Scrapy-redis源码学习

Scrapy 是 Python 的一个非常强大的爬虫库，但是当我们要爬取的页面非常多的时候，单个主机的处理能力就不能满足我们的需求了（无论是处理速度还是网络请求的并发数），这时候分布式爬虫的优势就显现出来。而 scrapy_redis 就是结合了分布式数据库 Redis，重写了 Scrapy 的任务调度器，将 Scrapy 变成一个可以在多个主机上同时运行的分布式爬虫。

scrapy_redis 工程的主体还是是 Redis 和 Scrapy 两个库，工程本身实现的东西不是很多，这个工程就像胶水一样，把这两个插件粘结了起来。下面我们来看看，scrapy_redis 的每一个源代码文件都实现了什么功能，最后如何实现分布式的爬虫系统。

defaults: 默认参数配置

本模块主要定义了一些默认参数，这些都可以在 Scrapy 的 settings 中设置：

• Redis 连接参数
  
  • REDIS_CLS
  • REDIS_ENCODING
  • REDIS_PARAMS
• PIPELINE_KEY: 数据存储在 Redis 的 key
• SCHEDULER_*: 参与任务调度的 Redis key，包括 requests 存储，调度队列类型选择，去重指纹存储
• START_URLS_*: 任务注入 Redis 的 key 和 数据类型

import redis

# For standalone use.
DUPEFILTER_KEY = 'dupefilter:%(timestamp)s'

PIPELINE_KEY = '%(spider)s:items'

REDIS_CLS = redis.StrictRedis
REDIS_ENCODING = 'utf-8'
# Sane connection defaults.
REDIS_PARAMS = {
    'socket_timeout': 30,
    'socket_connect_timeout': 30,
    'retry_on_timeout': True,
    'encoding': REDIS_ENCODING,
}

SCHEDULER_QUEUE_KEY = '%(spider)s:requests'
SCHEDULER_QUEUE_CLASS = 'scrapy_redis.queue.PriorityQueue'
SCHEDULER_DUPEFILTER_KEY = '%(spider)s:dupefilter'
SCHEDULER_DUPEFILTER_CLASS = 'scrapy_redis.dupefilter.RFPDupeFilter'

START_URLS_KEY = '%(name)s:start_urls'
START_URLS_AS_SET = False

connection: 建立 Redis 连接

顾名思义，本模块用于建立 Redis 连接，其中定义了：

• SETTINGS_PARAMS_MAP: 用于将 Redis 参数名映射到 redis 库的参数名
• get_redis_from_settings 函数: 从 Scrapy 的 settings 对象获取连接参数并调用 get_redis 建立 Redis 连接
• get_redis 函数: 辅助函数，从传入参数建立 Redis 连接

SETTINGS_PARAMS_MAP = {
    'REDIS_URL': 'url',
    'REDIS_HOST': 'host',
    'REDIS_PORT': 'port',
    'REDIS_ENCODING': 'encoding',
}

def get_redis_from_settings(settings):
    """Returns a redis client instance from given Scrapy settings object.

    This function uses ``get_client`` to instantiate the client and uses
    ``defaults.REDIS_PARAMS`` global as defaults values for the parameters. You
    can override them using the ``REDIS_PARAMS`` setting.

    Parameters
    ----------
    settings : Settings
        A scrapy settings object. See the supported settings below.

    Returns
    -------
    server
        Redis client instance.

    Other Parameters
    ----------------
    REDIS_URL : str, optional
        Server connection URL.
    REDIS_HOST : str, optional
        Server host.
    REDIS_PORT : str, optional
        Server port.
    REDIS_ENCODING : str, optional
        Data encoding.
    REDIS_PARAMS : dict, optional
        Additional client parameters.

    """

def get_redis(**kwargs):
    """Returns a redis client instance.

    Parameters
    ----------
    redis_cls : class, optional
        Defaults to ``redis.StrictRedis``.
    url : str, optional
        If given, ``redis_cls.from_url`` is used to instantiate the class.
    **kwargs
        Extra parameters to be passed to the ``redis_cls`` class.

    Returns
    -------
    server
        Redis client instance.

    """

utils

模块内只有一个方法 bytes_to_str，用于将字节序列转换为字符串，因为使用 redis 模块读取的字符数据都为 bytes，所以使用前需要解码为 str 。

queue: 任务调度队列

本模块定义了三中队列（FifoQueue、PriorityQueue、LifoQueue）用于任务调度。

• Base 基类:
  
  • _encode_request: 将 request 对象转为字典并序列化（默认 pickle）
  • _decode_request: 和上面相反，反序列化回 request 对象

class Base(object):
    """Per-spider base queue class"""

    def __init__(self, server, spider, key, serializer=None):
        """Initialize per-spider redis queue.

