Getting started =============== Setting up a working Erlang application is a little more complex than for most other languages. The reason is that Erlang is designed to build systems and not just simple applications. An Erlang system is typically comprised of many different nodes, each containing many different OTP applications, each containing many different modules and running many different processes. Nodes may or may not be identical depending on the nature of the system. To get started though, we only need one node that contains your own HTTP application, plus the dependencies that it needs, like Cowboy. To create our node, we need to build what is called a release. A release is a set of files that contain the Erlang VM plus all the applications required to run our node. Let's start by creating this application. We will simply call it `hello_erlang`. This application will have the following directory structure: ``` hello_erlang/ src/ hello_erlang.app.src hello_erlang_app.erl hello_erlang_sup.erl hello_handler.erl erlang.mk Makefile relx.config ``` Once the release is generated, we will also have the following files added: ``` hello_erlang/ ebin/ hello_erlang.app hello_erlang_app.beam hello_erlang_sup.beam hello_handler.beam _rel/ ``` As you can probably guess, the `.app.src` file end up becoming the `.app` file, and the `.erl` files are compiled into `.beam`. Then, the whole release will be copied into the `_rel/` directory. The `.app` file contains various informations about the application. It contains its name, a description, a version, a list of modules, default configuration and more. Using a build system like [erlang.mk](https://github.com/extend/erlang.mk), the list of modules will be included automatically in the `.app` file, so you don't need to manually put them in your `.app.src` file. For generating the release, we will use [relx](https://github.com/erlware/relx) as it is a much simpler alternative to the tool coming with Erlang. First, create the `hello_erlang` directory. It should have the same name as the application within it. Then we create the `src` directory inside it, which will contain the source code for our application. ``` bash $ mkdir hello_erlang $ cd hello_erlang $ mkdir src ``` Let's first create the `hello_erlang.app.src` file. It should be pretty straightforward for the most part. You can use the following template and change what you like in it. ``` erlang {application, hello_erlang, [ {description, "Hello world with Cowboy!"}, {vsn, "0.1.0"}, {modules, []}, {registered, [hello_erlang_sup]}, {applications, [ kernel, stdlib, cowboy ]}, {mod, {hello_erlang_app, []}}, {env, []} ]}. ``` The `modules` line will be replaced with the list of modules during compilation. Make sure to leave this line even if you do not use it directly. The `registered` value indicates which processes are registered by this application. You will often only register the top-level supervisor of the application. The `applications` value lists the applications that must be started for this application to work. The Erlang release will start all the applications listed here automatically. The `mod` value defines how the application should be started. Erlang will use the `hello_erlang_app` module for starting the application. The `hello_erlang_app` module is what we call an application behavior. The application behavior must define two functions: `start/2` and `stop/1`, for starting and stopping the application. A typical application module would look like this: ``` erlang -module(hello_erlang_app). -behavior(application). -export([start/2]). -export([stop/1]). start(_Type, _Args) -> hello_erlang_sup:start_link(). stop(_State) -> ok. ``` That's not enough however. Since we are building a Cowboy based application, we also need to initialize Cowboy when we start our application. Cowboy does nothing by default. 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](http://ninenines.eu/docs/en/ranch/HEAD/guide/toc) 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](routing.md) chapter. For the purpose of this example we will simply map all URLs to our handler `hello_handler`, using the wildcard `_` for both the hostname and path parts of the URL. This is how the `hello_erlang_app:start/2` function looks like with Cowboy initialized. ``` erlang start(_Type, _Args) -> Dispatch = cowboy_router:compile([ %% {URIHost, list({URIPath, Handler, Opts})} {'_', [{'_', hello_handler, []}]} ]), %% Name, NbAcceptors, TransOpts, ProtoOpts cowboy:start_http(my_http_listener, 100, [{port, 8080}], [{env, [{dispatch, Dispatch}]}] ), hello_erlang_sup:start_link(). ``` Do note that we told Cowboy to start listening on port 8080. You can change this value if needed. Our application doesn't need to start any process, as Cowboy will automatically start processes for every incoming connections. We are still required to have a top-level supervisor however, albeit a fairly small one. ``` erlang -module(hello_erlang_sup). -behavior(supervisor). -export([start_link/0]). -export([init/1]). start_link() -> supervisor:start_link({local, ?MODULE}, ?MODULE, []). init([]) -> {ok, {{one_for_one, 10, 10}, []}}. ``` Finally, we need to write the code for handling incoming requests. 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](http://ninenines.eu/docs/en/cowboy/HEAD/manual/cowboy_http_handler). Our handler will only send a friendly hello back to the client. ``` erlang -module(hello_handler). -behavior(cowboy_http_handler). -export([init/3]). -export([handle/2]). -export([terminate/3]). init(_Type, Req, _Opts) -> {ok, Req, undefined_state}. handle(Req, State) -> {ok, Req2} = cowboy_req:reply(200, [ {<<"content-type">>, <<"text/plain">>} ], <<"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 documented in the [cowboy_req function reference](http://ninenines.eu/docs/en/cowboy/HEAD/manual/cowboy_req). The code for our application is ready, so let's build a release! First we need to download `erlang.mk`. ``` bash $ wget https://raw.github.com/extend/erlang.mk/master/erlang.mk $ ls src/ erlang.mk ``` Then we need to create a Makefile that will include `erlang.mk` for building our application. We need to define the Cowboy dependency in the Makefile. Thankfully `erlang.mk` already knows where to find Cowboy as it features a package index, so we can just tell it to look there. ``` Makefile PROJECT = hello_erlang DEPS = cowboy dep_cowboy = pkg://cowboy master include erlang.mk ``` Note that when creating production nodes you will most likely want to use a specific version of Cowboy instead of `master`, and properly test your release every time you update Cowboy. If you type `make` in a shell now, your application should build as expected. If you get compilation errors, double check that you haven't made any typo when creating the previous files. ``` bash $ make ``` That's not all however, as we want to create a working release. For that purpose we will need `relx`. You can download it directly [from Github](https://github.com/erlware/relx). After downloading it, you will need to build it using `make`, which should give you a `relx` executable that you can then put in your `$PATH`. You only need to do this once. We are almost ready to build the release. All that's left is the `relx.config` file! In it, we only need to tell `relx` that we want the release to include the `hello_erlang` application, and that we want an extended start script for convenience. `relx` will figure out which other applications are required by looking into the `.app` files for dependencies. ``` erlang {release, {hello_erlang, "1"}, [hello_erlang]}. {extended_start_script, true}. ``` The `release` value is used to specify the release name, its version, and the applications to be included. We can now build and start the release. ``` bash $ relx $ ./_rel/bin/hello_erlang console ``` If you then access `http://localhost:8080` using your browser, you should receive a nice greet! You can find many more examples in the `examples/` directory of the Cowboy repository.