers).

monitor_workers(Workers) ->
    receive
        {'DOWN', Ref, process, Pid, Why} ->
            Worker = {Pid, Ref},
            case is_my_worker(Worker, Workers) of
                true  ->  
                    NewWorkers = replace_worker(Worker, Workers, Why),
                    io:format("Old Workers:~n~p~n", [Workers]),
                    io:format("New Workers:~n~p~n", [NewWorkers]),
                    monitor_workers(NewWorkers);
                false -> 
                    monitor_workers(Workers)
            end;
        _Other -> 
            monitor_workers(Workers)
    end.
    
is_my_worker(Worker, Workers) ->
    lists:keymember(Worker, 1, Workers).

replace_worker(Worker, Workers, Why) ->
    {{Pid, _}, Id} = lists:keyfind(Worker, 1, Workers),
    io:format("Worker~w (~w) went down: ~s~n", [Id, Pid, Why]),
    NewWorkers = lists:keydelete(Worker, 1, Workers),
    NewWorker = spawn_monitor(?MODULE, worker, [Id]),
    [{NewWorker, Id}|NewWorkers].

start() ->
    observer:start(),  %%In the Processes tab, you can right click on a worker and kill it.
    create_workers(4).

In the shell:

$ ./run
Erlang/OTP 19 [erts-8.2] [source] [64-bit] [smp:4:4] [async-threads:10] [hipe] [kernel-poll:false]

Eshell V8.2  (abort with ^G)


1> Worker3: I'm still alive
Worker1: I'm still alive
Worker2: I'm still alive
Worker4: I'm still alive
Worker3: I'm still alive
Worker1: I'm still alive
Worker4: I'm still alive
Worker2: I'm still alive
Worker3: I'm still alive
Worker1: I'm still alive
Worker4: I'm still alive
Worker3 (<0.87.0>) went down: killed
Old Workers:
[{{<0.85.0>,#Ref<0.0.4.292>},1},
 {{<0.86.0>,#Ref<0.0.4.293>},2},
 {{<0.87.0>,#Ref<0.0.4.294>},3},
 {{<0.88.0>,#Ref<0.0.4.295>},4}]
New Workers:
[{{<0.2386.0>,#Ref<0.0.1.416>},3},
 {{<0.85.0>,#Ref<0.0.4.292>},1},
 {{<0.86.0>,#Ref<0.0.4.293>},2},
 {{<0.88.0>,#Ref<0.0.4.295>},4}]
Worker2: I'm still alive
Worker1: I'm still alive
Worker2: I'm still alive
Worker1: I'm still alive
Worker1: I'm still alive
Worker4: I'm still alive
Worker3: I'm still alive
Worker2: I'm still alive
Worker1: I'm still alive
Worker3: I'm still alive
Worker4: I'm still alive
Worker1: I'm still alive
Worker4 (<0.88.0>) went down: killed
Old Workers:
[{{<0.2386.0>,#Ref<0.0.1.416>},3},
 {{<0.85.0>,#Ref<0.0.4.292>},1},
 {{<0.86.0>,#Ref<0.0.4.293>},2},
 {{<0.88.0>,#Ref<0.0.4.295>},4}]
New Workers:
[{{<0.5322.0>,#Ref<0.0.1.9248>},4},
 {{<0.2386.0>,#Ref<0.0.1.416>},3},
 {{<0.85.0>,#Ref<0.0.4.292>},1},
 {{<0.86.0>,#Ref<0.0.4.293>},2}]
Worker3: I'm still alive
Worker2: I'm still alive
Worker4: I'm still alive
Worker1: I'm still alive
Worker3: I'm still alive
Worker3: I'm still alive
Worker2: I'm still alive
Worker1 (<0.85.0>) went down: killed
Old Workers:
[{{<0.5322.0>,#Ref<0.0.1.9248>},4},
 {{<0.2386.0>,#Ref<0.0.1.416>},3},
 {{<0.85.0>,#Ref<0.0.4.292>},1},
 {{<0.86.0>,#Ref<0.0.4.293>},2}]
New Workers:
[{{<0.5710.0>,#Ref<0.0.1.10430>},1},
 {{<0.5322.0>,#Ref<0.0.1.9248>},4},
 {{<0.2386.0>,#Ref<0.0.1.416>},3},
 {{<0.86.0>,#Ref<0.0.4.293>},2}]
Worker2: I'm still alive
Worker3: I'm still alive
Worker4: I'm still alive
Worker3: I'm still alive

I think the version below is probably more efficient: it uses lists:map() to both search for and replace the crashed worker, so it only traverses the Worker's list once:

-module(mo).
-compile(export_all).

worker(Id) ->
    timer:sleep(1000 * rand:uniform(5)),
    io:format("Worker~w: I'm still alive~n", [Id]),
    worker(Id).

create_workers(N) ->
    Workers = [  % { {Pid, Ref}, Id }
        { spawn_monitor(?MODULE, worker, [Id]), Id }
        || Id <- lists:seq(1,N)
    ],
    monitor_workers(Workers).

monitor_workers(Workers) ->
    receive
        {'DOWN', Ref, process, Pid, Why} ->
            CrashedWorker = {Pid, Ref},
            NewWorkers = replace(CrashedWorker, Workers, Why),
            io:format("Old Workers:~n~p~n", [Workers]),
            io:format("New Workers:~n~p~n", [NewWorkers]),
            monitor_workers(NewWorkers);
        _Other -> 
            monitor_workers(Workers)
    end.

replace(CrashedWorker, Workers, Why) ->
    lists:map(fun(PidRefId) ->
                      { {Pid,_Ref}=Worker, Id} = PidRefId,
                      case Worker =:= CrashedWorker of
                          true ->  %replace worker
                              io:format("Worker~w (~w) went down: ~s~n", 
                                        [Id, Pid, Why]),
                              {spawn_monitor(?MODULE, worker, [Id]), Id}; %=> { {Pid,Ref}, Id }
                          false ->  %leave worker alone
                              PidRefId  
                      end
              end,
              Workers).

start() ->
    observer:start(),  %%In the Processes tab, you can right click on a worker and kill it.
    create_workers(4).

If so then what happens if one of those spawned monitors failed/crashed?

Erlang owns several server farms in different countries, and erlang has acquired several redundant power grids, so erlang will restart everything in a fault tolerant, distributed system that will never fail. It's all built in. You don't have to worry about anything. :)

Actually...anywhere that you can imagine something failing, then it has to be backed up, e.g. by another monitoring process on another computer.