Class DashWebsocketCallback

dash/backends/ws.py:36–186

WebSocket callback communication via queues. Provides methods for real-time bidirectional communication between the server and renderer during callback execution. Uses janus.Queue for outbound messages (serialized with to_json) and queue.Queue for get_props responses, enabling thre

class DashWebsocketCallback:
    """WebSocket callback communication via queues.

    Provides methods for real-time bidirectional communication between
    the server and renderer during callback execution.

    Uses janus.Queue for outbound messages (serialized with to_json) and
    queue.Queue for get_props responses, enabling thread-safe communication
    between worker threads and the main event loop.

    IMPORTANT: For long-running callbacks that use loops (e.g., streaming updates),
    you MUST check `ws.is_shutdown` in your loop to detect disconnections:

        @callback(Input('btn', 'n_clicks'))  # No Output - uses set_props only
        async def long_running(n_clicks):
            ws = ctx.websocket
            while True:
                if ws and ws.is_shutdown:
                    raise PreventUpdate  # Exit gracefully on disconnect
                set_props('progress', {'value': get_data()})
                await asyncio.sleep(0.1)

    Without this check, callbacks will continue running after the client disconnects,
    wasting server resources.

    Note: Only "persistent callbacks" (callbacks with no Output and no Input that use
    only set_props) are automatically restarted when the WebSocket reconnects. Regular
    callbacks with outputs are not restarted.
    """

    def __init__(
        self,
        pending_get_props: Dict[str, queue.Queue[Any]],
        renderer_id: str,
        outbound_queue: janus.Queue[str],
        shutdown_event: "threading.Event",
    ):
        """Initialize the WebSocket callback interface.

        Args:
            pending_get_props: Dict to track pending get_props requests.
                Values are queue.Queue instances for blocking response retrieval.
            renderer_id: The renderer ID for routing messages back to the correct client
            outbound_queue: janus.Queue for thread-safe outbound messaging.
            shutdown_event: Event signaling the websocket connection has closed.
        """
        self._pending_get_props = pending_get_props
        self._renderer_id = renderer_id
        self._outbound_queue = outbound_queue
        self._shutdown_event = shutdown_event

    @property
    def is_shutdown(self) -> bool:
        """Check if the websocket connection has been shut down."""
        return self._shutdown_event.is_set()

    def _get_outbound_queue(self) -> janus.Queue[str] | None:
        """Get the outbound queue."""