WSGI的理解 perfect

https://blog.csdn.net/hzrandd/article/details/10099871

https://blog.csdn.net/cloudxli/article/details/79210333 perfect

https://www.cnblogs.com/eric-nirnava/p/wsgi.html

------------------

前置技能 — WSGI

在具体读源码之前，这里先需要说一个概念，什么是WSGI。

WSGI，全称 Web Server Gateway Interface，或者 Python Web Server Gateway Interface ，是为 Python 语言定义的 Web 服务器和 Web 应用程序或框架之间的一种简单而通用的接口。自从 WSGI 被开发出来以后，许多其它语言中也出现了类似接口。

WSGI 的官方定义是，the Python Web Server Gateway Interface。从名字就可以看出来，这东西是一个Gateway，也就是网关。网关的作用就是在协议之间进行转换。

WSGI 是作为 Web 服务器与 Web 应用程序或应用框架之间的一种低级别的接口，以提升可移植 Web 应用开发的共同点。WSGI 是基于现存的 CGI 标准而设计的。

很多框架都自带了 WSGI server ，比如 Flask，webpy，Django、CherryPy等等。当然性能都不好，自带的 web server 更多的是测试用途，发布时则使用生产环境的 WSGI server或者是联合 nginx 做 uwsgi 。

在网上搜过WSGI的人应该会看到一个图，左边是Server，右边是APP，中间有一个连接纽带是WSGI。

不过，我看了源码以后的理解和这个有些不同，我个人觉得，实际上不应该单独写一个APP，因为，这个WSGI的使用方法实际上也是包含在APP里面的，最右端的app实际上应该指的是逻辑功能，包括URL和view function的对应关系。

所以我个人的理解如下自己画的图

WSGI其实是作为一个接口，来接受Server传递过来的信息, 然后通过这个接口调用后台app里的view function进行响应。

WSGI具体的功能

上面讲到了WSGI可以起到一个接口的功能，前面对接服务器，后面对接app的具体功能

我们先来看看最简单的一个wsgi_app的实现

[python] view plain copy

def application(environ, start_response): #一个符合wsgi协议的应用程序写法应该接受2个参数
start_response(’200 OK’, [(‘Content-Type’, ‘text/html’)]) #environ为http的相关信息，如请求头等 start_response则是响应信息
return [b‘<h1>Hello, web!</h1>’] #return出来是响应内容

def application(environ, start_response):               #一个符合wsgi协议的应用程序写法应该接受2个参数
    start_response('200 OK', [('Content-Type', 'text/html')])  #environ为http的相关信息，如请求头等 start_response则是响应信息
    return [b'<h1>Hello, web!</h1>']        #return出来是响应内容

但是，作为app本身，你就算启动了程序，你也没办法给application传递参数？

所以，实际上，调用application和传递2个参数的动作，是服务器来进行的，比如Gunicorn.

而这个叫做application的东西，在Flask框架内部的名字，叫做wsgi_app，请看下面章节的源码。

Flask和WSGI

生成Flask实例

我们再来看下生成flask应用的操作写法，用过的人肯定都非常熟悉。

[python] view plain copy

from flask import Flask
app = Flask(__name__) #生成app实例
@app.route(‘/’)
def index():
return ‘Hello World’

from flask import Flask

app = Flask(__name__)         #生成app实例

@app.route('/')
def index():
        return 'Hello World'

这样，一个flask app就生成了

但是这里有一个概念必须要搞清楚，就是当你的gunicorn收到http请求，去调用app的时候，他实际上是用了Flask 的 __call__方法，这点非常重要！！！

因为__call__方法怎么写，决定了你整个流程从哪里开始。

那我们来看下Flask类的__call__方法的源码

[python] view plain copy

class Flask(_PackageBoundObject): #Flask类
#中间省略一些代码
def __call__(self, environ, start_response): #Flask实例的__call__方法
”“”Shortcut for :attr:`wsgi_app`.”“”
return self.wsgi_app(environ, start_response) #注意他的return，他返回的时候，实际上是调用了wsgi_app这个功能

class Flask(_PackageBoundObject):        #Flask类

中间省略一些代码

def __call__(self, environ, start_response):    #Flask实例的__call__方法
    """Shortcut for :attr:`wsgi_app`."""
    return self.wsgi_app(environ, start_response)  #注意他的return，他返回的时候，实际上是调用了wsgi_app这个功能</pre>

