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Greatly improve the guide introduction

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+Erlang for beginners
+Chances are you are interested in using Cowboy, but have
+no idea how to write an Erlang program. Fear not! This
+chapter will help you get started.
+We recommend two books for beginners. You should read them
+both at some point, as they cover Erlang from two entirely
+different perspectives.
+Learn You Some Erlang for Great Good!
+The quickest way to get started with Erlang is by reading
+a book with the funny name of [LYSE](,
+as we affectionately call it.
+It will get right into the syntax and quickly answer the questions
+a beginner would ask themselves, all the while showing funny
+pictures and making insightful jokes.
+You can read an early version of the book online for free,
+but you really should buy the much more refined paper and
+ebook versions.
+Programming Erlang
+After writing some code, you will probably want to understand
+the very concepts that make Erlang what it is today. These
+are best explained by Joe Armstrong, the godfather of Erlang,
+in his book [Programming Erlang](
+Instead of going into every single details of the language,
+Joe focuses on the central concepts behind Erlang, and shows
+you how they can be used to write a variety of different
+At the time of writing, the 2nd edition of the book is in beta,
+and includes a few details about upcoming Erlang features that
+cannot be used today. Choose the edition you want, then get
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+Erlang and the Web
+The Web is concurrent
+When you access a website there is little concurrency
+involved. A few connections are opened and requests
+are sent through these connections. Then the web page
+is displayed on your screen. Your browser will only
+open up to 4 or 8 connections to the server, depending
+on your settings. This isn't much.
+But think about it. You are not the only one accessing
+the server at the same time. There can be hundreds, if
+not thousands, if not millions of connections to the
+same server at the same time.
+Even today a lot of systems used in production haven't
+solved the C10K problem (ten thousand concurrent connections).
+And the ones who did are trying hard to get to the next
+step, C100K, and are pretty far from it.
+Erlang meanwhile has no problem handling millions of
+connections. At the time of writing there are application
+servers written in Erlang that can handle more than two
+million connections on a single server in a real production
+application, with spare memory and CPU!
+The Web is concurrent, and Erlang is a language designed
+for concurrency, so it is a perfect match.
+Of course, various platforms need to scale beyond a few
+million connections. This is where Erlang's built-in
+distribution mechanisms come in. If one server isn't
+enough, add more! Erlang allows you to use the same code
+for talking to local processes or to processes in other
+parts of your cluster, which means you can scale very
+quickly if the need arises.
+The Web has large userbases, and the Erlang platform was
+designed to work in a distributed setting, so it is a
+perfect match.
+Or is it? Surely you can find solutions to handle that many
+concurrent connections with my favorite language... But all
+these solutions will break down in the next few years. Why?
+Firstly because servers don't get any more powerful, they
+instead get a lot more cores and memory. This is only useful
+if your application can use them properly, and Erlang is
+light-years away from anything else in that area. Secondly,
+today your computer and your phone are online, tomorrow your
+watch, goggles, bike, car, fridge and tons of other devices
+will also connect to various applications on the Internet.
+Only Erlang is prepared to deal with what's coming.
+The Web is soft real time
+What does soft real time mean, you ask? It means we want the
+operations done as quickly as possible, and in the case of
+web applications, it means we want the data propagated fast.
+In comparison, hard real time has a similar meaning, but also
+has a hard time constraint, for example an operation needs to
+be done in under N milliseconds otherwise the system fails
+Users aren't that needy yet, they just want to get access
+to their content in a reasonable delay, and they want the
+actions they make to register at most a few seconds after
+they submitted them, otherwise they'll start worrying about
+whether it successfully went through.
+The Web is soft real time because taking longer to perform an
+operation would be seen as bad quality of service.
+Erlang is a soft real time system. It will always run
+processes fairly, a little at a time, switching to another
+process after a while and preventing a single process to
+steal resources from all others. This means that Erlang
+can guarantee stable low latency of operations.
+Erlang provides the guarantees that the soft real time Web
+The Web is asynchronous
+Long ago, the Web was synchronous because HTTP was synchronous.
+You fired a request, and then waited for a response. Not anymore.
+It all started when XmlHttpRequest started being used. It allowed
+the client to perform asynchronous calls to the server.
+Then Websocket appeared and allowed both the server and the client
+to send data to the other endpoint completely asynchronously. The
+data is contained within frames and no response is necessary.
+Erlang processes work the same. They send each other data contained
+within messages and then continue running without needing a response.
+They tend to spend most of their time inactive, waiting for a new
+message, and the Erlang VM happily activate them when one is received.
+It is therefore quite easy to imagine Erlang being good at receiving
+Websocket frames, which may come in at unpredictable times, pass the
+data to the responsible processes which are always ready waiting for
+new messages, and perform the operations required by only activating
+the required parts of the system.
