scrapy源码分析（十一）----------下载器Downloader

经过前面几篇的分析，scrapy的五大核心组件已经介绍了4个：engine,scheduler,scraper,spidemw。

还剩最后一个downloader，这个下载器关系到了网页如何下载，内容相对来说是最为复杂的一部分，这篇教程就逐步分析其源码。

下载操作开始于engine的_next_request_from_scheduler，这个方法已经不止一次提到过,这次只列出关键代码:

scrapy/core/engine.py:

def _next_request_from_scheduler(self, spider):
    slot = self.slot
    request = slot.scheduler.next_request()
    if not request:
        return
    d = self._download(request, spider)

调用_download方法:

def _download(self, request, spider):
    slot = self.slot
    slot.add_request(request)
    def _on_success(response):
        assert isinstance(response, (Response, Request))
        if isinstance(response, Response):
            response.request = request # tie request to response received
            logkws = self.logformatter.crawled(request, response, spider)
            logger.log(*logformatter_adapter(logkws), extra={'spider': spider})
            self.signals.send_catch_log(signal=signals.response_received, \
                response=response, request=request, spider=spider)
        return response

    def _on_complete(_):
        slot.nextcall.schedule()
        return _
    dwld = self.downloader.fetch(request, spider)
    dwld.addCallbacks(_on_success)
    dwld.addBoth(_on_complete)
    return dwld

_download方法首先将request加入slot的inprogress集合记录正在进行的request,然后调用下载器downloader的fetch方法，给fetch返回的deferred添加一个'_on_success'方法，这样在下载完成后会打印日志并发送一个response_received消息给关心者。

我们看下这个默认的downloader是什么:

scrapy/settings/default_settings.py:

scrapy.core.downloader.Downloader

我们先来看下它的构造函数，再看fetch方法的实现:

scrapy/core/downloader/__init__.py:

class Downloader(object):

    def __init__(self, crawler):
        self.settings = crawler.settings
        self.signals = crawler.signals
        self.slots = {}
        self.active = set()
        self.handlers = DownloadHandlers(crawler)
        self.total_concurrency = self.settings.getint('CONCURRENT_REQUESTS')
        self.domain_concurrency = self.settings.getint('CONCURRENT_REQUESTS_PER_DOMAIN')
        self.ip_concurrency = self.settings.getint('CONCURRENT_REQUESTS_PER_IP')
        self.randomize_delay = self.settings.getbool('RANDOMIZE_DOWNLOAD_DELAY')
        self.middleware = DownloaderMiddlewareManager.from_crawler(crawler)
        self._slot_gc_loop = task.LoopingCall(self._slot_gc)
        self._slot_gc_loop.start(60)

关键对象4个:slots,active,DownloadHandlers,middleware以及一些配置选项。先依次分析4个对象的作用：

1.slots:

这个slots是一个存储Slot对象的字典，key是request对应的域名，值是一个Slot对象。

Slot对象用来控制一种Request下载请求，通常这种下载请求是对于同一个域名。

这个Slot对象还控制了访问这个域名的并发度，下载延迟控制，随机延时等，主要是为了控制对一个域名的访问策略，一定程度上避免流量过大被封IP，不能继续爬取。

通过代码来详细了解:

def _get_slot(self, request, spider):
    key = self._get_slot_key(request, spider)
    if key not in self.slots:
        conc = self.ip_concurrency if self.ip_concurrency else self.domain_concurrency
        conc, delay = _get_concurrency_delay(conc, spider, self.settings)
        self.slots[key] = Slot(conc, delay, self.randomize_delay)

    return key, self.slots[key]

可以看到，对于一个request，先调用'_get_slot_key'获取request对应的key.

看下其中的'_get_slot_key'函数，可以看到我们可以通过给request的meta中添加'download_slot'来控制request的key值，这样增加了灵活性。如果没有定制request的key,则key值来源于request要访问的域名。

另外对于request对应的域名也增加了缓存机制:urlparse_cached,dnscahe.

def _get_slot_key(self, request, spider):
    if 'download_slot' in request.meta:
        return request.meta['download_slot']

    key = urlparse_cached(request).hostname or ''
    if self.ip_concurrency:
        key = dnscache.get(key, key)

    return key

同时也通过slots集合达到了缓存的目的，对于同一个域名的访问策略可以通过slots获取而不用每次都解析配置。

然后根据key从slots里取对应的Slot对象，如果还没有，则构造一个新的对象。

if key not in self.slots:
        conc = self.ip_concurrency if self.ip_concurrency else self.domain_concurrency
        conc, delay = _get_concurrency_delay(conc, spider, self.settings)
        self.slots[key] = Slot(conc, delay, self.randomize_delay)

