scrapy分布式Spider源码分析及实现过程

分布式框架scrapy_redis实现了一套完整的组件，其中也实现了spider，RedisSpider是在继承原scrapy的Spider的基础上略有改动，初始URL不在从start_urls列表中读取，而是从redis起始队列中读取。

scrapy_redis源码在scrapy.redis.spider中，不仅实现了RedisSpider（分布式爬虫）还实现了RedisCrawlSpider（分布式深度爬虫）的逻辑，不过二者很多方法是一致的。

源码如下：

from scrapy import signals
from scrapy.exceptions import DontCloseSpider
from scrapy.spiders import Spider, CrawlSpider

from . import connection

# Default batch size matches default concurrent requests setting.

DEFAULT_START_URLS_BATCH_SIZE = 16

DEFAULT_START_URLS_KEY = '%(name)s:start_urls'

class RedisMixin(object):

    """Mixin class to implement reading urls from a redis queue."""

    # Per spider redis key, default to DEFAULT_START_URLS_KEY.

    redis_key = None

    # Fetch this amount of start urls when idle. Default to DEFAULT_START_URLS_BATCH_SIZE.

    redis_batch_size = None

    # Redis client instance.

    server = None

    def start_requests(self):

        """Returns a batch of start requests from redis."""

        return self.next_requests()

    def setup_redis(self, crawler=None):

        """Setup redis connection and idle signal.

        This should be called after the spider has set its crawler object.

        """

        if self.server is not None:

            return

        if crawler is None:

            # We allow optional crawler argument to keep backwards

            # compatibility.

            # XXX: Raise a deprecation warning.

            crawler = getattr(self, 'crawler', None)

        if crawler is None:

            raise ValueError("crawler is required")

        settings = crawler.settings

        if self.redis_key is None:

            self.redis_key = settings.get(

                'REDIS_START_URLS_KEY', DEFAULT_START_URLS_KEY,

            )

        self.redis_key = self.redis_key % {'name': self.name}

        if not self.redis_key.strip():

            raise ValueError("redis_key must not be empty")

        if self.redis_batch_size is None:

            self.redis_batch_size = settings.getint(

                'REDIS_START_URLS_BATCH_SIZE', DEFAULT_START_URLS_BATCH_SIZE,

            )

        try:

            self.redis_batch_size = int(self.redis_batch_size)

        except (TypeError, ValueError):

            raise ValueError("redis_batch_size must be an integer")

        self.logger.info("Reading start URLs from redis key '%(redis_key)s' "

                         "(batch size: %(redis_batch_size)s)", self.__dict__)

        self.server = connection.from_settings(crawler.settings)

        # The idle signal is called when the spider has no requests left,

        # that's when we will schedule new requests from redis queue

        crawler.signals.connect(self.spider_idle, signal=signals.spider_idle)

    def next_requests(self):

        """Returns a request to be scheduled or none."""

        use_set = self.settings.getbool('REDIS_START_URLS_AS_SET')

        fetch_one = self.server.spop if use_set else self.server.lpop

        # XXX: Do we need to use a timeout here?

        found = 0

        while found < self.redis_batch_size:

            data = fetch_one(self.redis_key)

            if not data:

                # Queue empty.

                break

            req = self.make_request_from_data(data)

            if req:

                yield req

                found += 1

            else:

                self.logger.debug("Request not made from data: %r", data)

        if found:

            self.logger.debug("Read %s requests from '%s'", found, self.redis_key)

    def make_request_from_data(self, data):

        # By default, data is an URL.

        if '://' in data:

            return self.make_requests_from_url(data)

        else:

            self.logger.error("Unexpected URL from '%s': %r", self.redis_key, data)

    def schedule_next_requests(self):

        """Schedules a request if available"""

        for req in self.next_requests():

            self.crawler.engine.crawl(req, spider=self)

    def spider_idle(self):

        """Schedules a request if available, otherwise waits."""

        # XXX: Handle a sentinel to close the spider.

        self.schedule_next_requests()

        raise DontCloseSpider

class RedisSpider(RedisMixin, Spider):

    """Spider that reads urls from redis queue when idle."""

