spider.py
设计的这个spider从redis中读取要爬的url,然后执行爬取，若爬取过程中返回更多的url，那么继续进行直至所有的request完成。之后继续从redis中读取url，循环这个过程。
分析：在这个spider中通过connect signals.spider_idle信号实现对crawler状态的监视。当idle时，返回新的make_requests_from_url(url)给引擎，进而交给调度器调度。
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
spider的改动也不是很大，主要是通过connect接口，给spider绑定了spider_idle信号，spider初始化时，通过setup_redis函数初始化好和redis的连接，之后通过next_requests函数从redis中取出strat url，使用的key是settings中REDIS_START_URLS_AS_SET定义的（注意了这里的初始化url池和我们上边的queue的url池不是一个东西，queue的池是用于调度的，初始化url池是存放入口url的，他们都存在redis中，但是使用不同的key来区分，就当成是不同的表吧），spider使用少量的start url，可以发展出很多新的url，这些url会进入scheduler进行判重和调度。直到spider跑到调度池内没有url的时候，会触发spider_idle信号，从而触发spider的next_requests函数，再次从redis的start url池中读取一些url。
总结
最后总结一下scrapy-redis的总体思路：这个工程通过重写scheduler和spider类，实现了调度、spider启动和redis的交互。实现新的dupefilter和queue类，达到了判重和调度容器和redis的交互，因为每个主机上的爬虫进程都访问同一个redis数据库，所以调度和判重都统一进行统一管理，达到了分布式爬虫的目的。 当spider被初始化时，同时会初始化一个对应的scheduler对象，这个调度器对象通过读取settings，配置好自己的调度容器queue和判重工具dupefilter。每当一个spider产出一个request的时候，scrapy内核会把这个reuqest递交给这个spider对应的scheduler对象进行调度，scheduler对象通过访问redis对request进行判重，如果不重复就把他添加进redis中的调度池。当调度条件满足时，scheduler对象就从redis的调度池中取出一个request发送给spider，让他爬取。当spider爬取的所有暂时可用url之后，scheduler发现这个spider对应的redis的调度池空了，于是触发信号spider_idle，spider收到这个信号之后，直接连接redis读取strart url池，拿去新的一批url入口，然后再次重复上边的工作。