Erlang cowboy 入门参考
cheungmine,2014-10-28
本文翻译自：
http://ninenines.eu/docs/en/cowboy/HEAD/guide/getting_started/
我没有按原文逐句翻译。仅仅是我自己的理解，力求简单明了。本文程序在RHEL6.4上写成并运行，参考下面的文章，安装Erlang：
1 引言
Erlang不仅仅是一门语言，更是一个操作平台。Erlang开发者很少写单独的模块，而是写库和程序，然后把它们打包在一起进行发布。一个产品发布包含Erlang虚拟机和所有用来运行的程序，因此可以直接成为产品。Cowboy是Erlang编写的WebServer，提供Http,Https,WebSocket,TCP等各种高性能服务框架。本文将说明如何安装Cowboy，写第一个程序并发布。通过阅读本文，读者可以了解发布你的第一个Cowboy程序的全部内容。
2 开始第一个产品
当我们要开发一个程序(产品)，我们希望用一个工程去管理它。代码构建，打包，发布等一系列行为。做Java开发使用的是Eclipse+Maven+Ant等工具。做Erlang开发可以使用Gnu Make。由于每个工具所完成的功能都类似，因此推荐使用下面这个Make模板。
https://github.com/ninenines/erlang.mk
下面开始我们的第一个Erlang工程。创建一个hello_erlang的目录，进入这个目录：
$ mkdir hello_erlang && cd hello_erlang
下载erlang.mk文件。这是erlang工程的构建脚本。关于这个脚本的细节可以在各位成为erlang专家的时候去理解。
$ wget --no-check-certificate https://raw.githubusercontent.com/ninenines/erlang.mk/master/erlang.mk
之后我们会看到目录下多了一个文件：erlang.mk。然后我们就可以生成我们的程序，同时生成发布版本(bootstrap,bootstrap-rel是target)：
$ make -f erlang.mk bootstrap bootstrap-rel
现在就可以构建我们的工程，生成发布版本：
$ make
这个命令会花费较长时间执行完毕。然后启动运行产品程序：
$ ./_rel/hello_erlang_release/bin/hello_erlang_release console
输入下面的命令可以看到运行的进程，其中包括：hello_erlang_sup。这个是我们的程序的督程(supervisor)。
(hello_erlang@127.0.0.1)1> i().
现在这个程序什么也没做，接下来我们给它添加Cowboy依赖，然后把它变成一个简单的HelloWorld程序。整个过程就是如下样子：
[root@rhel64-origin workspace]# mkdir hello_erlang
[root@rhel64-origin workspace]# cd hello_erlang/
[root@rhel64-origin hello_erlang]# ls
[root@rhel64-origin hello_erlang]# wget --no-check-certificate https://raw.githubusercontent.com/ninenines/erlang.mk/master/erlang.mk
--2014-10-28 17:42:55-- https://raw.githubusercontent.com/ninenines/erlang.mk/master/erlang.mk
Resolving raw.githubusercontent.com... 103.245.222.133
Connecting to raw.githubusercontent.com|103.245.222.133|:443... connected.
WARNING: certificate common name “www.github.com” doesn’t match requested host name “raw.githubusercontent.com”.
HTTP request sent, awaiting response... 200 OK
Length: 20522 (20K) [text/plain]
Saving to: “erlang.mk”
100%[======================================================>] 20,522 --.-K/s in 0.04s
2014-10-28 17:42:55 (473 KB/s) - “erlang.mk” saved [20522/20522]
[root@rhel64-origin hello_erlang]# ls
erlang.mk
[root@rhel64-origin hello_erlang]# make -f erlang.mk bootstrap bootstrap-rel
[root@rhel64-origin hello_erlang]# make
ERLC hello_erlang_app.erl hello_erlang_sup.erl
APP hello_erlang.app.src
GEN distclean-relx-rel
--2014-10-28 17:43:08-- https://github.com/erlware/relx/releases/download/v1.0.2/relx
Resolving github.com... 192.30.252.131
Connecting to github.com|192.30.252.131|:443... connected.
HTTP request sent, awaiting response... 302 Found
Location: https://s3.amazonaws.com/github-cloud/releases/9900098/f6fcb596-e38d-11e3-9e8b-c953e4c1e42a?response-content-disposition=attachment%3B%20filename%3Drelx&response-content-type=application/octet-stream&AWSAccessKeyId=AKIAISTNZFOVBIJMK3TQ&Expires=1414489636&Signature=adyKnn8IPqauqWSnsXhJTc2vBio%3D [following]
--2014-10-28 17:43:12-- https://s3.amazonaws.com/github-cloud/releases/9900098/f6fcb596-e38d-11e3-9e8b-c953e4c1e42a?response-content-disposition=attachment%3B%20filename%3Drelx&response-content-type=application/octet-stream&AWSAccessKeyId=AKIAISTNZFOVBIJMK3TQ&Expires=1414489636&Signature=adyKnn8IPqauqWSnsXhJTc2vBio%3D
Resolving s3.amazonaws.com... 54.231.244.0
Connecting to s3.amazonaws.com|54.231.244.0|:443... connected.