这个Slot对象有3个参数，并发度，延迟时间和随机延迟。下面分别看下3个参数的获取:

a.并发度

我们看下这个并发度先取ip并发度控制，如果没有则取域名的并发配置。默认配置如下:

ip并发度:

CONCURRENT_REQUESTS_PER_IP = 0

域名并发度:

CONCURRENT_REQUESTS_PER_DOMAIN = 8

b.延迟:

def _get_concurrency_delay(concurrency, spider, settings):
    delay = settings.getfloat('DOWNLOAD_DELAY')
    if hasattr(spider, 'DOWNLOAD_DELAY'):
        warnings.warn("%s.DOWNLOAD_DELAY attribute is deprecated, use %s.download_delay instead" %
                      (type(spider).__name__, type(spider).__name__))
        delay = spider.DOWNLOAD_DELAY
    if hasattr(spider, 'download_delay'):
        delay = spider.download_delay

    if hasattr(spider, 'max_concurrent_requests'):
        concurrency = spider.max_concurrent_requests

    return concurrency, delay

先从配置中取'DOWNLOAD_DELAY':

DOWNLOAD_DELAY = 0

如果spider定义了'DOWNLOAD_DELAY'则取它，这个大写的配置已经过期，如果需要请定义小写的值.

然后取spider定义的'max_concurrent_requests'.

综上可知，并发度优先取spider定义的'max_concurrent_request'，如果未定义则取配置中的ip并发度或域名并发度。

对于延迟则优先取spider中定义的'download_delay',如果示定义则取配置中的.

c.随机延迟

RANDOMIZE_DOWNLOAD_DELAY = True

取配置中的值，是否开启随机下载延迟。如果开启的话，会给前面2中的延迟值增加一个随机性。

综上，对这个Slot对象的作用应该清楚了，就是控制一个域名的request的访问策略。

如果一个域名的request已经爬取完了，如果清除slots中的缓存呢？

后面通过task.LoopingCall安装了一个60s的定时心跳函数_slot_gc,这个函数用于对slots中的对象进行定期的回收。

垃圾回收：
def _slot_gc(self, age=60):
    mintime = time() - age
    for key, slot in list(self.slots.items()):
        if not slot.active and slot.lastseen + slot.delay < mintime:
            self.slots.pop(key).close()

可以看到垃圾回收的策略:如果一个Slot对象没有正在活动的下载request,且距离上次活动的时间已经过去了60s则进行回收。

2.active

active是一个活动集合，用于记录当前正在下载的request集合。

3.handlers:

它是一个DownloadHandlers对象，它控制了许多handlers,对于不同的下载协议使用不同的handlers.

默认支持handlers如下:

DOWNLOAD_HANDLERS_BASE = {
    'file': 'scrapy.core.downloader.handlers.file.FileDownloadHandler',
    'http': 'scrapy.core.downloader.handlers.http.HTTPDownloadHandler',
    'https': 'scrapy.core.downloader.handlers.http.HTTPDownloadHandler',
    's3': 'scrapy.core.downloader.handlers.s3.S3DownloadHandler',
    'ftp': 'scrapy.core.downloader.handlers.ftp.FTPDownloadHandler',
}

后面下载网页会调用handler的download_request方法，后面讲fetch源码时再详细讲解。

4.middleware

这个middleware前面已经讲解过很多次，对于下载器，它使用的中间件管理器是

DownloaderMiddlewareManager

当然，也通过调用其from_crawler方法生成下载器中间件管理器对象。

self.middleware = DownloaderMiddlewareManager.from_crawler(crawler)

前面讲过，中间件要自己实现'_get_mwlist_from_settings'构造自己的中间件列表。还可以实现‘_add_middleware'方法来添加特有的中间件方法。我们看下DownloaderMiddlewareManager的实现:

scrapy/core/downloader/middleware.py:

@classmethod
def _get_mwlist_from_settings(cls, settings):
    return build_component_list(
        settings.getwithbase('DOWNLOADER_MIDDLEWARES'))

def _add_middleware(self, mw):
    if hasattr(mw, 'process_request'):
        self.methods['process_request'].append(mw.process_request)
    if hasattr(mw, 'process_response'):
        self.methods['process_response'].insert(0, mw.process_response)
    if hasattr(mw, 'process_exception'):
        self.methods['process_exception'].insert(0, mw.process_exception)

