Scrapy 源码分析之 RetryMiddleware 模块-技术圈

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作者：TheWeiJun

来源：逆向与爬虫的故事

时隔一个多月，scrapy 章节又迎来了重大更新，今天分享的主题是 RetryMiddleware 中间件。文中若有错误内容，欢迎各位读者多多指正。在阅读的同时不要忘记点赞+关注哦⛽️

一、问题思考

二、文档查寻

三、源码分析

四、源码重写

五、总结分享

趣味模块

娜娜是一名爬虫工程师，最近小娜在采集数据过程中遇到了难题。原因是因为任务积压代理超时了，所有的 request 全部无法下载了。娜娜很是苦恼，不知道如何解决这类型问题。后来小娜看了 TheWeiJun 发表的文章，存在的问题立马迎刃而解，接下来，让我们一起去看看他们是怎么做的吧。

一、问题思考

Question

①使用 scrapy 框架时，如果请求失败，如何保证该请求成功率？

Question

②scrapy 的重试机制是否了解，默认是几次？在什么样的情况下触发？

Question

③scrapy 重试机制，重试状态码有哪些，我们是否可以动态定义？

Question

④scrapy 在重试过程中，如何实时更换代理？如何清除失效的代理？

前言：那么带着这些问题，接下来我们对 Scrapy 源码进行分析探索吧，我相信这篇文章会让大家受益匪浅！

二、文档查寻

1、查看官网文档，搜索指定的模块 RetryMiddleware，搜索结果如下：

说明：观察搜索结果，我们发现官方文档中存在对 RetryMiddleware 模块的解释，接下来让我们点进去，一起去看看官方说明吧。

2、点击搜索结果，查看官方对当前模块的说明解释截图如下：

说明：观察上面的截图，我们发现上面提到的问题大家应该已经知道了部分答案吧。但是还是不够清晰，接下来，让我带大家进入源码分析环节一探究竟吧！

三、源码分析

RetryMiddleware 模块源码如下：

def get_retry_request(    request: Request,    *,    spider: Spider,    reason: Union[str, Exception] = 'unspecified',    max_retry_times: Optional[int] = None,    priority_adjust: Optional[int] = None,    logger: Logger = retry_logger,    stats_base_key: str = 'retry',):    settings = spider.crawler.settings    stats = spider.crawler.stats    retry_times = request.meta.get('retry_times', 0) + 1    if max_retry_times is None:        max_retry_times = request.meta.get('max_retry_times')        if max_retry_times is None:            max_retry_times = settings.getint('RETRY_TIMES')    if retry_times <= max_retry_times:        logger.debug(            "Retrying %(request)s (failed %(retry_times)d times): %(reason)s",            {'request': request, 'retry_times': retry_times, 'reason': reason},            extra={'spider': spider}        )        new_request: Request = request.copy()        new_request.meta['retry_times'] = retry_times        new_request.dont_filter = True        if priority_adjust is None:            priority_adjust = settings.getint('RETRY_PRIORITY_ADJUST')        new_request.priority = request.priority + priority_adjust
        if callable(reason):            reason = reason()        if isinstance(reason, Exception):            reason = global_object_name(reason.__class__)
        stats.inc_value(f'{stats_base_key}/count')        stats.inc_value(f'{stats_base_key}/reason_count/{reason}')        return new_request    else:        stats.inc_value(f'{stats_base_key}/max_reached')        logger.error(            "Gave up retrying %(request)s (failed %(retry_times)d times): "            "%(reason)s",            {'request': request, 'retry_times': retry_times, 'reason': reason},            extra={'spider': spider},        )        return None