HTTP request sent, awaiting response... 200 OK
Length: 743699 (726K) [application/octet-stream]
Saving to: “/root/workspace/hello_erlang/relx”
100%[======================================================>] 743,699 147K/s in 5.0s
2014-10-28 17:43:19 (145 KB/s) - “/root/workspace/hello_erlang/relx” saved [743699/743699]
===> Starting relx build process ...
===> Resolving OTP Applications from directories:
/root/workspace/hello_erlang/ebin
/usr/local/lib/erlang/lib
===> Resolving available OTP Releases from directories:
/root/workspace/hello_erlang/ebin
/usr/local/lib/erlang/lib
===> Resolved hello_erlang_release-1
===> Including Erts from /usr/local/lib/erlang
===> release successfully created!
3 cowboy安装
为了使我们的hello_erlang使用cowboy，我们需要修改2个文件：Makefile和src/hello_erlang.app.src。修改后的Makefile如下：
PROJECT = hello_erlang DEPS = cowboy include erlang.mk
我们在src/hello_erlang.app.src文件中添加cowboy，这让我们的程序发布时自动将cowboy打包。有些仅仅是开发阶段的依赖不必要添加在这里(applications[...])。修改后的文件如下：
{application, hello_erlang, [
{description, "Hello Erlang!"},
{vsn, "0.1.0"},
{modules, []},
{registered, []},
{applications, [
kernel,
stdlib,
cowboy
]},
{mod, {hello_erlang_app, []}},
{env, []}
]}.
现在我们运行make命令，cowboy就被自动加入到产品中，当然它什么也没做。输出如下：
[root@rhel64-origin hello_erlang]# make
--2014-10-28 17:53:58-- https://raw.githubusercontent.com/ninenines/erlang.mk/master/packages.v2.tsv
Resolving raw.githubusercontent.com... 103.245.222.133
Connecting to raw.githubusercontent.com|103.245.222.133|:443... connected.
WARNING: certificate common name “www.github.com” doesn’t match requested host name “raw.githubusercontent.com”.
HTTP request sent, awaiting response... 200 OK
Length: 7115 (6.9K) [text/plain]
Saving to: “/root/workspace/hello_erlang/.erlang.mk.packages.v2”
100%[======================================================>] 7,115 --.-K/s in 0.001s
2014-10-28 17:54:06 (12.6 MB/s) - “/root/workspace/hello_erlang/.erlang.mk.packages.v2” saved [7115/7115]
Initialized empty Git repository in /root/workspace/hello_erlang/deps/cowboy/.git/
remote: Counting objects: 7063, done.
remote: Total 7063 (delta 0), reused 0 (delta 0)
Receiving objects: 100% (7063/7063), 5.26 MiB | 530 KiB/s, done.
Resolving deltas: 100% (4198/4198), done.
make[1]: Entering directory `/root/workspace/hello_erlang/deps/cowboy'
DEPPKG=$(awk 'BEGIN { FS = "\t" }; $1 == "cowlib" { print $2 " " $3 " " $4 }' /root/workspace/hello_erlang/.erlang.mk.packages.v2;) VS=$(echo $DEPPKG | cut -d " " -f1); REPO=$(echo $DEPPKG | cut -d " " -f2); COMMIT=$(echo $DEPPKG | cut -d " " -f3); if [ "$VS" = "git" ]; then git clone -n -- $REPO /root/workspace/hello_erlang/deps/cowlib; cd /root/workspace/hello_erlang/deps/cowlib && git checkout -q $COMMIT; else exit 78; fi
Initialized empty Git repository in /root/workspace/hello_erlang/deps/cowlib/.git/
remote: Counting objects: 202, done.
remote: Total 202 (delta 0), reused 0 (delta 0)
Receiving objects: 100% (202/202), 87.64 KiB | 100 KiB/s, done.
Resolving deltas: 100% (119/119), done.
DEPPKG=$(awk 'BEGIN { FS = "\t" }; $1 == "ranch" { print $2 " " $3 " " $4 }' /root/workspace/hello_erlang/.erlang.mk.packages.v2;) VS=$(echo $DEPPKG | cut -d " " -f1); REPO=$(echo $DEPPKG | cut -d " " -f2); COMMIT=$(echo $DEPPKG | cut -d " " -f3); if [ "$VS" = "git" ]; then git clone -n -- $REPO /root/workspace/hello_erlang/deps/ranch; cd /root/workspace/hello_erlang/deps/ranch && git checkout -q $COMMIT; else exit 78; fi
Initialized empty Git repository in /root/workspace/hello_erlang/deps/ranch/.git/
remote: Counting objects: 905, done.