如此一来，我们便知道，当http请求从server发送过来的时候，他会启动__call__功能，最终实际是调用了wsgi_app功能并传入environ和start_response

Flask的wsgi_app定义

[python] view plain copy

class Flask(_PackageBoundObject):
#中间省略一些代码
#请注意函数的说明，说得非常准确，这个wsgi_app是一个真正的WSGI应用
def wsgi_app(self, environ, start_response): #他扮演的是一个中间角色
”“”The actual WSGI application. This is not implemented in
`__call__` so that middlewares can be applied without losing a
reference to the class. So instead of doing this::
app = MyMiddleware(app)
It’s a better idea to do this instead::
app.wsgi_app = MyMiddleware(app.wsgi_app)
Then you still have the original application object around and
can continue to call methods on it.
:param environ: a WSGI environment
:param start_response: a callable accepting a status code,
a list of headers and an optional
exception context to start the response
”“”
ctx = self.request_context(environ)
ctx.push()
error = None
try:
try:
response = self.full_dispatch_request() #full_dispatch_request起到了预处理和错误处理以及分发请求的作用
except Exception as e:
error = e
response = self.make_response(self.handle_exception(e)) #如果有错误发生，则生成错误响应
return response(environ, start_response) #如果没有错误发生，则正常响应请求，返回响应内容
finally:
if self.should_ignore_error(error):
error = None
ctx.auto_pop(error)

class Flask(_PackageBoundObject):

中间省略一些代码

                            #请注意函数的说明，说得非常准确，这个wsgi_app是一个真正的WSGI应用
def wsgi_app(self, environ, start_response):    #他扮演的是一个中间角色
    """The actual WSGI application.  This is not implemented in
    `__call__` so that middlewares can be applied without losing a
    reference to the class.  So instead of doing this::

        app = MyMiddleware(app)

    It's a better idea to do this instead::

        app.wsgi_app = MyMiddleware(app.wsgi_app)

    Then you still have the original application object around and
    can continue to call methods on it.

    :param environ: a WSGI environment
    :param start_response: a callable accepting a status code,
                           a list of headers and an optional
                           exception context to start the response
    """
    ctx = self.request_context(environ)
    ctx.push()
    error = None
    try:
        try:
            response = self.full_dispatch_request()    #full_dispatch_request起到了预处理和错误处理以及分发请求的作用
        except Exception as e:
            error = e
            response = self.make_response(self.handle_exception(e))  #如果有错误发生，则生成错误响应
        return response(environ, start_response)       #如果没有错误发生，则正常响应请求，返回响应内容
    finally:
        if self.should_ignore_error(error):
            error = None
        ctx.auto_pop(error)</pre>

Ok, 这个wsgi_app的函数定义，基本上包含了整个流程的功能

WSGI_APP的内部流程

第一步：生成request请求对象和请求上下文环境

首先，你会看到ctx = self.request_context(environ)的语句，这个涉及到Flask使用了请求上下文和应用上下文的概念，结构为栈结构，这部分比较难，后面第二篇会单独写。

这里只需要理解为，上面语句产生的所用是生成了一个request请求对象以及包含请求信息在内的request context

第二部：请求进入预处理，错误处理及请求转发到响应的过程

进入wsgi_app的函数内部,生成了request对象和上下文环境之后，进入到try

[python] view plain copy

response = self.full_dispatch_request()

response = self.full_dispatch_request()

我们看到，响应被赋值成了full_dispatch_request()方法的返回内容，所以我们来看一下full_dispatch_request方法

[python] view plain copy

class Flask(_PackageBoundObject):
#此处省略一些代码
def full_dispatch_request(self):
”“”Dispatches the request and on top of that performs request
pre and postprocessing as well as HTTP exception catching and
error handling.
.. versionadded:: 0.7
”“”
self.try_trigger_before_first_request_functions() #进行发生真实请求前的处理
try:
request_started.send(self) #socket部分的操作
rv = self.preprocess_request() #进行请求的预处理
if rv is None:
rv = self.dispatch_request()
except Exception as e:
rv = self.handle_user_exception(e)
response = self.make_response(rv)
response = self.process_response(response)
request_finished.send(self, response=response)
return response

class Flask(_PackageBoundObject):

此处省略一些代码

def full_dispatch_request(self):
    """Dispatches the request and on top of that performs request
    pre and postprocessing as well as HTTP exception catching and
    error handling.