class RetryMiddleware:
    # IOError is raised by the HttpCompression middleware when trying to    # decompress an empty response    EXCEPTIONS_TO_RETRY = (defer.TimeoutError, TimeoutError, DNSLookupError,                           ConnectionRefusedError, ConnectionDone, ConnectError,                           ConnectionLost, TCPTimedOutError, ResponseFailed,                           IOError, TunnelError)
    def __init__(self, settings):        if not settings.getbool('RETRY_ENABLED'):            raise NotConfigured        self.max_retry_times = settings.getint('RETRY_TIMES')        self.retry_http_codes = set(int(x) for x in settings.getlist('RETRY_HTTP_CODES'))        self.priority_adjust = settings.getint('RETRY_PRIORITY_ADJUST')
    @classmethod    def from_crawler(cls, crawler):        return cls(crawler.settings)
    def process_response(self, request, response, spider):        if request.meta.get('dont_retry', False):            return response        if response.status in self.retry_http_codes:            reason = response_status_message(response.status)            return self._retry(request, reason, spider) or response        return response
    def process_exception(self, request, exception, spider):        if (            isinstance(exception, self.EXCEPTIONS_TO_RETRY)            and not request.meta.get('dont_retry', False)        ):            return self._retry(request, exception, spider)
    def _retry(self, request, reason, spider):        max_retry_times = request.meta.get('max_retry_times', self.max_retry_times)        priority_adjust = request.meta.get('priority_adjust', self.priority_adjust)        return get_retry_request(            request,            reason=reason,            spider=spider,            max_retry_times=max_retry_times,            priority_adjust=priority_adjust,        )

环节说明：代码一共也就 94 行，但是却能实现多个功能。在好奇心的驱使下，我们还是对源码进行一一讲解分析吧。

from_crawler 函数

# 类方法,创建当前class的实例对象，参数：当前spider settings对象@classmethoddef from_crawler(cls, crawler):    return cls(crawler.settings)

__init__ 函数

"""这里涉及到了settings.py配置文件中定义的一些参数。RETRY_ENABLED: 用于开启中间件，默认为TrueRETRY_TIMES: 重试次数, 默认为2RETRY_HTTP_CODES: 遇到哪些返回状态码需要重试, 一个列表，默认为[500, 503, 504, 400, 408]RETRY_PRIORITY_ADJUST：调整相对于原始请求的重试请求优先级，默认为-1"""def __init__(self, settings):    if not settings.getbool('RETRY_ENABLED'):        raise NotConfigured    self.max_retry_times = settings.getint('RETRY_TIMES')    self.retry_http_codes = set(int(x) for x in settings.getlist('RETRY_HTTP_CODES'))    self.priority_adjust = settings.getint('RETRY_PRIORITY_ADJUST')

process_response 函数
process_exception 函数

EXCEPTIONS_TO_RETRY = (defer.TimeoutError, TimeoutError, DNSLookupError,                           ConnectionRefusedError, ConnectionDone, ConnectError,                           ConnectionLost, TCPTimedOutError, ResponseFailed,                           IOError, TunnelError)                           def process_response(self, request, response, spider):    # 处理request请求，确定是否需要请求重试，重试触发机制，前面提到的问题.    if request.meta.get('dont_retry', False):        return response    # 检查response状态码是否在重试机制list中,如果存在就要调用_retry方法进行重试    if response.status in self.retry_http_codes:        reason = response_status_message(response.status)        return self._retry(request, reason, spider) or response    # 不存在会返回response，但会被spider parse方法是过滤掉，只处理200状态码    return response
def process_exception(self, request, exception, spider):    # 如果产生了EXCEPTIONS_TO_RETRY列表中的异常错误并且重试机制为开启状态,则会调用_retry方法进行重试。    if (        isinstance(exception, self.EXCEPTIONS_TO_RETRY)        and not request.meta.get('dont_retry', False)    ):        return self._retry(request, exception, spider)

