# Gateway - the Protocol between Applications and Web Servers

## WSGI

### What is WSGI?

> WSGI is the Web Server Gateway Interface. It is a specification that describes how a web server communicates with web applications, and how web applications can be chained together to process one request.
>
> WSGI is a Python standard described in detail in PEP 3333.

从[定义](https://wsgi.readthedocs.io/en/latest/what.html)来看，WSGI做了两件事情：

* 定义 web服务器 <-> web应用 之间的交流规则
* 处理请求时，web应用 <-> web应用 之间如何串联

另外，WSGI是针对Python定义的，下面就来部署一个简单的Python "Hello World" web应用。

### Deploy a WSGI Application

这是一个简单的，遵循WSGI协议的Python应用(<https://werkzeug.palletsprojects.com/en/2.3.x/tutorial/#step-0-a-basic-wsgi-introduction>)

```python
def application(environ, start_response):
    start_response('200 OK', [('Content-Type', 'text/plain')])
    return ['Hello World!'.encode('utf-8')]
```

![](/files/Z9UFAZE5CV1O79WSmssT)

#### gunicorn

> Gunicorn 'Green Unicorn' is a Python WSGI HTTP Server for UNIX.

gunicorn原生支持WSGI协议，所以用它来部署python应用很方便，不需要额外的配置。

```
[root@uco-pdc-mgmt01 tmp]# gunicorn wsgi:app --pythonpath /root
[2020-05-17 16:51:10 +0800] [28843] [INFO] Starting gunicorn 19.9.0
[2020-05-17 16:51:10 +0800] [28843] [INFO] Listening at: http://127.0.0.1:8000 (28843)
[2020-05-17 16:51:10 +0800] [28843] [INFO] Using worker: sync
[2020-05-17 16:51:10 +0800] [28846] [INFO] Booting worker with pid: 28846
```

```
[bwu@uco-pdc-mgmt01 ~]$curl localhost:8000
Hello World!
```

#### httpd

httpd原生不支持WSGI，所以需要安装`mod_wsgi`模块，相比gunicorn还要进行一些额外配置

<https://flask.palletsprojects.com/en/1.1.x/deploying/mod\\_wsgi/>

## CGI

CGI是个比较古老的技术，也是用来定义应用和web服务器之间如何交互，但是它并不限定于某种语言，相比WSGI更加通用。"C"就是Common的意思。

下面还是用httpd来部署一个CGI应用

### Deploy a CGI Application

## 小结

这些"X"GI本质上是一个协议，约定了双方交流的一个规则。不支持这些协议的应用，可以通过添加额外的模块/插件(e.g. httpd的mod\_cgi)，从而支持这个协议。有的应用从设计之初就支持，比如gunicorn原生支持WSGI，应用程序代码会按照这个协议规则来写，也算原生支持，不过为了开发方便，关于协议实现部分，都被包装在框架里，代码只要专注于业务逻辑实现就行了。


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