    .. versionadded:: 0.7
    """
    self.try_trigger_before_first_request_functions()  #进行发生真实请求前的处理
    try:
        request_started.send(self)                     #socket部分的操作
        rv = self.preprocess_request()                 #进行请求的预处理
        if rv is None:
            rv = self.dispatch_request()
    except Exception as e:
        rv = self.handle_user_exception(e)
    response = self.make_response(rv)
    response = self.process_response(response)
    request_finished.send(self, response=response)
    return response</pre><br><p></p>

他首先会触发 try_trigger_before_first_request_function()方法

在方法内部 ———->会触发 _got_first_request 属性，这个属性的返回值是True或者False. True的话就代表了程序开始处理请求了.

来看看 try_trigger_before_first_request_function()的代码，他的目的是，最后将_got_first_request属性置为True.

[python] view plain copy

class Flask(_PackageBoundObject):
#省略一些代码
def try_trigger_before_first_request_functions(self):
”“”Called before each request and will ensure that it triggers
the :attr:`before_first_request_funcs` and only exactly once per
application instance (which means process usually).
:internal:
”“”
if self._got_first_request:
return
with self._before_request_lock:
if self._got_first_request:
return
for func in self.before_first_request_funcs:
func()
self._got_first_request = True

class Flask(_PackageBoundObject):

省略一些代码

def try_trigger_before_first_request_functions(self):
    """Called before each request and will ensure that it triggers
    the :attr:`before_first_request_funcs` and only exactly once per
    application instance (which means process usually).

    :internal:
    """
    if self._got_first_request:
        return
    with self._before_request_lock:
        if self._got_first_request:
            return
        for func in self.before_first_request_funcs:
            func()
        self._got_first_request = True</pre>

再来看看_got_first_request 的定义，他的默认值是False

他的定义中可以明显看到， if the application started，this attribute is set to True.

[python] view plain copy

class Flask(_PackageBoundObject):
#省略一些代码
@property
def got_first_request(self):
”“”This attribute is set to “True“ if the application started
handling the first request.
.. versionadded:: 0.8
”“”
return self._got_first_request

class Flask(_PackageBoundObject):

省略一些代码

@property
def got_first_request(self):
    """This attribute is set to ``True`` if the application started
    handling the first request.

    .. versionadded:: 0.8
    """
    return self._got_first_request</pre>

接着，当_got_first_request 属性被设置完以后，我们就需要再次回到 full_dispatch_request函数内部，继续往下走

下面一段代码是request_started.send()，他是继承自signal模块，大致作用是进行socket部分的功能，暂时不详细追溯。

preprocess_request()方法的话，主要是进行flask的hook钩子, before_request功能的实现，也就是在真正发生请求之前，有些事情需要提前做

Flask一共有4个hook钩子，另外再写吧

随后，继续往下走，来到了一个至关重要的功能 dispatch_request()

[python] view plain copy

try:
request_started.send(self)
rv = self.preprocess_request()
if rv is None:
rv = self.dispatch_request()

        try:
            request_started.send(self)
            rv = self.preprocess_request()
            if rv is None:
                rv = self.dispatch_request()

为什么说至关重要，因为一个http请求到了这里，实际上已经完成了从wsgi部分的过渡，进入到了寻找响应的阶段了,一个请求通过url进来以后，app怎么知道要如何响应呢？

就是通过dispatch_request方法来进行请求判定和分发。

第三步：请求分发 dispatch_request

来看源码

[python] view plain copy

class Flask(_PackageBoundObject):
#省略一些代码
def dispatch_request(self): #看函数定义，matches the URL and returns the value of the view or error.
”“”Does the request dispatching. Matches the URL and returns the
return value of the view or error handler. This does not have to
be a response object. In order to convert the return value to a
proper response object, call :func:`make_response`.
.. versionchanged:: 0.7
This no longer does the exception handling, this code was
moved to the new :meth:`full_dispatch_request`.
”“”
req = _request_ctx_stack.top.request
if req.routing_exception is not None:
self.raise_routing_exception(req)
rule = req.url_rule
# if we provide automatic options for this URL and the
# request came with the OPTIONS method, reply automatically
if getattr(rule, ‘provide_automatic_options’, False)
and req.method == ‘OPTIONS’:
return self.make_default_options_response()
# otherwise dispatch to the handler for that endpoint
return self.view_functions[rule.endpoint](**req.view_args) #最终进入view_functions，取出url对应的视图函数的返回值

class Flask(_PackageBoundObject):

省略一些代码

def dispatch_request(self):   #看函数定义，matches the URL and returns the value of the view or error.
    """Does the request dispatching.  Matches the URL and returns the
    return value of the view or error handler.  This does not have to
    be a response object.  In order to convert the return value to a
    proper response object, call :func:`make_response`.

