# Concurrency and async / await { #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? { #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()
```

app = FastAPI()


@app.get("/")
async def read_main():
    return {"msg": "Hello World"}


@app.websocket("/ws")
async def websocket(websocket: WebSocket):
    await websocket.accept()
    await websocket.send_json({"msg": "Hello WebSocket"})
    await websocket.close()


def test_read_main():
    client = TestClient(app)
    response = client.get("/")
    assert response.status_code == 200

    assertTrue(listenerLatch.await(5, SECONDS));
    assertTrue(task.isDone());
    assertFalse(task.isCancelled());
  }

  public void testListenerCalledOnException() throws Exception {
    throwException = true;

    // Start up the task and unblock the latch to finish the task.
    exec.execute(task);
    runLatch.await();
    taskLatch.countDown();

            }

            else if (type == 'progress') {
                await this.progress(line);
                await this._wait(delay);
            }

            else {
                this.container.appendChild(line);
                await this._wait(delay);
            }

            line.removeAttribute(`${this.pfx}-cursor`);
        }
        this.addRestart()

</html>
"""


@app.get("/")
async def get():
    return HTMLResponse(html)


@app.websocket("/ws")
async def websocket_endpoint(websocket: WebSocket):
    await websocket.accept()
    while True:
        data = await websocket.receive_text()

        executor.execute(wrapper);
      }
      barrier.await(); // release the threads!
      barrier.await(); // wait for them all to complete
      assertEquals(1, task.get().intValue());
      assertEquals(1, counter.get());
    }
    executor.shutdown();
  }

  @J2ktIncompatible
  @GwtIncompatible // blocking wait
  public void testToString() throws Exception {

    assertTrue(listenerLatch.await(5, SECONDS));
    assertTrue(task.isDone());
    assertFalse(task.isCancelled());
  }

  public void testListenerCalledOnException() throws Exception {
    throwException = true;

    // Start up the task and unblock the latch to finish the task.
    exec.execute(task);
    runLatch.await();
    taskLatch.countDown();

    await websocket.accept()
    await websocket.send_text(pathparam)
    await websocket.send_text(queryparam)
    await websocket.close()


async def ws_dependency():
    return "Socket Dependency"


@router.websocket("/router-ws-depends/")
async def router_ws_decorator_depends(
    websocket: WebSocket, data=Depends(ws_dependency)
):
    await websocket.accept()
    await websocket.send_text(data)

@app.websocket("/function-scope")
async def function_scope(websocket: WebSocket, session: SessionFuncDep) -> Any:
    await websocket.accept()
    await websocket.send_json({"is_open": session.open})


@app.websocket("/request-scope")
async def request_scope(websocket: WebSocket, session: SessionRequestDep) -> Any:
    await websocket.accept()
    await websocket.send_json({"is_open": session.open})


@app.websocket("/two-scopes")

            await connection.send_text(message)


manager = ConnectionManager()


@app.get("/")
async def get():
    return HTMLResponse(html)


@app.websocket("/ws/{client_id}")
async def websocket_endpoint(websocket: WebSocket, client_id: int):
    await manager.connect(websocket)
    try:
        while True:
            data = await websocket.receive_text()