        Parameters
        ----------
        server : StrictRedis
            Redis client instance.
        spider : Spider
            Scrapy spider instance.
        key: str
            Redis key where to put and get messages.
        serializer : object
            Serializer object with ``loads`` and ``dumps`` methods.

        """
        if serializer is None:
            # Backward compatibility.
            # TODO: deprecate pickle.
            serializer = picklecompat
        if not hasattr(serializer, 'loads'):
            raise TypeError("serializer does not implement 'loads' function: %r"
                            % serializer)
        if not hasattr(serializer, 'dumps'):
            raise TypeError("serializer '%s' does not implement 'dumps' function: %r"
                            % serializer)

        self.server = server
        self.spider = spider
        self.key = key % {'spider': spider.name}
        self.serializer = serializer

    def _encode_request(self, request):
        """Encode a request object"""
        obj = request_to_dict(request, self.spider)
        return self.serializer.dumps(obj)

    def _decode_request(self, encoded_request):
        """Decode an request previously encoded"""
        obj = self.serializer.loads(encoded_request)
        return request_from_dict(obj, self.spider)

    def __len__(self):
        """Return the length of the queue"""
        raise NotImplementedError

    def push(self, request):
        """Push a request"""
        raise NotImplementedError

    def pop(self, timeout=0):
        """Pop a request"""
        raise NotImplementedError

    def clear(self):
        """Clear queue/stack"""
        self.server.delete(self.key)

• FifoQueue: 继承 Base，重写了 push 和 pop 方法实现先进先出队列。
• PriorityQueue: 继承 Base，重写了 push 和 pop 方法实现优先级队列。
• LifoQueue: 继承 Base，重写了 push 和 pop 方法实现后进先出队列。

dupefilter: 实现 request 去重

重要模块，继承自 Scrapy BaseDupeFilter，实现了 request 去重功能，基于 Scrapy 的 request_fingerprint 生成指纹，并在 Redis 上存储。当收到新的 request，首先生成指纹判断是否存在于已爬取的指纹库内（Redis set），若存在则返回 False，不存在返回 True。

class RFPDupeFilter(BaseDupeFilter):
    """Redis-based request duplicates filter.

    This class can also be used with default Scrapy's scheduler.

    """

    def request_seen(self, request):
        """Returns True if request was already seen.

        Parameters
        ----------
        request : scrapy.http.Request

        Returns
        -------
        bool

        """
        fp = self.request_fingerprint(request)
        # This returns the number of values added, zero if already exists.
        added = self.server.sadd(self.key, fp)
        return added == 0

    def request_fingerprint(self, request):
        """Returns a fingerprint for a given request.

        Parameters
        ----------
        request : scrapy.http.Request

        Returns
        -------
        str

        """
        return request_fingerprint(request)

scheduler: 远程任务调度

作为最核心的调度，本模块实现了和 Redis 的交互进行任务管理调度，使得多机在 Redis 上协调存取工作（与 redis 建立连接，创建任务队列，request 的入队、出对和过滤等操作）。

Scheduler 类主要方法有：

• open(): 调度器启动时的自动的操作，这里主要实例化了任务队列 queue 和过滤器 dupefilter。
• close(): 调度器关闭时执行的操作，清理资源。
• enqueue_request(): 过滤 request 并加入任务队列 queue
• next_request(): 从任务队列取出 request

class Scheduler(object):
    """Redis-based scheduler

    Settings
    --------
    SCHEDULER_PERSIST : bool (default: False)
        Whether to persist or clear redis queue.
    SCHEDULER_FLUSH_ON_START : bool (default: False)
        Whether to flush redis queue on start.
    SCHEDULER_IDLE_BEFORE_CLOSE : int (default: 0)
        How many seconds to wait before closing if no message is received.
    SCHEDULER_QUEUE_KEY : str
        Scheduler redis key.
    SCHEDULER_QUEUE_CLASS : str
        Scheduler queue class.
    SCHEDULER_DUPEFILTER_KEY : str
        Scheduler dupefilter redis key.
    SCHEDULER_DUPEFILTER_CLASS : str
        Scheduler dupefilter class.
    SCHEDULER_SERIALIZER : str
        Scheduler serializer.