    .. versionchanged:: 0.7
       This no longer does the exception handling, this code was
       moved to the new :meth:`full_dispatch_request`.
    """
    req = _request_ctx_stack.top.request
    if req.routing_exception is not None:
        self.raise_routing_exception(req)
    rule = req.url_rule
    # if we provide automatic options for this URL and the
    # request came with the OPTIONS method, reply automatically
    if getattr(rule, 'provide_automatic_options', False) 
       and req.method == 'OPTIONS':
        return self.make_default_options_response()
    # otherwise dispatch to the handler for that endpoint
    return self.view_functions[rule.endpoint](**req.view_args)   #最终进入view_functions，取出url对应的视图函数的返回值</pre>

中间不需要过多考虑，req = _request_ctx_stack.top.request 可以暂时理解为，将请求对象赋值给req

这里先简单讲下，每个url进来以后，他都会对应一个view_function

比如下面的一个简单视图函数，路径 ‘/’ 对应的是index函数

但是，实际上当中是分2步走的，第一步是’/’对应的endpoint为’index’ ，第二部是endpoint ‘index’ 对应到index()视图函数

这个也是放在第二篇文章里面具体写flask 路由的实现，这里暂时可以忽略中间步骤，只要知道URL———–>VIEW FUNCTION的逻辑步骤就ok

[python] view plain copy

@app.route(‘/’)
def index():
return ‘Hello world’

@app.route('/')
def index():
    return 'Hello world'

另外说下view_functions 是一个字典形式，他的key和value的关系是endpoint ——> view function

所以每个有效的URL进来，都能找到他对应的视图函数view function，取得返回值并赋值给 rv

比如上面简单的index，他取得的就是 ‘Hello world’ 值

请求分发完成后，已经取得了返回的值，再看下一步是如何做

我们再次回到 full_dispatch_request方法内往下走

[html] view plain copy

response = self.make_response(rv)
response = self.process_response(response)
request_finished.send(self, response=response)
return response

        response = self.make_response(rv)
        response = self.process_response(response)
        request_finished.send(self, response=response)
        return response

这时候，通过make_response函数，将刚才取得的 rv 生成响应，重新赋值response

再通过process_response功能主要是处理一个after_request的功能，比如你在请求后，要把数据库连接关闭等动作，和上面提到的before_request对应和类似。

之后再进行request_finished.send的处理，也是和socket处理有关，暂时不详细深入。

之后返回新的response对象

这里特别需要注意的是，make_response函数是一个非常重要的函数，他的作用是返回一个response_class的实例对象，也就是可以接受environ和start_reponse两个参数的对象

非常重要！！！

Converts the return value from a view function to a real response object that is an instance of :attr:`response_class

[python] view plain copy

class Flask(_PackageBoundObject): #注意函数说明，converts the return value from view function to a real response object
#省略一部分代码
def make_response(self, rv):
”“”Converts the return value from a view function to a real
response object that is an instance of :attr:`response_class`.
The following types are allowed for `rv`:
.. tabularcolumns:: |p{3.5cm}|p{9.5cm}|
======================= ===========================================
:attr:`response_class` the object is returned unchanged
:class:`str` a response object is created with the
string as body
:class:`unicode` a response object is created with the
string encoded to utf-8 as body
a WSGI function the function is called as WSGI application
and buffered as response object
:class:`tuple` A tuple in the form “(response, status,
headers)“ or “(response, headers)“
where `response` is any of the
types defined here, `status` is a string
or an integer and `headers` is a list or
a dictionary with header values.
======================= ===========================================
:param rv: the return value from the view function
.. versionchanged:: 0.9
Previously a tuple was interpreted as the arguments for the
response object.
”“”
status_or_headers = headers = None
if isinstance(rv, tuple):
rv, status_or_headers, headers = rv + (None,) * (3 - len(rv))
if rv is None:
raise ValueError(‘View function did not return a response’)
if isinstance(status_or_headers, (dict, list)):
headers, status_or_headers = status_or_headers, None
if not isinstance(rv, self.response_class):
# When we create a response object directly, we let the constructor
# set the headers and status. We do this because there can be
# some extra logic involved when creating these objects with
# specific values (like default content type selection).
if isinstance(rv, (text_type, bytes, bytearray)):
rv = self.response_class(rv, headers=headers,
status=status_or_headers)
headers = status_or_headers = None
else:
rv = self.response_class.force_type(rv, request.environ)
if status_or_headers is not None:
if isinstance(status_or_headers, string_types):
rv.status = status_or_headers
else:
rv.status_code = status_or_headers
if headers:
rv.headers.extend(headers)
return rv