Receiving objects: 100% (905/905), 288.85 KiB | 170 KiB/s, done.
remote: Total 905 (delta 0), reused 0 (delta 0)
Resolving deltas: 100% (532/532), done.
make[2]: Entering directory `/root/workspace/hello_erlang/deps/cowlib'
ERLC cow_http_te.erl cow_http.erl cow_http_hd.erl cow_date.erl cow_mimetypes.erl cow_multipart.erl cow_qs.erl cow_cookie.erl cow_spdy.erl
APP cowlib.app.src
make[2]: Leaving directory `/root/workspace/hello_erlang/deps/cowlib'
make[2]: Entering directory `/root/workspace/hello_erlang/deps/ranch'
ERLC ranch_acceptors_sup.erl ranch_conns_sup.erl ranch_server.erl ranch_tcp.erl ranch.erl ranch_acceptor.erl ranch_listener_sup.erl ranch_app.erl ranch_protocol.erl ranch_ssl.erl ranch_sup.erl ranch_transport.erl
APP ranch.app.src
make[2]: Leaving directory `/root/workspace/hello_erlang/deps/ranch'
ERLC cowboy_router.erl cowboy_rest.erl cowboy_http_handler.erl cowboy_sup.erl cowboy_websocket_handler.erl cowboy_static.erl cowboy_sub_protocol.erl cowboy_websocket.erl cowboy.erl cowboy_app.erl cowboy_handler.erl cowboy_loop_handler.erl cowboy_protocol.erl cowboy_bstr.erl cowboy_clock.erl cowboy_http.erl cowboy_spdy.erl cowboy_req.erl cowboy_middleware.erl
APP cowboy.app.src
make[1]: Leaving directory `/root/workspace/hello_erlang/deps/cowboy'
APP hello_erlang.app.src
GEN distclean-relx-rel
===> Starting relx build process ...
===> Resolving OTP Applications from directories:
/root/workspace/hello_erlang/ebin
/root/workspace/hello_erlang/deps
/usr/local/lib/erlang/lib
===> Resolving available OTP Releases from directories:
/root/workspace/hello_erlang/ebin
/root/workspace/hello_erlang/deps
/usr/local/lib/erlang/lib
===> Resolved hello_erlang_release-1
===> Including Erts from /usr/local/lib/erlang
===> release successfully created!
4 监听连接
当一个客户端请求到来，我们需要根据请求内容将请求分发(dispatch)到处理器(handler)。因此我们需要一个路由表(dispatch list)，cowboy根据路由表分发请求到对应的处理模块程序。然后我们让cowboy开始监听连接。
编辑Erlang程序源代码文件：src/hello_erlang_app.erl，修改start/2函数，最后的src/hello_erlang_app.erl如下：
-module(hello_erlang_app).
-behaviour(application).
-export([start/2]).
-export([stop/1]).
start(_Type, _Args) ->
Dispatch = cowboy_router:compile([
{'_', [{"/", hello_handler, []}]}
]),
cowboy:start_http(my_http_listener, 100, [{port, 8080}],
[{env, [{dispatch, Dispatch}]}]
),
hello_erlang_sup:start_link().
stop(_State) ->
ok.
现在我们把根路由"/"定义到hello_handler，下面我们就需要为此写这个处理器。
5 处理请求
cowboy具有处理各种请求的特色，例如：REST，WebSocket等。这里我们使用一个简单的HTTP处理器。我们使用处理器模板生成这个处理器：
$ make new t=cowboy_http n=hello_handler
这将创建src/hello_handler.erl代码文件。打开它，修改成下面的样子(只修改了init/3)：
-module(hello_handler).
-behaviour(cowboy_http_handler).
-export([init/3]).
-export([handle/2]).
-export([terminate/3]).
-record(state, {
}).
init(_, Req, Opts) ->
Req2 = cowboy_req:reply(200,
[{<<"content-type">>, <<"text/plain">>}],
<<"Hello Erlang!">>,
Req),
{ok, Req2, Opts}.
handle(Req, State=#state{}) ->
{ok, Req2} = cowboy_req:reply(200, Req),
{ok, Req2, State}.
terminate(_Reason, _Req, _State) ->
ok.
6 构建并运行
$ make $ ./_rel/hello_erlang_release/bin/hello_erlang_release console
在浏览器中打开：http://your-ip-addr:8080/，显示如下：
7 总结
cowboy这个框架和nodejs很像。erlang的语法有点生疏，但是erlang比nodejs不知道强多少倍。erlang是C写的，nodejs的底层是C++，可见多数C++写的产品最后都不如C写的，这是偶然么？“一只鸟长得像鸭子，叫声像鸭子，走路也像鸭子，那它就是鸭子！”