How can Erlang be used to create fault-tolerant concurrent systems?

2 The let-it-crash philosophy

Erlang adopts a let-it-crash philosophy, which means that instead of trying to handle every possible error or exception within an actor, it simply lets the actor crash and restarts it with a clean state. This way, the system can recover from failures quickly and avoid propagating errors to other parts of the system. Erlang also provides mechanisms for supervising and restarting crashed actors, such as supervisors and supervision trees, which can define different strategies and policies for fault recovery.

3 The OTP framework

Erlang comes with a set of libraries and tools called the Open Telecom Platform (OTP), which provides a standard way of building fault-tolerant concurrent systems. OTP includes common patterns and abstractions for actors, such as generic servers, finite state machines, event handlers, and applications. OTP also provides features for distribution, communication, configuration, logging, testing, and documentation. By using OTP, you can leverage the best practices and conventions of Erlang and focus on the domain-specific logic of your system.

4 The hot code loading

Erlang supports hot code loading, which means that you can update the code of your system without stopping or restarting it. This allows for continuous deployment and maintenance of your system, as well as for dynamic adaptation to changing requirements and environments. Erlang also ensures that the old and new versions of the code can coexist and communicate with each other until all the actors have switched to the new version.

5 The pattern matching

Erlang uses pattern matching, which is a powerful way of expressing and manipulating data structures. Pattern matching allows you to decompose and match complex data types, such as tuples, lists, records, and maps, by using simple and concise syntax. Pattern matching also enables you to write elegant and expressive code for handling messages, errors, and control flow. Pattern matching is one of the core features of functional programming, which promotes immutability, purity, and simplicity.

6 The example

To illustrate how Erlang can be used to create fault-tolerant concurrent systems, let's look at a simple example of a chat server. The chat server consists of a supervisor that creates and monitors a pool of chat rooms, each of which is an actor that handles messages from clients. The chat room actor uses pattern matching to process different types of messages, such as join, leave, send, and broadcast. The chat room actor also uses hot code loading to update its behavior without affecting the existing clients. The chat server uses OTP to implement the common patterns and features of a fault-tolerant concurrent system.

-module(chat_room).
-behaviour(gen_server).
-export([start_link/1, join/2, leave/2, send/3]).
-export([init/1, handle_call/3, handle_cast/2, handle_info/2, terminate/2, code_change/3]).
-record(state, {name, clients}).
start_link(Name) ->
    gen_server:start_link({local, Name}, ?MODULE, [Name], []).
join(Name, Pid) ->
    gen_server:call(Name, {join, Pid}).
leave(Name, Pid) ->
    gen_server:cast(Name, {leave, Pid}).
send(Name, Pid, Message) ->
    gen_server:cast(Name, {send, Pid, Message}).
init([Name]) ->
    {ok, #state{name = Name, clients = []}}.
handle_call({join, Pid}, _From, State) ->
    {reply, ok, add_client(Pid, State)}.
handle_cast({leave, Pid}, State) ->
    {noreply, remove_client(Pid, State)}.
handle_cast({send, Pid, Message}, State) ->
    {noreply, broadcast(Message, State)}.
handle_info(_Info, State) ->
    {noreply, State}.
terminate(_Reason, _State) ->
    ok.
code_change(_OldVsn, State, _Extra) ->
    {ok, State}.
add_client(Pid, State) ->
    State#state{clients = [Pid | State#state.clients]}.
remove_client(Pid, State) ->
    State#state{clients = lists:delete(Pid, State#state.clients)}.
broadcast(Message, State) ->
    lists:foreach(fun(Pid) -> Pid ! Message end, State#state.clients),
    State.        

7 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?