    """

    def open(self, spider):
        self.spider = spider

        try:
            self.queue = load_object(self.queue_cls)(
                server=self.server,
                spider=spider,
                key=self.queue_key % {'spider': spider.name},
                serializer=self.serializer,
            )
        except TypeError as e:
            raise ValueError("Failed to instantiate queue class '%s': %s",
                            self.queue_cls, e)

        try:
            self.df = load_object(self.dupefilter_cls)(
                server=self.server,
                key=self.dupefilter_key % {'spider': spider.name},
                debug=spider.settings.getbool('DUPEFILTER_DEBUG'),
            )
        except TypeError as e:
            raise ValueError("Failed to instantiate dupefilter class '%s': %s",
                            self.dupefilter_cls, e)

        if self.flush_on_start:
            self.flush()
        # notice if there are requests already in the queue to resume the crawl
        if len(self.queue):
            spider.log("Resuming crawl (%d requests scheduled)" % len(self.queue))

    def close(self, reason):
        if not self.persist:
            self.flush()

    def enqueue_request(self, request):
        if not request.dont_filter and self.df.request_seen(request):
            self.df.log(request, self.spider)
            return False
        if self.stats:
            self.stats.inc_value('scheduler/enqueued/redis', spider=self.spider)
        self.queue.push(request)
        return True

    def next_request(self):
        block_pop_timeout = self.idle_before_close
        request = self.queue.pop(block_pop_timeout)
        if request and self.stats:
            self.stats.inc_value('scheduler/dequeued/redis', spider=self.spider)
        return request

spiders: 赋予 Scrapy spiders 远程调度

本模块定义了 RedisMixin 类用于从 Redis 服务器读取 url 构造为 Request，同时绑定了 idle 信号实现本地空闲时再次从 Redis 调度任务继续爬取。

• RedisMixin 类
  
  • setup_redis(): 建立 Redis 连接并绑定 idle 信号
  • spider_idle(): idle 信号处理，这里调用 schedule_next_requests 完成从 Redis 调度
  • schedule_next_requests(): 调用 next_requests 从 Redis 获取 url 包装为 HttpRequest
  • start_requests(): 重写了 Spider 的 start_requests 函数，直接调用 next_requests 从 Redis 取任务
• RedisSpider(RedisMixin, Spiser) 类: 多继承，用 RedisMixin 调度功能覆盖 Spider 原生
• RedisSpider(RedisMixin, CrawlSpiser) 类: 多继承，用 RedisMixin 调度功能覆盖 CrawlSpider 原生

class RedisMixin(object):
    """Mixin class to implement reading urls from a redis queue."""
    redis_key = None
    redis_batch_size = None
    redis_encoding = None

    # Redis client placeholder.
    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', defaults.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:
            # TODO: Deprecate this setting (REDIS_START_URLS_BATCH_SIZE).
            self.redis_batch_size = settings.getint(
                'REDIS_START_URLS_BATCH_SIZE',
                settings.getint('CONCURRENT_REQUESTS'),
            )

        try:
            self.redis_batch_size = int(self.redis_batch_size)
        except (TypeError, ValueError):
            raise ValueError("redis_batch_size must be an integer")

        if self.redis_encoding is None:
            self.redis_encoding = settings.get('REDIS_ENCODING', defaults.REDIS_ENCODING)

        self.logger.info("Reading start URLs from redis key '%(redis_key)s' "
                        "(batch size: %(redis_batch_size)s, encoding: %(redis_encoding)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', defaults.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
        # TODO: Use redis pipeline execution.
        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):
        """Returns a Request instance from data coming from Redis.

        By default, ``data`` is an encoded URL. You can override this method to
        provide your own message decoding.

        Parameters
        ----------
        data : bytes
            Message from redis.

        """
        url = bytes_to_str(data, self.redis_encoding)
        return self.make_requests_from_url(url)

    def schedule_next_requests(self):
        """Schedules a request if available"""
        # TODO: While there is capacity, schedule a batch of redis requests.
        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

pipelines: 默认数据收集存储

scrapy_redis 默认提供的 pipeline，将爬取的数据上传在 Reids 列表内。

其中使用 twisted 处理数据，详情移步：deferToThread

from twisted.internet.threads import deferToThread


class RedisPipeline(object):
    """Pushes serialized item into a redis list/queue

    Settings
    --------
    REDIS_ITEMS_KEY : str
        Redis key where to store items.
    REDIS_ITEMS_SERIALIZER : str
        Object path to serializer function.

    """

    def process_item(self, item, spider):
        return deferToThread(self._process_item, item, spider)

    def _process_item(self, item, spider):
        key = self.item_key(item, spider)
        data = self.serialize(item)
        self.server.rpush(key, data)
        return item