erlang + rebar + cowboy Hello World

readme.md

This is the process of setting up erlang, rebar3, and cowboy for a Hello World, starting with a clean Debian 8 install.

Update apt and install deps:

root@046edcaea45a:~# apt-get update
root@046edcaea45a:~# apt-get install erlang erlang-dev gcc
root@046edcaea45a:~# wget https://s3.amazonaws.com/rebar3/rebar3
root@046edcaea45a:~# mkdir ~/bin/
root@046edcaea45a:~# mv rebar3 ~/bin/
root@046edcaea45a:~# chmod +x ~/bin/rebar3 

Create the rebar project:

root@046edcaea45a:~# ~/bin/rebar3 new release cowboy_hello_world 
root@046edcaea45a:~# cd cowboy_hello_world/

Put the following entry in the rebar.config deps tuple and add the rebar3_run plugin:

root@046edcaea45a:~/cowboy_hello_world# vi rebar.config 

{deps, [
        {cowboy, {git, "https://github.com/ninenines/cowboy", {tag, "2.0.0-pre.1"}}}
]}.

{plugins, [rebar3_run]}.

Add cowboy to the list {application, [{applications, [kernel, stdlib, cowboy]}]}

root@046edcaea45a:~/cowboy_hello_world# vi apps/cowboy_hello_world/src/cowboy_hello_world.app.src 

Register a handler for the / url

root@046edcaea45a:~/cowboy_hello_world# vi apps/cowboy_hello_world/src/cowboy_hello_world_app.erl  

start(_StartType, _StartArgs) ->
    {ok, Pid} = 'cowboy_hello_world_sup':start_link(),
    Routes = [ {
        '_',
        [
            {"/", cowboy_hello_world_root, []}
        ]
    } ],
    Dispatch = cowboy_router:compile(Routes),

    NumAcceptors = 10,
    TransOpts = [ {ip, {0,0,0,0}}, {port, 2938} ],
    ProtoOpts = [{env, [{dispatch, Dispatch}]}],

    {ok, _} = cowboy:start_http(chicken_poo_poo,
        NumAcceptors, TransOpts, ProtoOpts),

    {ok, Pid}.

Create the handler referenced above:

root@046edcaea45a:~/cowboy_hello_world# vi apps/cowboy_hello_world/src/cowboy_hello_world_root.erl

-module(cowboy_hello_world_root).

-export([init/2]).

init(Req, Opts) ->
    Req2 = cowboy_req:reply(200,
        [{<<"content-type">>, <<"text/plain">>}],
        <<"Hello Erlang!">>,
        Req),
    {ok, Req2, Opts}.

root@046edcaea45a:~/cowboy_hello_world# ~/bin/rebar3 run

Browse to http://localhost:2938 and win.

If you want to try this with Cowboy 2.0.0-rc.1, currently the latest preview release of Cowboy 2.x, you'll need to make a few changes. Since version 2.0.0-pre.1 in many places in the API property list arguments ([{key1, value1}, ..., {keyN, valueN}]) have been replaced with maps (#{key1 => value1, ..., keyN => valueN}), and cowboy:start_http/4 has been replaced with cowboy:start_clear/3. Update the Cowboy version in rebar.config, run rebar3 upgrade, then apply the following change to your files.

apps/cowboy_hello_world/src/cowboy_hello_world_app.erl

--- a/apps/cowboy_hello_world/src/cowboy_hello_world_app.erl.old
+++ b/apps/cowboy_hello_world/src/cowboy_hello_world_app.erl
@@ -24,12 +24,11 @@ start(_StartType, _StartArgs) ->
     } ],
     Dispatch = cowboy_router:compile(Routes),
 
-    NumAcceptors = 10,
     TransOpts = [ {ip, {0,0,0,0}}, {port, 2938} ],
-    ProtoOpts = [{env, [{dispatch, Dispatch}]}],
+    ProtoOpts = #{env => #{dispatch => Dispatch}},
 
-    {ok, _} = cowboy:start_http(chicken_poo_poo,
-        NumAcceptors, TransOpts, ProtoOpts),
+    {ok, _} = cowboy:start_clear(chicken_poo_poo,
+        TransOpts, ProtoOpts),
 
     {ok, Pid}.
 %%--------------------------------------------------------------------

apps/cowboy_hello_world/src/cowboy_hello_world_root.erl

--- a/apps/cowboy_hello_world/src/cowboy_hello_world_root.erl.old
+++ b/apps/cowboy_hello_world/src/cowboy_hello_world_root.erl
@@ -4,7 +4,7 @@
 
 init(Req, Opts) ->
     Req2 = cowboy_req:reply(200,
-        [{<<"content-type">>, <<"text/plain">>}],
+        #{<<"content-type">> => <<"text/plain">>},
         <<"Hello Erlang!">>,
         Req),
     {ok, Req2, Opts}.