class Flask(_PackageBoundObject):   #注意函数说明，converts the return value from view function to a real response object

    #省略一部分代码

    def make_response(self, rv):
        """Converts the return value from a view function to a real
        response object that is an instance of :attr:`response_class`.

        The following types are allowed for `rv`:

        .. tabularcolumns:: |p{3.5cm}|p{9.5cm}|

        ======================= ===========================================
        :attr:`response_class`  the object is returned unchanged
        :class:`str`            a response object is created with the
                                string as body
        :class:`unicode`        a response object is created with the
                                string encoded to utf-8 as body
        a WSGI function         the function is called as WSGI application
                                and buffered as response object
        :class:`tuple`          A tuple in the form ``(response, status,
                                headers)`` or ``(response, headers)``
                                where `response` is any of the
                                types defined here, `status` is a string
                                or an integer and `headers` is a list or
                                a dictionary with header values.
        ======================= ===========================================

        :param rv: the return value from the view function

        .. versionchanged:: 0.9
           Previously a tuple was interpreted as the arguments for the
           response object.
        """
        status_or_headers = headers = None
        if isinstance(rv, tuple):
            rv, status_or_headers, headers = rv + (None,) * (3 - len(rv))

        if rv is None:
            raise ValueError('View function did not return a response')

        if isinstance(status_or_headers, (dict, list)):
            headers, status_or_headers = status_or_headers, None

        if not isinstance(rv, self.response_class):
            # When we create a response object directly, we let the constructor
            # set the headers and status.  We do this because there can be
            # some extra logic involved when creating these objects with
            # specific values (like default content type selection).
            if isinstance(rv, (text_type, bytes, bytearray)):
                rv = self.response_class(rv, headers=headers,
                                         status=status_or_headers)
                headers = status_or_headers = None
            else:
                rv = self.response_class.force_type(rv, request.environ)

        if status_or_headers is not None:
            if isinstance(status_or_headers, string_types):
                rv.status = status_or_headers
            else:
                rv.status_code = status_or_headers
        if headers:
            rv.headers.extend(headers)

        return rv

第四步：返回到wsgi_app内部

终于快进行到了最后一步，流程走回到了wsgi_app的内部

下面这段是wsgi_app内部的代码

[python] view plain copy

try:
try:
response = self.full_dispatch_request()
except Exception as e:
error = e
response = self.make_response(self.handle_exception(e))
return response(environ, start_response)
finally:
if self.should_ignore_error(error):
error = None
ctx.auto_pop(error)

        try:
            try:
                response = self.full_dispatch_request()
            except Exception as e:
                error = e
                response = self.make_response(self.handle_exception(e))
            return response(environ, start_response)
        finally:
            if self.should_ignore_error(error):
                error = None
            ctx.auto_pop(error)

当response从刚刚的full_dispatch_request功能返回之后，函数会对这个response加上environ, start_response的参数并返回给Gunicorn

至此，一个HTTP从请求到响应的流程就完毕了.

总的来说，一个流程的关键步骤可以简单归结如下：

后一篇，将会记录一下flask的route实现，里面的url如何和endpoint对应起来，endpoint和view function又是如何对应起来

本篇参考资料：

http://docs.jinkan.org/docs/flask/api.html?highlight=wsgi_app#flask.Flask.wsgi_app

https://segmentfault.com/a/1190000004223296

http://docs.gunicorn.org/en/stable/run.html