    @classmethod

    def from_crawler(self, crawler, *args, **kwargs):

        obj = super(RedisSpider, self).from_crawler(crawler, *args, **kwargs)

        obj.setup_redis(crawler)

        return obj

class RedisCrawlSpider(RedisMixin, CrawlSpider):

    """Spider that reads urls from redis queue when idle."""

    @classmethod

    def from_crawler(self, crawler, *args, **kwargs):

        obj = super(RedisCrawlSpider, self).from_crawler(crawler, *args, **kwargs)

        obj.setup_redis(crawler)

        return obj

开始start_requests方法是从Redis中获取所有连接，然后与URL去重列表比对去重，不被去重的通过make_requests_from_url(url)传递给引擎，然后到调度器，这里和一般的爬虫思路是一致的。

当引擎收到任务队列为空的消息后会调用Spider_idle方法，进而调用schedule_next_requests迭代获取到的起始URL递交给引擎，然后就开始任务，直到任务完成后在次调用起始队列中的URL。

   def schedule_next_requests(self):

        """Schedules a request if available"""

        for req in self.next_requests():

            self.crawler.engine.crawl(req, spider=self)

    def spider_idle(self):

        """Schedules a request if available, otherwise waits."""

        # XXX: Handle a sentinel to close the spider.

        self.schedule_next_requests()

        raise DontCloseSpider

RedisSpider和RedisCrawlSpider很简单，继承了Spider和RedisMiXine，实现了setup_redis方法，该方法会根据不同的crawler初始化setting、redis_key，及通过connect接口，给spider绑定了spider_idle信号绑定。

class RedisSpider(RedisMixin, Spider):

    """Spider that reads urls from redis queue when idle."""

    @classmethod

    def from_crawler(self, crawler, *args, **kwargs):

        obj = super(RedisSpider, self).from_crawler(crawler, *args, **kwargs)

        obj.setup_redis(crawler)

        return obj

class RedisCrawlSpider(RedisMixin, CrawlSpider):

    """Spider that reads urls from redis queue when idle."""

    @classmethod

    def from_crawler(self, crawler, *args, **kwargs):

        obj = super(RedisCrawlSpider, self).from_crawler(crawler, *args, **kwargs)

        obj.setup_redis(crawler)

        return obj

 def setup_redis(self, crawler=None):

        """Setup redis connection and idle signal.

        This should be called after the spider has set its crawler object.

        """

        if self.server is not None:

            return

        if crawler is None:

            # We allow optional crawler argument to keep backwards

            # compatibility.

            # XXX: Raise a deprecation warning.

            crawler = getattr(self, 'crawler', None)

        if crawler is None:

            raise ValueError("crawler is required")

        settings = crawler.settings

        if self.redis_key is None:

            self.redis_key = settings.get(

                'REDIS_START_URLS_KEY', DEFAULT_START_URLS_KEY,

            )

        self.redis_key = self.redis_key % {'name': self.name}

        if not self.redis_key.strip():

            raise ValueError("redis_key must not be empty")

        if self.redis_batch_size is None:

            self.redis_batch_size = settings.getint(

                'REDIS_START_URLS_BATCH_SIZE', DEFAULT_START_URLS_BATCH_SIZE,

            )

        try:

            self.redis_batch_size = int(self.redis_batch_size)

        except (TypeError, ValueError):

            raise ValueError("redis_batch_size must be an integer")

        self.logger.info("Reading start URLs from redis key '%(redis_key)s' "

                         "(batch size: %(redis_batch_size)s)", self.__dict__)

        self.server = connection.from_settings(crawler.settings)

        # The idle signal is called when the spider has no requests left,

        # that's when we will schedule new requests from redis queue

        crawler.signals.connect(self.spider_idle, signal=signals.spider_idle)

总结：RedisSpider从初始Redis队列中获取start_url生成Request，然后递交给scrapy引擎交给对应的Redis scheduler调度，完成去重和入队列。当调度条件满足是队列中的Request交由引擎给下载器，生成Response对象递交给Spider解析，产生新的Request，继续抓取；直到没有新的Request产生后，Spider由从起始队列中去获取URL，继续上一轮循环，当然如果队列中没有了，那么爬虫就结束了。