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Erlang OTP: Distributed & Fault-Tolerant Systems Programming · Lezione

Gestione dinamica dei processi

Padroneggi l’avvio e l’arresto dinamici dei processi figli all’interno dei supervisor, creando sistemi adattabili ed efficienti nell’uso delle risorse

Gestione dinamica dei processi è una lezione Erlang OTP: Distributed & Fault-Tolerant Systems Programming gratuita su CoddyKit. Questa è la lezione 2 di 4. Puoi leggere la lezione completa qui gratuitamente — poi esercitati direttamente nel browser con un editor di codice integrato e un tutor IA disponibile 24/7. Fa parte del percorso di apprendimento Erlang OTP: Distributed & Fault-Tolerant Systems Programming, e i tuoi progressi si sincronizzano tra il web e l'app CoddyKit. Il corso Erlang OTP: Distributed & Fault-Tolerant Systems Programming include 4 lezioni in totale.

Parti di questa lezione non sono ancora state tradotte e vengono mostrate in inglese.

Dynamic Processes Overview

In Erlang, not all processes need to be started when your application first boots. Sometimes, you need processes that are created and destroyed on demand.

These are called dynamic processes, and they are crucial for building adaptable and resource-efficient systems. Think of them as temporary workers that supervisors can hire and fire as needed.

Why Dynamic Management?

Dynamic process management offers several key advantages:

  • Resource Efficiency: Only start processes when they are actually needed, saving memory and CPU.
  • Adaptability: Respond to varying loads by scaling up or down the number of workers.
  • On-Demand Tasks: Ideal for handling transient tasks like a new client connection, a file conversion request, or a single database query.

This contrasts with static children, which are always part of the supervisor's initial setup.

The `simple_one_for_one` Strategy

To manage dynamic children, supervisors often employ the simple_one_for_one restart strategy. This strategy is specifically designed for supervisors that will dynamically add children after startup.

  • It allows you to add children without pre-defining them in the supervisor's init/1 function.
  • Each dynamically added child is treated as a unique entity, even if they share the same underlying module.
  • If a dynamic child crashes, simple_one_for_one will restart only that child, not all of them.

Starting Dynamic Children

You start a dynamic child process using the supervisor:start_child/2 function. It takes two arguments:

  • SupRef: The name or PID of the supervisor.
  • ChildSpec: A map (or a list of tuples in older Erlang) describing the child process.

This function returns {ok, ChildPid} or an error if the child couldn't be started.

Dynamic Child Specification

A ChildSpec for a dynamic child is similar to a static one, but the id field is crucial for distinguishing instances. Here's a typical structure:

  • id: A unique atom or term to identify this specific child instance (e.g., client_123).
  • start: A tuple {Module, Function, Args} to call for starting the child (e.g., {gen_server, start_link, [{local, TaskId}, ?MODULE, [], []]}).
  • type: Either worker or supervisor.
  • restart: Often transient for dynamic workers, meaning they only restart if they crash unexpectedly, not if they exit normally.
  • shutdown: Timeout for graceful shutdown.

Code: Create Dynamic Workers

This example shows a supervisor starting two unique 'task' processes dynamically. Each task maintains its own count.

Run it to see how new processes are spawned and how you can interact with them individually.

-module(dynamic_start_example).
-behaviour(supervisor).

-export([start_link/0, init/1, start_task/1, call_task/2, run/0]).
-export([task_init/1, task_handle_call/3, task_terminate/2]).

% --- Supervisor part ---
start_link() ->
    supervisor:start_link({local, ?MODULE}, ?MODULE, []).

init([]) ->
    Strategy = {simple_one_for_one, 0, 1},
    Children = [],
    {ok, {Strategy, Children}}.

start_task(TaskId) ->
    io:format("Supervisor: Starting task ~p~n", [TaskId]),
    ChildSpec = #{
        id => TaskId,
        start => {gen_server, start_link, [{local, TaskId}, ?MODULE, [], []]},
        type => worker,
        restart => transient,
        shutdown => 5000
    },
    supervisor:start_child(?MODULE, ChildSpec).

call_task(TaskId, Message) ->
    gen_server:call(TaskId, Message).

% --- Worker part (this module acts as a gen_server for the tasks) ---
task_init([]) ->
    io:format("Task ~p: Initializing with count 0~n", [self()]),
    {ok, 0}. % Initial state for the task

task_handle_call(get_count, _From, State) ->
    io:format("Task ~p: Getting count ~p~n", [self(), State]),
    {reply, State, State}.
task_handle_call({increment, Value}, _From, State) ->
    NewState = State + Value,
    io:format("Task ~p: Incrementing by ~p to ~p~n", [self(), Value, NewState]),
    {reply, NewState, NewState}.

task_terminate(_Reason, State) ->
    io:format("Task ~p: Terminating with final state ~p~n", [self(), State]),
    ok.

% --- Entry point for runnable example ---
run() ->
    io:format("~n--- Starting Dynamic Task Example ---~n"),
    {ok, SupPid} = dynamic_start_example:start_link(),
    io:format("Supervisor started: ~p~n", [SupPid]),

    io:format("~nStarting Task 'task_alpha':~n"),
    dynamic_start_example:start_task(task_alpha),
    timer:sleep(100), % Give it a moment to start

    Count1 = dynamic_start_example:call_task(task_alpha, get_count),
    io:format("Task 'task_alpha' count: ~p~n", [Count1]),

    dynamic_start_example:call_task(task_alpha, {increment, 7}),
    Count2 = dynamic_start_example:call_task(task_alpha, get_count),
    io:format("Task 'task_alpha' count after increment: ~p~n", [Count2]),

    io:format("~nStarting Task 'task_beta':~n"),
    dynamic_start_example:start_task(task_beta),
    timer:sleep(100),

    Count3 = dynamic_start_example:call_task(task_beta, get_count),
    io:format("Task 'task_beta' count: ~p~n", [Count3]),

    dynamic_start_example:call_task(task_beta, {increment, 12}),
    Count4 = dynamic_start_example:call_task(task_beta, get_count),
    io:format("Task 'task_beta' count after increment: ~p~n", [Count4]),

    % In a real app, you'd stop the supervisor or individual tasks here.
    % For this example, we'll let them run.
    io:format("--- Dynamic Task Start Demo Finished ---~n"),
    ok.

