Applications

• Networking (Switching, Routing, SDN)
• Messaging (SMS, MMS, IM) services
• Distributed systems (Databases, cluster management)
• Web services (analytics, ad servers)
• Simulation (testing)
• Game servers

Advertising

• Mochi Media
• AdRoll
• OpenX
• AdGear
• AOL

Cloud / Data Center Automation

• Rackspace
• Opscode
• Heroku
• VMWare

Mobile/VoIP services

• WhatsApp
• 2600hz
• TigerText

Monitoring / Ops

• Boundary
• BugSense
• Alert Logic

Games

• Spilgames
• Wooga
• MuchDifferent / Pikkotekk
• Linden Lab

Money

• Klarna (payments)
• Smarkets (sports betting)

Open Source

• Riak (Basho)
• CouchDB (CouchBase, Cloudant)
• ejabberd (Process-One)
• RabbitMQ (LShift → VMWare)
• Disco (Nokia)

Ancestry

• Syntax mostly from Prolog and ML
• List comprehensions from Miranda
• Message passing inspired by Smalltalk

Erlang was prototyped in Prolog:

% factorial.pl (Prolog)
:- module(factorial, [factorial/2]).

factorial(0, 1).
factorial(N, F) :-
  N>0,
  N1 is N-1,
  factorial(N1, F1),
  F is N * F1.

% factorial.erl (Erlang)
-module(factorial).
-export([factorial/2]).

factorial(0) -> 1;
factorial(N) when N > 0 ->
  N * factorial(N - 1).

List comprehensions from Miranda (like Haskell)

-- Subsets.hs
module Subsets (subsets) where

subsets :: [a] -> [[a]]
subsets []     = [[]]
subsets (x:xs) = [(x:y) | y <- ys] ++ ys
  where ys = subsets xs

% subsets.erl
-module(subsets).
-export([subsets/1]).

subsets([]) ->
    [[]];
subsets([X | Xs]) ->
    Ys = subsets(Xs),
    [[X | Y] || Y <- Ys] ++ Ys.

Pattern matching like ML

(* reverse.sml *)
fun reverse(l : 'a list) : 'a list =
    reverse1(l, [])

fun reverse1(l : 'a list, acc : 'a list) =
    case l of
        [] => acc
      | (x::xs) => reverse1(xs, x::acc)

% reverse.erl
-module(reverse).
-export([reverse/1]).

reverse(L) ->
    reverse(L, []).

reverse(L, Acc) ->
    case L of
        [] -> Acc;
        [X | Rest] -> reverse(Rest, [X | Acc])
    end.

Declarative

• "Describe the problem, not the solution"
• Great syntax for this (but not C-like)!
• Functional code is (often) easier to understand and test
• … but not 100% pure (message passing is a side-effect)

-module(merge_sort).
-export([merge_sort/1]).

% bottom-up merge sort
merge_sort([]) ->
    [];
merge_sort(L) ->
    iterate([[X] || X <- L]).

iterate([Xs]) ->
    Xs;
iterate(Lists) ->
    iterate(merge_lists(Lists)).

merge_lists([Xs, Ys | Rest]) ->
    [merge2(Xs, Ys) | merge_lists(Rest)];
merge_lists(Lists) ->
    Lists.

merge2(Xs=[X | _], [Y | Ys]) when X > Y ->
    [Y | merge2(Xs, Ys)];
merge2([X | Xs], Ys) ->
    [X | merge2(Xs, Ys)];
merge2([], Ys) ->
    Ys.

# merge_sort.py
def merge_sort(lst):
    if not lst:
        return []
    lists = [[x] for x in lst]
    while len(lists) > 1:
        lists = merge_lists(lists)
    return lists[0]

def merge_lists(lists):
    result = []
    for i in range(0, len(lists) // 2):
        result.append(merge2(lists[i*2], lists[i*2 + 1]))
    if len(lists) % 2:
        result.append(lists[-1])
    return result

def merge2(xs, ys):
    i = 0
    j = 0
    result = []
    while i < len(xs) and j < len(ys):
        x = xs[i]
        y = ys[j]
        if x > y:
            result.append(y)
            j += 1
        else:
            result.append(x)
            i += 1
    result.extend(xs[i:])
    result.extend(ys[j:])
    return result

Convenient bit syntax for parsing binary data

%% This parses a TCP packet header!
<< SourcePort:16, DestinationPort:16, SequenceNumber:32,
   AckNumber:32, DataOffset:4, _Reserved:4, Flags:8,
   WindowSize:16, Checksum:16, UrgentPointer:16,
   Payload/binary >> = Segment,
OptSize = (DataOffset - 5)*32,
<< Options:OptSize, Message/binary >> = Payload,
<< CWR:1, ECE:1, URG:1, ACK:1, PSH:1,
   RST:1, SYN:1, FIN:1>> = <<Flags:8>>,

%% Can now process the Message according to the 
%% Options (if any) and the flags CWR, …, FIN etc.

Concurrent

• Erlang concurrency is cheap
• Per-process heaps with incremental GC
• Message passing, not mutexes
• Straightforward control flow (not callbacks!)

RAM footprint per unit of concurrency (approx)

Per-process heaps

• No sharing
• GC is per-process, and not "stop the world"!
• Process references do not prevent GC
• Explicitly hibernate idle processes for compaction

RPC with a Counter process

Counter ! {self(), {add, 1}},
receive
  {Counter, {result, N}} ->
    io:format("~p~n", [N])
end

Multi-core

• One scheduler per core, scales well to 32+ cores
• Better scalability to more cores is in-progress
• Schedulers understand IO (disk, network calls)
• No implicit synchronization

%% Parse HTTP headers from Socket
headers(Socket, Request, Headers) ->
    ok = inet:setopts(Socket, [{active, once}]),
    receive
        {http, _, http_eoh} ->
            %% All of the HTTP headers are parsed
            handle_request(Socket, Request, Headers);
        {http, _, {http_header, _, Name, _, Value}} ->
            headers(Socket, Request, [{Name, Value} | Headers]);
        {tcp_closed, _} ->
            exit(normal);
        _Other ->
            %% Invalid request
            exit(normal)
    after ?HEADERS_RECV_TIMEOUT ->
        exit(normal)
    end.

Distributed

• Built-in distributed computing support
• Easy to get started, "no configuration"
• Same syntax, similar semantics, whether local or not
• Necessary for true fault tolerant systems (2+ nodes)
• Scales well to hundreds of nodes (LAN only)

Connect to other nodes by name

(node1@res)1> net_adm:ping(node2@res).
pong
(node1@res)2> self().
<0.38.0>
(node1@res)3> rpc:call(node2@res, erlang, self, []).
<6943.43.0>
(node1@res)4> nodes().
[node2@res]
(node1@res)5> rpc:cast(node2@res, init, stop, []).
true
(node1@res)6> nodes().
[]

Robust

• OTP framework encodes these best practices
• Processes can be linked for exit signal propagation
• Errors are localized and do not corrupt application state
• Supervisor processes start/stop/monitor other processes
• "Let it crash" means far less error handling code

OTP

Open Telephony Platform

Language Overview

Functional

• No mutable data structures
• Linked lists (like LISP)
• All state is on the stack
• Tail-call elimiation

Modules

• Flat namespace
• Explicit export of public functions
• Referenced by name (atom)
• Compiled to ".beam" file
• Can be reloaded in a running system

%% hello.erl
%% USAGE: hello:world().
-module(hello).
-export([world/0]).

world() -> io:format("hello world~n", []).