该文件实现了几个容器类，可以看这些容器和redis交互频繁，同时使用了我们上边picklecompat中定义的序列化器。这个文件实现的几个容器大体相同，只不过一个是队列，一个是栈，一个是优先级队列，这三个容器到时候会被scheduler对象实例化，来实现request的调度。比如我们使用SpiderQueue最为调度队列的类型，到时候request的调度方法就是先进先出，而实用SpiderStack就是先进后出了。

从SpiderQueue的实现看出来，他的push函数就和其他容器的一样，只不过push进去的request请求先被scrapy的接口request_to_dict变成了一个dict对象（因为request对象实在是比较复杂，有方法有属性不好串行化），之后使用picklecompat中的serializer串行化为字符串，然后使用一个特定的key存入redis中（该key在同一种spider中是相同的）。而调用pop时，其实就是从redis用那个特定的key去读其值（一个list），从list中读取最早进去的那个，于是就先进先出了。 这些容器类都会作为scheduler调度request的容器，scheduler在每个主机上都会实例化一个，并且和spider一一对应，所以分布式运行时会有一个spider的多个实例和一个scheduler的多个实例存在于不同的主机上，但是，因为scheduler都是用相同的容器，而这些容器都连接同一个redis服务器，又都使用spider名加queue来作为key读写数据，所以不同主机上的不同爬虫实例公用一个request调度池，实现了分布式爬虫之间的统一调度。

from scrapy.utils.reqser import request_to_dict, request_from_dict

from . import picklecompat


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

    def __init__(self, server, spider, key, serializer=None):
        """Initialize per-spider redis queue.
        Parameters:
            server -- redis connection
            spider -- spider instance
            key -- key for this queue (e.g. "%(spider)s:queue")
        """
        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)


class SpiderQueue(Base):
    """Per-spider FIFO queue"""

    def __len__(self):
        """Return the length of the queue"""
        return self.server.llen(self.key)

    def push(self, request):
        """Push a request"""
        self.server.lpush(self.key, self._encode_request(request))

    def pop(self, timeout=0):
        """Pop a request"""
        if timeout > 0:
            data = self.server.brpop(self.key, timeout)
            if isinstance(data, tuple):
                data = data[1]
        else:
            data = self.server.rpop(self.key)
        if data:
            return self._decode_request(data)


class SpiderPriorityQueue(Base):
    """Per-spider priority queue abstraction using redis' sorted set"""

    def __len__(self):
        """Return the length of the queue"""
        return self.server.zcard(self.key)

    def push(self, request):
        """Push a request"""
        data = self._encode_request(request)
        score = -request.priority
        # We don't use zadd method as the order of arguments change depending on
        # whether the class is Redis or StrictRedis, and the option of using
        # kwargs only accepts strings, not bytes.
        self.server.execute_command('ZADD', self.key, score, data)

    def pop(self, timeout=0):
        """
        Pop a request
        timeout not support in this queue class
        """
        # use atomic range/remove using multi/exec
        pipe = self.server.pipeline()
        pipe.multi()
        pipe.zrange(self.key, 0, 0).zremrangebyrank(self.key, 0, 0)
        results, count = pipe.execute()
        if results:
            return self._decode_request(results[0])


class SpiderStack(Base):
    """Per-spider stack"""

    def __len__(self):
        """Return the length of the stack"""
        return self.server.llen(self.key)

    def push(self, request):
        """Push a request"""
        self.server.lpush(self.key, self._encode_request(request))

    def pop(self, timeout=0):
        """Pop a request"""
        if timeout > 0:
            data = self.server.blpop(self.key, timeout)
            if isinstance(data, tuple):
                data = data[1]
        else:
            data = self.server.lpop(self.key)

        if data:
            return self._decode_request(data)


__all__ = ['SpiderQueue', 'SpiderPriorityQueue', 'SpiderStack']