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。