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

Erlang Processes & Messaging

Understand Erlang's core concurrency primitive: the process. Learn how processes communicate via asynchronous message passing and build isolated, lightweight concurrent units.

Erlang Processes & Messaging is a free Erlang OTP: Distributed & Fault-Tolerant Systems Programming lesson on CoddyKit — lesson 2 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Erlang OTP: Distributed & Fault-Tolerant Systems Programming learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Erlang's Core: The Process

The fundamental unit of Erlang concurrency is the process: a tiny, independent program running alongside others, far lighter than an OS thread.

Why Erlang Processes?

Erlang processes are lightweight (millions per machine), isolated (no shared memory), concurrent, and talk only via async messages.

Spawning a New Process

You create a process with spawn, passing a module, function, and args. It returns a unique PID identifying the new process.

-module(process_spawn).
-export([start/0, hello_world_process/0]).

hello_world_process() ->
    io:format("Hello from a new Erlang process!~n").

start() ->
    % Spawns a new process running hello_world_process/0
    Pid = spawn(process_spawn, hello_world_process, []),
    io:format("Spawned process with PID: ~p~n", [Pid]).

Understanding Process IDs (PIDs)

Every process gets a unique PID — its postal address. You need that PID to send it any message; it looks like <0.80.0>.

Sending Messages with '!'

Processes talk by messages, never shared memory. Send one with the ! operator: Pid ! Message, where the message is any Erlang term.

-module(message_sender).
-export([start/0, receiver_process/0, sender_process/1]).

receiver_process() ->
    io:format("Receiver process ~p started.~n", [self()]),
    % This process will just print its PID for now.
    % It doesn't receive messages in this version.
    timer:sleep(1000). % Keep process alive for a moment

sender_process(ReceiverPid) ->
    io:format("Sender process ~p sending message to ~p...~n", [self(), ReceiverPid]),
    ReceiverPid ! {hello, "from sender"},
    io:format("Message sent!~n").

start() ->
    ReceiverPid = spawn(message_sender, receiver_process, []),
    io:format("Receiver spawned with PID: ~p~n", [ReceiverPid]),
    sender_process(ReceiverPid).

Receiving Messages: 'receive'

A process pulls messages from its mailbox with receive, which pattern-matches each one and waits (or times out) until a match arrives.

-module(message_receiver).
-export([start/0, my_receiver/0, my_sender/1]).

my_receiver() ->
    io:format("Receiver ~p waiting for messages...~n", [self()]),
    receive
        {hello, Msg} ->
            io:format("Receiver ~p received: ~p~n", [self(), Msg]);
        _AnyOtherMessage ->
            io:format("Receiver ~p received an unexpected message: ~p~n", [self(), _AnyOtherMessage])
    end,
    io:format("Receiver ~p finished.~n", [self()]).

my_sender(ReceiverPid) ->
    io:format("Sender ~p sending message to ~p.~n", [self(), ReceiverPid]),
    ReceiverPid ! {hello, "World"},
    io:format("Sender ~p sent message.~n", [self()]).

start() ->
    ReceiverPid = spawn(message_receiver, my_receiver, []),
    timer:sleep(100), % Give receiver time to start
    my_sender(ReceiverPid).

A Full Messaging Example

Here is the full flow in one example: spawn a server, the client sends a request, and the server receives and replies.

-module(full_message_example).
-export([start/0, server_loop/0, client_action/1]).

server_loop() ->
    io:format("Server ~p started, waiting for requests...~n", [self()]),
    receive
        {request, ClientPid, Message} ->
            io:format("Server ~p received request from ~p: ~p~n", [self(), ClientPid, Message]),
            ClientPid ! {response, self(), "Got your message!"};
        _Other ->
            io:format("Server ~p received unexpected: ~p~n", [self(), _Other])
    end.

client_action(ServerPid) ->
    io:format("Client ~p sending request to server ~p...~n", [self(), ServerPid]),
    ServerPid ! {request, self(), "Can you hear me?"},
    receive
        {response, ServerPid, Reply} ->
            io:format("Client ~p received reply from ~p: ~p~n", [self(), ServerPid, Reply])
    end.

start() ->
    ServerPid = spawn(full_message_example, server_loop, []),
    timer:sleep(100), % Give server time to start
    client_action(ServerPid).

Asynchronous Communication

Erlang messaging is asynchronous: the sender drops the message in the mailbox and keeps going, never blocking on the receiver.

The Process Mailbox

Each process has a private mailbox where incoming messages queue. A receive scans it for a pattern match, usually in arrival order.

Knowing Your Own PID: `self()`

The built-in self() returns the current process's PID — handy for messaging yourself or embedding a reply address in a message.

-module(self_example).
-export([start/0, print_self/0]).

print_self() ->
    io:format("Hello, my PID is: ~p~n", [self()]).

start() ->
    io:format("Parent process PID: ~p~n", [self()]),
    spawn(self_example, print_self, []).

Quick Check: Message Flow

Consider the following sequence of events in Erlang:

  1. Process A spawns Process B.
  2. Process A sends a message to Process B.
  3. Process B receives the message.

Which of the following statements about this interaction are true?

Recap: Processes & Messaging

Recap: processes communicate by async messages — spawn returns a PID, ! sends, receive matches, each has a mailbox, and self() gives your own PID.

Frequently asked questions

Is the “Erlang Processes & Messaging” lesson free?

Yes — the full text of “Erlang Processes & Messaging” is free to read here on the web, and the Erlang OTP: Distributed & Fault-Tolerant Systems Programming course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Erlang OTP: Distributed & Fault-Tolerant Systems Programming course, upgrade to CoddyKit PRO.

What will I learn in “Erlang Processes & Messaging”?

Understand Erlang's core concurrency primitive: the process. Learn how processes communicate via asynchronous message passing and build isolated, lightweight concurrent units. You practise Erlang OTP: Distributed & Fault-Tolerant Systems Programming with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Erlang OTP: Distributed & Fault-Tolerant Systems Programming?

No prior experience is required. Erlang OTP: Distributed & Fault-Tolerant Systems Programming on CoddyKit is structured for beginners through advanced learners; this is — lesson 2 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Erlang Processes & Messaging” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Erlang OTP: Distributed & Fault-Tolerant Systems Programming lesson?

Yes. Every Erlang OTP: Distributed & Fault-Tolerant Systems Programming lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

  1. Introduction to Erlang & VM
  2. Erlang Processes & Messaging
  3. Basic Concurrency Patterns
  4. Pattern Matching & Guards
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