+The more recent Web technologies, like Websocket of course, but also
+SPDY and HTTP/2.0, are all fully asynchronous protocols. The concept
+of requests and responses is retained of course, but anything could
+be sent in between, by both the client or the browser, and the
+responses could also be received in a completely different order.
+Erlang is by nature asynchronous and really good at it thanks to the
+great engineering that has been done in the VM over the years. It's
+only natural that it's so good at dealing with the asynchronous Web.
+The Web is omnipresent
+The Web has taken a very important part of our lives. We're
+connected at all times, when we're on our phone, using our computer,
+passing time using a tablet while in the bathroom... And this
+isn't going to slow down, every single device at home or on us
+will be connected.
+All these devices are always connected. And with the number of
+alternatives to give you access to the content you seek, users
+tend to not stick around when problems arise. Users today want
+their applications to be always available and if it's having
+too many issues they just move on.
+Despite this, when developers choose a product to use for building
+web applications, their only concern seem to be "Is it fast?",
+and they look around for synthetic benchmarks showing which one
+is the fastest at sending "Hello world" with only a handful
+concurrent connections. Web benchmarks haven't been representative
+of reality in a long time, and are drifting further away as
+time goes on.
+What developers should really ask themselves is "Can I service
+all my users with no interruption?" and they'd find that they have
+two choices. They can either hope for the best, or they can use
+Erlang is built for fault tolerance. When writing code in any other
+language, you have to check all the return values and act accordingly
+to avoid any unforeseen issues. If you're lucky, you won't miss
+anything important. When writing Erlang code, you can just check
+the success condition and ignore all errors. If an error happen,
+the Erlang process crashes and is then restarted by a special
+process called a supervisor.
+The Erlang developer thus has no need to fear about unhandled
+errors, and can focus on handling only the errors that should
+give some feedback to the user and let the system take care of
+the rest. This also has the advantage of allowing him to write
+a lot less code, and letting him sleep at night.
+Erlang's fault tolerance oriented design is the first piece of
+what makes it the best choice for the omnipresent, always available
+The second piece is Erlang's built-in distribution. Distribution
+is a key part of building a fault tolerant system, because it
+allows you to handle bigger failures, like a whole server going
+down, or even a data center entirely.
+Fault tolerance and distribution are important today, and will be
+vital in the future of the Web. Erlang is ready.
+Erlang is the ideal platform for the Web
+Erlang provides all the important features that the Web requires
+or will require in the near future. Erlang is a perfect match
+for the Web, and it only makes sense to use it to build web
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+Getting started
+Cowboy does nothing by default.
+Cowboy requires the `crypto` and `ranch` applications to be started.
+``` erlang
+ok = application:start(crypto).
+ok = application:start(ranch).
+ok = application:start(cowboy).
+Cowboy uses Ranch for handling the connections and provides convenience
+functions to start Ranch listeners.
+The `cowboy:start_http/4` function starts a listener for HTTP connections
+using the TCP transport. The `cowboy:start_https/4` function starts a
+listener for HTTPS connections using the SSL transport.
+Listeners are a group of processes that are used to accept and manage
+connections. The processes used specifically for accepting connections
+are called acceptors. The number of acceptor processes is unrelated to
+the maximum number of connections Cowboy can handle. Please refer to
+the [Ranch guide](
+for in-depth information.
+Listeners are named. They spawn a given number of acceptors, listen for
+connections using the given transport options and pass along the protocol
+options to the connection processes. The protocol options must include
+the dispatch list for routing requests to handlers.
+The dispatch list is explained in greater details in the
+[Routing]( chapter.
+``` erlang
+Dispatch = cowboy_router:compile([
+ %% {URIHost, list({URIPath, Handler, Opts})}
+ {'_', [{'_', my_handler, []}]}
+%% Name, NbAcceptors, TransOpts, ProtoOpts
+cowboy:start_http(my_http_listener, 100,
+ [{port, 8080}],
+ [{env, [{dispatch, Dispatch}]}]
+Cowboy features many kinds of handlers. For this simple example,
+we will just use the plain HTTP handler, which has three callback
+functions: init/3, handle/2 and terminate/3. You can find more information
+about the arguments and possible return values of these callbacks in the
+[cowboy_http_handler function reference](
+Following is an example of a simple HTTP handler module.
+``` erlang
+init({tcp, http}, Req, Opts) ->
+ {ok, Req, undefined_state}.
+handle(Req, State) ->
+ {ok, Req2} = cowboy_req:reply(200, [], <<"Hello World!">>, Req),
+ {ok, Req2, State}.
+terminate(Reason, Req, State) ->
+ ok.
+The `Req` variable above is the Req object, which allows the developer
+to obtain information about the request and to perform a reply. Its usage
+is explained in the [cowboy_req function reference](
+You can find many examples in the `examples/` directory of the
+Cowboy repository. A more complete "Hello world" example can be
+found in the `examples/hello_world/` directory.
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