_retry 函数
get_retry_request 函数

# 该方法获取最大重试次数，和请求重试优先级，然后调用get_retry_request方法def _retry(self, request, reason, spider):    max_retry_times = request.meta.get('max_retry_times', self.max_retry_times)    priority_adjust = request.meta.get('priority_adjust', self.priority_adjust)    return get_retry_request(        request,        reason=reason,        spider=spider,        max_retry_times=max_retry_times,        priority_adjust=priority_adjust,    )"""读取当前重试次数和最大重试次数进行比较，
如果小于等于最大重试次数：利用copy方法在原来的request上复制一个新request，并更新其retry_times，并将dont_filter设为True来防止因url重复而被过滤。
如果超出最大重试次数：记录重试失败请求量，并放弃该请求记录到logger日志中,logger级别为：error
"""def get_retry_request(    request: Request,    *,    spider: Spider,    reason: Union[str, Exception] = 'unspecified',    max_retry_times: Optional[int] = None,    priority_adjust: Optional[int] = None,    logger: Logger = retry_logger,    stats_base_key: str = 'retry',):    settings = spider.crawler.settings    stats = spider.crawler.stats    retry_times = request.meta.get('retry_times', 0) + 1    if max_retry_times is None:        max_retry_times = request.meta.get('max_retry_times')        if max_retry_times is None:            max_retry_times = settings.getint('RETRY_TIMES')    if retry_times <= max_retry_times:        logger.debug(            "Retrying %(request)s (failed %(retry_times)d times): %(reason)s",            {'request': request, 'retry_times': retry_times, 'reason': reason},            extra={'spider': spider}        )        new_request: Request = request.copy()        new_request.meta['retry_times'] = retry_times        new_request.dont_filter = True        if priority_adjust is None:            priority_adjust = settings.getint('RETRY_PRIORITY_ADJUST')        new_request.priority = request.priority + priority_adjust
        if callable(reason):            reason = reason()        if isinstance(reason, Exception):            reason = global_object_name(reason.__class__)
        stats.inc_value(f'{stats_base_key}/count')        stats.inc_value(f'{stats_base_key}/reason_count/{reason}')        return new_request    else:        stats.inc_value(f'{stats_base_key}/max_reached')        logger.error(            "Gave up retrying %(request)s (failed %(retry_times)d times): "            "%(reason)s",            {'request': request, 'retry_times': retry_times, 'reason': reason},            extra={'spider': spider},        )        return None

环节总结：整个源码分析流程到这里就结束了，接下来我们一起进入源码重写环节来解决下娜娜遇到的问题吧，我相信大家会豁然开朗的。

四、源码重写

重写 RetryMiddleware 源码后完整代码如下：

class RetryMiddleware:    EXCEPTIONS_TO_RETRY = (defer.TimeoutError, TimeoutError, DNSLookupError,                           ConnectionRefusedError, ConnectionDone, ConnectError,                           ConnectionLost, TCPTimedOutError, ResponseFailed,                           IOError, TunnelError)
    def __init__(self, settings):        if not settings.getbool('RETRY_ENABLED'):            raise NotConfigured        self.max_retry_times = settings.getint('RETRY_TIMES')        self.retry_http_codes = set(int(x) for x in settings.getlist('RETRY_HTTP_CODES'))        self.priority_adjust = settings.getint('RETRY_PRIORITY_ADJUST')
    @classmethod    def from_crawler(cls, crawler):        return cls(crawler.settings)
    def process_response(self, request, response, spider):        if request.meta.get('dont_retry', False):            return response        if response.status in self.retry_http_codes: # 可以自定义重试状态码            reason = response_status_message(response.status)            response.last_content = request.meta            return self._retry(request, reason, spider) or response        return response
    def process_exception(self, request, exception, spider):        if (                isinstance(exception, self.EXCEPTIONS_TO_RETRY)                and not request.meta.get('dont_retry', False)        ):            return self._retry(request, exception, spider)
    def _retry(self, request, reason, spider):        max_retry_times = request.meta.get('max_retry_times', self.max_retry_times)        priority_adjust = request.meta.get('priority_adjust', self.priority_adjust)        request.meta['proxy'] = "xxx:xxxx"        request.headers['Proxy-Authorization'] = "proxyauth"        return get_retry_request(            request,            reason=reason,            spider=spider,            max_retry_times=max_retry_times,            priority_adjust=priority_adjust,        )

重写总结：我们只需要在 _retry 函数中实时更换代理即可，如果涉及到代理池需要剔除失败代理的问题，同样在 _retry 函数中删除代理池中指定代理即可。我们还可以自定义重试机制状态码，大家可自行添加即可！

五、总结分享

通过本次案例分析，上面的几个问题我们都已经得到了答案。今天分享到这里就结束了，欢迎大家关注下期文章，我们不见不散⛽️。最后希望大家多多转发、点赞、在看支持一波

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Scrapy 源码分析之 RetryMiddleware 模块

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