可以看到，加入的中间件为'DOWNLOADER_MIDDLEWARES',默认有以下几个:

DOWNLOADER_MIDDLEWARES_BASE = {
    # Engine side
    'scrapy.downloadermiddlewares.robotstxt.RobotsTxtMiddleware': 100,
    'scrapy.downloadermiddlewares.httpauth.HttpAuthMiddleware': 300,
    'scrapy.downloadermiddlewares.downloadtimeout.DownloadTimeoutMiddleware': 350,
    'scrapy.downloadermiddlewares.useragent.UserAgentMiddleware': 400,
    'scrapy.downloadermiddlewares.retry.RetryMiddleware': 500,
    'scrapy.downloadermiddlewares.defaultheaders.DefaultHeadersMiddleware': 550,
    'scrapy.downloadermiddlewares.ajaxcrawl.AjaxCrawlMiddleware': 560,
    'scrapy.downloadermiddlewares.redirect.MetaRefreshMiddleware': 580,
    'scrapy.downloadermiddlewares.httpcompression.HttpCompressionMiddleware': 590,
    'scrapy.downloadermiddlewares.redirect.RedirectMiddleware': 600,
    'scrapy.downloadermiddlewares.cookies.CookiesMiddleware': 700,
    'scrapy.downloadermiddlewares.httpproxy.HttpProxyMiddleware': 750,
    'scrapy.downloadermiddlewares.chunked.ChunkedTransferMiddleware': 830,
    'scrapy.downloadermiddlewares.stats.DownloaderStats': 850,
    'scrapy.downloadermiddlewares.httpcache.HttpCacheMiddleware': 900,
    # Downloader side
}

另外，对于下载中间件，可以实现的方法有’process_request','process_response','process_exception'.

分析完了4个关键对象，我们通过fetch方法来看下下载器是如何使用它们工作的：

def fetch(self, request, spider):
    def _deactivate(response):
        self.active.remove(request)
        return response

    self.active.add(request)
    dfd = self.middleware.download(self._enqueue_request, request, spider)
    return dfd.addBoth(_deactivate)

首先，调用中间件管理器的download方法，同时传入了自己的_enqueue_request方法。

看下中间件管理器的download方法:

scrapy/core/downloader/middleware.py:

def download(self, download_func, request, spider):
    @defer.inlineCallbacks
    def process_request(request):
        for method in self.methods['process_request']:
            response = yield method(request=request, spider=spider)
            assert response is None or isinstance(response, (Response, Request)), \
                    'Middleware %s.process_request must return None, Response or Request, got %s' % \
                    (six.get_method_self(method).__class__.__name__, response.__class__.__name__)
            if response:
                defer.returnValue(response)
        defer.returnValue((yield download_func(request=request,spider=spider)))

    @defer.inlineCallbacks
    def process_response(response):
        assert response is not None, 'Received None in process_response'
        if isinstance(response, Request):
            defer.returnValue(response)

        for method in self.methods['process_response']:
            response = yield method(request=request, response=response,
                                    spider=spider)
            assert isinstance(response, (Response, Request)), \
                'Middleware %s.process_response must return Response or Request, got %s' % \
                (six.get_method_self(method).__class__.__name__, type(response))
            if isinstance(response, Request):
                defer.returnValue(response)
        defer.returnValue(response)

    @defer.inlineCallbacks
    def process_exception(_failure):
        exception = _failure.value
        for method in self.methods['process_exception']:
            response = yield method(request=request, exception=exception,
                                    spider=spider)
            assert response is None or isinstance(response, (Response, Request)), \
                'Middleware %s.process_exception must return None, Response or Request, got %s' % \
                (six.get_method_self(method).__class__.__name__, type(response))
            if response:
                defer.returnValue(response)
        defer.returnValue(_failure)

    deferred = mustbe_deferred(process_request, request)
    deferred.addErrback(process_exception)
    deferred.addCallback(process_response)
    return deferred

可以看出和上一节讲的spidermiddlewaremanager的scrape_reponse方法类似，先依次调用下载中间件的'process_request'方法处理request,然后调用Downloader的'_enqueue_request'方法进行下载，最后对response依次调用中间件的'process_response'方法。

