Lifespan Tasks

Added in v0.5.2

Lifespan tasks are coroutines that run when the backend server is running. They are useful for setting up the initial global state of the app, running periodic tasks, and cleaning up resources when the server is shut down.

Lifespan tasks are defined as async coroutines or async contextmanagers. To avoid blocking the event thread, never use time.sleep or perform non-async I/O within a lifespan task.

Tasks execute in the order they are registered.

In dev mode, lifespan tasks will stop and restart when a hot-reload occurs.

Lifespan tasks are not the only way to run work outside the normal event flow. Choose based on what starts the work and what it operates on:

• Use a lifespan task for automatic, continuous, application-wide work that runs independent of any user or session — it starts when the app starts and is not tied to UI state. Examples: polling an external API on an interval, monitoring a service, refreshing a shared cache, scheduled maintenance.
• Use a background event (@rx.event(background=True)) for user-triggered, session-bound work that reads or updates that user's UI state. Examples: processing a file after a user clicks submit, calling an API on demand, showing progress during a long-running operation the user initiated.

For instance, "check an external API every 5 minutes" is a lifespan task, while "process data when the user clicks submit" is a background event.

Any async coroutine can be used as a lifespan task. It will be started when the backend comes up and will run until it returns or is cancelled due to server shutdown. Long-running tasks should catch asyncio.CancelledError to perform any necessary clean up.

To register a lifespan task, use app.register_lifespan_task(coro_func, **kwargs). Any keyword arguments specified during registration will be passed to the task.

If the task accepts the special argument, app, it will be passed the Reflex app instance (rx.App/LifespanMixin).

If the task accepts the special argument, starlette_app, it will be passed the underlying Starlette application instance.

All tasks must be registered before the app starts. Calling register_lifespan_task after the lifespan has begun (for example, from an event handler or from within another lifespan task) will raise a RuntimeError.

To get the currently registered lifespan tasks, use app.get_lifespan_tasks(), which returns a tuple of tasks in registration order.

Lifespan tasks can also be defined as async contextmanagers. This is useful for setting up and tearing down resources and behaves similarly to the ASGI lifespan protocol.

Code up to the first yield will run when the backend comes up. As the backend is shutting down, the code after the yield will run to clean up.