Terminating Dynamic Children

Just as you can start processes dynamically, you can also stop them. The supervisor:terminate_child/2 function is used for this.

  • It takes the SupRef and the ChildId (the id from the child specification) as arguments.
  • The supervisor will send an exit signal to the child, initiating a graceful shutdown.
  • Once terminated, the child is removed from the supervisor's list, freeing up resources.

This is crucial for managing resources and ensuring processes don't linger unnecessarily.

Code: Terminate Dynamic Workers

This example demonstrates starting a task, interacting with it, and then gracefully stopping it using terminate_child/2. Notice the output when the task terminates.

-module(dynamic_stop_example).
-behaviour(supervisor).

-export([start_link/0, init/1, start_task/1, call_task/2, stop_task/1, run/0]).
-export([task_init/1, task_handle_call/3, task_terminate/2]).

% --- Supervisor part ---
start_link() ->
    supervisor:start_link({local, ?MODULE}, ?MODULE, []).

init([]) ->
    Strategy = {simple_one_for_one, 0, 1},
    Children = [],
    {ok, {Strategy, Children}}.

start_task(TaskId) ->
    io:format("Supervisor: Starting task ~p~n", [TaskId]),
    ChildSpec = #{
        id => TaskId,
        start => {gen_server, start_link, [{local, TaskId}, ?MODULE, [], []]},
        type => worker,
        restart => transient,
        shutdown => 5000
    },
    supervisor:start_child(?MODULE, ChildSpec).

call_task(TaskId, Message) ->
    gen_server:call(TaskId, Message).

stop_task(TaskId) ->
    io:format("Supervisor: Stopping task ~p~n", [TaskId]),
    supervisor:terminate_child(?MODULE, TaskId).

% --- Worker part (this module acts as a gen_server for the tasks) ---
task_init([]) ->
    io:format("Task ~p: Initializing with count 0~n", [self()]),
    {ok, 0}.

task_handle_call(get_count, _From, State) ->
    io:format("Task ~p: Getting count ~p~n", [self(), State]),
    {reply, State, State}.
task_handle_call({increment, Value}, _From, State) ->
    NewState = State + Value,
    io:format("Task ~p: Incrementing by ~p to ~p~n", [self(), Value, NewState]),
    {reply, NewState, NewState}.

task_terminate(_Reason, State) ->
    io:format("Task ~p: Terminating with final state ~p~n", [self(), State]),
    ok.

% --- Entry point for runnable example ---
run() ->
    io:format("~n--- Terminating Dynamic Task Example ---~n"),
    {ok, SupPid} = dynamic_stop_example:start_link(),
    io:format("Supervisor started: ~p~n", [SupPid]),

    io:format("~nStarting Task 'temp_task':~n"),
    dynamic_stop_example:start_task(temp_task),
    timer:sleep(100),

    dynamic_stop_example:call_task(temp_task, {increment, 100}),
    Count = dynamic_stop_example:call_task(temp_task, get_count),
    io:format("Task 'temp_task' current count: ~p~n", [Count]),

    io:format("~nStopping Task 'temp_task':~n"),
    dynamic_stop_example:stop_task(temp_task),
    timer:sleep(100), % Give it a moment to terminate

    io:format("~nAttempting to call 'temp_task' after termination (will fail):~n"),
    CatchResult = try dynamic_stop_example:call_task(temp_task, get_count) of
                      Result -> Result
                  catch
                      error:Reason -> {error, Reason}
                  end,
    io:format("Call result after stop: ~p~n", [CatchResult]),

    supervisor:stop(SupPid), % Clean up the supervisor
    io:format("Supervisor stopped.~n"),
    io:format("--- Dynamic Task Stop Demo Finished ---~n"),
    ok.

Practical Use Cases

Dynamic process management is a powerful tool in Erlang. Here are some common scenarios where it shines:

  • Web Servers: Spawning a new process for each incoming client connection.
  • Connection Pools: Managing a pool of database connections, creating new ones as needed.
  • Batch Processors: Starting worker processes to handle items from a queue.
  • User Sessions: Managing individual user sessions in a multi-user application.

It allows your system to adapt to varying loads and efficiently manage transient resources.

Quick Check

You've learned about starting and stopping dynamic child processes. Let's test your understanding.

Recap: Dynamic Processes

We've explored dynamic process management, a key technique for building adaptable Erlang systems:

  • Dynamic processes are created and destroyed on demand, saving resources.
  • The simple_one_for_one strategy is typically used by supervisors managing dynamic children.
  • supervisor:start_child/2 is used to add new processes, providing a unique child specification for each.
  • supervisor:terminate_child/2 ensures graceful shutdown and removal of dynamic processes.

Mastering this allows your applications to efficiently scale and respond to changing demands.

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Tutte le lezioni di questo corso

  1. Alberi di supervisione complessi
  2. Gestione dinamica dei processi
  3. Strategie avanzate di riavvio
  4. Bridge dei supervisor e gerarchie di processi miste
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