接着，分析Downloader的_enqueue_request方法：

def _enqueue_request(self, request, spider):
    key, slot = self._get_slot(request, spider)
    request.meta['download_slot'] = key

    def _deactivate(response):
        slot.active.remove(request)
        return response

    slot.active.add(request)
    deferred = defer.Deferred().addBoth(_deactivate)
    slot.queue.append((request, deferred))
    self._process_queue(spider, slot)
    return deferred

这个方法一开始调用前面分析的'_get_slot'方法获取request相对应的Slot对象（主要是分析域名），然后向对应的slot对应的活动集合active中添加一个request，并向slot的队列queue添加request和对应的deferred对象。然后调用'_process_queue'方法处理slot对象。

接着分析'_process_queue'方法:

这个方法主要用于从slot对象的队列queue中获取请求并下载。

def _process_queue(self, spider, slot):
    if slot.latercall and slot.latercall.active(): /*如果一个latercall正在运行则直接返回*/
        return

    # Delay queue processing if a download_delay is configured
    now = time()
    delay = slot.download_delay() /*获取slot对象的延迟时间*/
    if delay:
        penalty = delay - now + slot.lastseen /*距离上次运行还需要延迟则latercall*/
        if penalty > 0:
            slot.latercall = reactor.callLater(penalty, self._process_queue, spider, slot)
            return

    # Process enqueued requests if there are free slots to transfer for this slot
    while slot.queue and slot.free_transfer_slots() > 0: /*不停地处理slot队列queue中的请求，如果队列非空且有空闲的传输slot,则下载，如果需要延迟则继续调用'_process_queue'*/
        slot.lastseen = now
        request, deferred = slot.queue.popleft()
        dfd = self._download(slot, request, spider)
        dfd.chainDeferred(deferred)
        # prevent burst if inter-request delays were configured
        if delay:
            self._process_queue(spider, slot)
            break

这个函数通过最下面的while循环处理队列中的请求，并判断当前是否有空闲的传输slot，有空闲的才继续下载处理。

处理下载请求时，会不断更新slot的lastseen为当前时间，这个值代表了slot的最近一次活跃下载时间。

这里有个注意点就是如果当前没有空闲的传输slot而队列非空，那么未处理的request怎么办？（后面讲解）

如果需要delay则再次调用'_process_queue'，否则不停地继续下载request.

再次调用后，会先计算延迟时间距离上次活跃时间是否到时，如果还要延迟则启动一个latercall(通过twisted的reactor的callLater实现）。这个latercall会再次处理slot的队列queue.因此入口处判断如果有正在活动的latercall则不再处理。

这样，就不断地处理下载请求，并根据需要进行适当的延迟。

紧接着分析'_download'方法:

def _download(self, slot, request, spider):
    # The order is very important for the following deferreds. Do not change!

    # 1. Create the download deferred
    dfd = mustbe_deferred(self.handlers.download_request, request, spider)

    # 2. Notify response_downloaded listeners about the recent download
    # before querying queue for next request
    def _downloaded(response):
        self.signals.send_catch_log(signal=signals.response_downloaded,
                                    response=response,
                                    request=request,
                                    spider=spider)
        return response
    dfd.addCallback(_downloaded)

    # 3. After response arrives,  remove the request from transferring
    # state to free up the transferring slot so it can be used by the
    # following requests (perhaps those which came from the downloader
    # middleware itself)
    slot.transferring.add(request)

    def finish_transferring(_):
        slot.transferring.remove(request)
        self._process_queue(spider, slot)
        return _

    return dfd.addBoth(finish_transferring)

可以看到这里调用了DownloadHandlers的download_request方法，并向传输集合transferring中添加正在传输request.

并给返回的Deferred对象添加了finish_transferring方法。

这里finish_transferring方法解释了上面的疑问，每次下载一个request完成，都会从传输集合中移除request,并触发一次_process_queue操作，这样就保证了队列queue中的请求不会残留。

下面分析handler的download_request方法：

scrapy/core/downloader/handlers/__init__.py:

def download_request(self, request, spider):
    scheme = urlparse_cached(request).scheme
    handler = self._get_handler(scheme)
    if not handler:
        raise NotSupported("Unsupported URL scheme '%s': %s" %
                           (scheme, self._notconfigured[scheme]))
    return handler.download_request(request, spider)

这里根据url的scheme获取对应的handler,这里的handler前面已经讲过了。就是不同协议对应不同的handler.

这样下载器Downloader的工作流程和核心代码就分析完了，至于具体怎么通过网络下载网页，后面详细分析。