问题与解答

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小牛编辑

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2023-12-01

为什么在例子中调用 time.sleep() 不会并发执行?
我的代码是异步的, 但它不能在两个浏览器标签页上并行运行.

Many people’s first foray into Tornado’s concurrency looks something like this:

class BadExampleHandler(RequestHandler):
    def get(self):
        for i in range(5):
            print(i)
            time.sleep(1)

Fetch this handler twice at the same time and you’ll see that the second five-second countdown doesn’t start until the first one has completely finished. The reason for this is that is a blocking function: it doesn’t allow control to return to the so that other handlers can be run.

Of course, is really just a placeholder in these examples, the point is to show what happens when something in a handler gets slow. No matter what the real code is doing, to achieve concurrency blocking code must be replaced with non-blocking equivalents. This means one of three things:

Find a coroutine-friendly equivalent. For , use instead:
```
class CoroutineSleepHandler(RequestHandler):
    @gen.coroutine
    def get(self):
        for i in range(5):
            print(i)
            yield gen.sleep(1)
```
When this option is available, it is usually the best approach. See the Tornado wiki for links to asynchronous libraries that may be useful.
Find a callback-based equivalent. Similar to the first option, callback-based libraries are available for many tasks, although they are slightly more complicated to use than a library designed for coroutines. These are typically used with as an adapter:
```
class CoroutineTimeoutHandler(RequestHandler):
    @gen.coroutine
    def get(self):
        io_loop = IOLoop.current()
        for i in range(5):
            print(i)
            yield gen.Task(io_loop.add_timeout, io_loop.time() + 1)
```
Again, the Tornado wiki can be useful to find suitable libraries.
Run the blocking code on another thread. When asynchronous libraries are not available, can be used to run any blocking code on another thread. This is a universal solution that can be used for any blocking function whether an asynchronous counterpart exists or not:
```
executor = concurrent.futures.ThreadPoolExecutor(8)

class ThreadPoolHandler(RequestHandler):
    @gen.coroutine
    def get(self):
        for i in range(5):
            print(i)
            yield executor.submit(time.sleep, 1)
```

See the chapter of the Tornado user’s guide for more on blocking and asynchronous functions.

即使你是用了异步和非阻塞的控制器, 你会发现在测试过程中可能也会出现问题. 流量器将会发现你试图在两个不同的标签页加载同一个页面,这时浏览器会延迟加载第二个页面一直等到第一个页面加载完成. 如果你想要看到异步的效果, 请尝试以下两种方法中的任意一个:

在url上添加一些参数让请求变得不同. 之前是将 http://localhost:8888 在两个标签页中打开, 现在可以尝试在一个标签页中打开 http://localhost:8888/?x=1 而后在另一个标签页打开 http://localhost:8888/?x=2 .
使用两个不同的浏览器. 例如, Firefox 和 Chrome 将会同时加载同样的url而不会等待对方.