# Concurrency and async / await

Details about the `async def` syntax for *path operation functions* and some background about asynchronous code, concurrency, and parallelism.

## In a hurry?

<abbr title="too long; didn't read"><strong>TL;DR:</strong></abbr>

If you are using third party libraries that tell you to call them with `await`, like:

```Python
results = await some_library()
```

    waiter2.join();
  }

  public void testRemoveWaiter_polling() throws Exception {
    final AbstractFuture<String> future = new AbstractFuture<String>() {};
    WaiterThread waiter = new WaiterThread(future);
    waiter.start();
    waiter.awaitWaiting();
    PollingThread poller = new PollingThread(future);
    poller.start();
    PollingThread poller2 = new PollingThread(future);
    poller2.start();

    Thread waiter =
        new Thread() {
          @Override
          public void run() {
            try {
              service.awaitTermination(1, DAYS);
            } catch (InterruptedException e) {
              return;
            }
          }
        };
    waiter.start();
    awaitTimedWaiting(waiter);
    service.shutdown();

			outBufTrim := strings.TrimSpace(outBuf.String())

			if !c.isValid && strings.Compare(errMsgTrim, outBufTrim) != 0 {
				t.Fatalf("\nwanted: %v \nbut found: %v", errMsgTrim, outBufTrim)
			}

			if c.isValid && outBuf.Len() > 0 {
				t.Fatalf("\nwanted: %v \nbut found: %v", errMsgTrim, outBufTrim)
			}
			outBuf.Reset()
		})
	}

    Thread waiter =
        new Thread() {
          @Override
          public void run() {
            try {
              service.awaitTermination(1, DAYS);
            } catch (InterruptedException e) {
              return;
            }
          }
        };
    waiter.start();
    awaitTimedWaiting(waiter);
    service.shutdown();

* `seek(offset)`: Goes to the byte position `offset` (`int`) in the file.
    * E.g., `await myfile.seek(0)` would go to the start of the file.
    * This is especially useful if you run `await myfile.read()` once and then need to read the contents again.
* `close()`: Closes the file.

As all these methods are `async` methods, you need to "await" them.

For example, inside of an `async` *path operation function* you can get the contents with:

    try {
      // if waiting works, this should get stuck
      Queues.drain(q, newArrayList(), 1, MAX_VALUE, NANOSECONDS);
      fail();
    } catch (InterruptedException expected) {
      // we indeed waited; a slow thread had enough time to interrupt us
    }
  }

  // same as above; uninterruptible version
  @SuppressWarnings("ThreadPriorityCheck") // TODO: b/175898629 - Consider onSpinWait.

        try {
          latch.await();
        } catch (InterruptedException quittingTime) {
        }
        return TEST_STRING;
      }
    };
  }

  public Runnable awaiter(final CountDownLatch latch) {
    return new CheckedRunnable() {
      @Override
      public void realRun() throws InterruptedException {
        await(latch);
      }
    };
  }

        try {
          latch.await();
        } catch (InterruptedException quittingTime) {
        }
        return TEST_STRING;
      }
    };
  }

  public Runnable awaiter(final CountDownLatch latch) {
    return new CheckedRunnable() {
      @Override
      public void realRun() throws InterruptedException {
        await(latch);
      }
    };
  }

   *
   * @param q the blocking queue to be drained
   * @param buffer where to add the transferred elements
   * @param numElements the number of elements to be waited for
   * @param timeout how long to wait before giving up
   * @return the number of elements transferred
   * @throws InterruptedException if interrupted while waiting