aiohttp.TCPConnector(带限制参数)vs asyncio.Semaphore用于限制并发连接数

Question

我想我想通过制作一个允许你在一个下载多个资源的简单脚本来学习新的python异步等待语法,更具体地说是asyncio模块.

但是现在我被卡住了.

在研究时,我遇到了两个限制并发请求数量的选项:

1. 将aiohttp.TCPConnector(带限制参数)传递给aiohttp.ClientSession或
2. 使用asyncio.Semaphore.

是否有首选选项,或者如果您只想限制并发连接数,它们是否可以互换使用？性能方面(大致)是否相等？

两者似乎都有默认值100并发连接/操作.如果我只使用信号量限制为500,那么aiohttp内部会隐式地将我锁定为100个并发连接吗？

这对我来说都是非常新的和不清楚的.请随时指出我的任何误解或我的代码中的缺陷.

这是我的代码目前包含两个选项(我应该删除哪些？):

奖金问题:

1. 如何处理(最好重试x次)出现错误的coros？
2. coro完成后,保存返回数据(通知我的DataHandler)的最佳方法是什么？我不希望最后全部保存,因为我可以尽快开始处理结果.

小号

import asyncio
from tqdm import tqdm
import uvloop as uvloop
from aiohttp import ClientSession, TCPConnector, BasicAuth

# You can ignore this class
class DummyDataHandler(DataHandler):
    """Takes data and stores it somewhere"""

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)

    def take(self, origin_url, data):
        return True

    def done(self):
        return None

class AsyncDownloader(object):
    def __init__(self, concurrent_connections=100, silent=False, data_handler=None, loop_policy=None):

        self.concurrent_connections = concurrent_connections
        self.silent = silent

        self.data_handler = data_handler or DummyDataHandler()

        self.sending_bar = None
        self.receiving_bar = None

        asyncio.set_event_loop_policy(loop_policy or uvloop.EventLoopPolicy())
        self.loop = asyncio.get_event_loop()
        self.semaphore = asyncio.Semaphore(concurrent_connections)

    async def fetch(self, session, url):
        # This is option 1: The semaphore, limiting the number of concurrent coros,
        # thereby limiting the number of concurrent requests.
        with (await self.semaphore):
            async with session.get(url) as response:
                # Bonus Question 1: What is the best way to retry a request that failed?
                resp_task = asyncio.ensure_future(response.read())
                self.sending_bar.update(1)
                resp = await resp_task

                await  response.release()
                if not self.silent:
                    self.receiving_bar.update(1)
                return resp

    async def batch_download(self, urls, auth=None):
        # This is option 2: Limiting the number of open connections directly via the TCPConnector
        conn = TCPConnector(limit=self.concurrent_connections, keepalive_timeout=60)
        async with ClientSession(connector=conn, auth=auth) as session:
            await asyncio.gather(*[asyncio.ensure_future(self.download_and_save(session, url)) for url in urls])

    async def download_and_save(self, session, url):
        content_task = asyncio.ensure_future(self.fetch(session, url))
        content = await content_task
        # Bonus Question 2: This is blocking, I know. Should this be wrapped in another coro
        # or should I use something like asyncio.as_completed in the download function?
        self.data_handler.take(origin_url=url, data=content)

    def download(self, urls, auth=None):
        if isinstance(auth, tuple):
            auth = BasicAuth(*auth)
        print('Running on concurrency level {}'.format(self.concurrent_connections))
        self.sending_bar = tqdm(urls, total=len(urls), desc='Sent    ', unit='requests')
        self.sending_bar.update(0)

        self.receiving_bar = tqdm(urls, total=len(urls), desc='Reveived', unit='requests')
        self.receiving_bar.update(0)

        tasks = self.batch_download(urls, auth)
        self.loop.run_until_complete(tasks)
        return self.data_handler.done()


### call like so ###

URL_PATTERN = 'https://www.example.com/{}.html'

def gen_url(lower=0, upper=None):
    for i in range(lower, upper):
        yield URL_PATTERN.format(i)   

ad = AsyncDownloader(concurrent_connections=30)
data = ad.download([g for g in gen_url(upper=1000)])

Answer 1

有首选的选择吗？

是的，见下图：

aiohttp 内部结构会隐式将我的并发连接数限制为 100 个吗？

是的，默认值 100 会锁定您，除非您指定其他限制。您可以在此处的源代码中看到它： https: //github.com/aio-libs/aiohttp/blob/master/aiohttp/connector.py#L1084

它们在性能方面（大致）相等吗？

否（但性能差异应该可以忽略不计），因为aiohttp.TCPConnector无论如何都会检查可用连接，无论它是否被信号量包围，在这里使用信号量只是不必要的开销。

如何处理（最好重试 x 次）抛出错误的 coros？

我不相信有一种标准方法可以做到这一点，但一种解决方案是将您的调用包装在如下方法中：

async def retry_requests(...):
    for i in range(5):
        try:
            return (await session.get(...)
        except aiohttp.ClientResponseError:
            pass