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Communicating with Background Tasks

Each instance of a Background Task is associated with a Task object that can be used to get the task’s current status, and any data the task has passed to the main program.

The Task object can also be used to control the running of the task.

Background tasks run in a separate session from the code that launched them, so the Task object is the only way to communicate with a task.

Communicating back to the main program

From within the task, modify anvil.server.task_state to share data with the main program.

The main program gets a Task object as the return value of anvil.server.launch_background_task(). This object has a get_state() method that returns the task_state.

By default, anvil.server.task_state is an empty dict, so you can immediately set key/value pairs: anvil.server.task_state['progress'] = 42.

The Task can also raise exceptions; the main program can access these using the is_completed and get_error methods of the Task object.

@anvil.server.callable
def launch_my_task():
  task = anvil.server.launch_background_task('train_my_network', 200)

  # Output some state from the task
  print(task.get_state()['false_positive_rate'])

  # Is the task complete yet?
  if task.is_completed():
    return

  # Store the ID of the task so we can see it later
  app_tables.tasks.add_row(
    when=datetime.now(),
    task_id=task.get_id(),
  )

  # If the task raised an exception, re-raise it here
  task.get_error()

  # Otherwise, kill the task
  task.kill()

  # Check the termination status
  if task.get_termination_status() == "killed":
    print("Yes, the task was killed!")

  # Finally, return the task to the client
  return task

Task object

The Task object reports on task status and gives you control over the task.

There are three ways to get the Task object for a particular task:

  • When launching a task. It is the return value of anvil.server.launch_background_task().
  • Later on, using anvil.server.get_background_task(task_id). You need to know the task_id, which you can get using task.get_id(). You may wish to store this ID in a Data Table or elsewhere.
  • Using anvil.server.list_background_tasks(). (See Listing Background Tasks from code)

The Task object’s methods are:

  • get_state(): returns an object that you can modify from within the task (by modifying anvil.server.task_state inside the task).
  • get_id(): returns the ID of a task, which is a string.
  • get_task_name(): returns the name of the task. Usually the name of the function decorated with anvil.server.background_task.
  • get_start_time(): returns a DateTime representing the time this task was started.
  • get_return_value(): returns the return value of the task, or None if the task has not yet returned.
  • get_error(): re-throws whatever exception caused the task to terminate.
  • kill(): stops the task and makes its status "killed" rather than "failed". This can only be executed on the server side.
  • get_termination_status() returns:
    • None if the task is still running
    • "completed" if the task completed successfully
    • "failed" if the task terminated with an exception
    • "killed" if the task was explicitly killed with task.kill(), or by clicking “Kill” in the development environment
    • "missing" if an Uplink disconnection or infrastructure failure caused us to lose track of the task.
  • is_running(): returns True if the task is still running. Otherwise returns False, regardless of how the task ended (completed, killed, failed or missing).
  • is_completed(): returns True if the task completed successfully, False if the task is still running. If the task has failed or been killed, it raises whatever caused the task to terminate.

Output from Background Tasks

When you call print() in a Background Task, the output is visible in the App Logs.

Advanced

Timeouts

For those on the Free Plan or using Basic Python, the maximum run-time for a background task is 30 seconds.

Runtime for background tasks is not limited for those on the Personal Plan and up.

Users on the Personal Plan and up will need to select one of the Full Python server environments to remove the background task timeout.

Separation of processes

Background Tasks are run in a separate Python process to your server functions. This means that they’ll never share global variables or anvil.server.session with your server calls. (They also do not share implicit session information, such as any user logged in via the Users Service.)

The reason we run Background Tasks in a separate process is to do with timeouts: If a server call takes too long, we need to kill it rather than letting it spin forever. Background tasks, however, can run for a long time. If we ran them in the same Python interpreter as your standard server functions, we would have to kill your background tasks whenever a server call timed out.

If the Task needs access to information from anvil.server.session, then it is the caller’s responsibility to furnish it, via additional parameters passed to anvil.server.launch_background_task().

The anvil.server.session object is not portable across server calls. If you need to share session data with a background task, pass only the arguments you need to pass.

Task object methods

The data in anvil.server.task_state is specific to the task - that is, a background task’s own anvil.server.task_state is read/write within the Task, and read-only everywhere else, including in the code of other Tasks.

When a background task is defined on the Uplink, it will execute in its own thread on the Uplink, just like a server call, and anvil.server.task_state will be a thread-local variable.

If an Uplink is disconnected from Anvil, we lose track of any background tasks running on that uplink. The termination status for those tasks will be "missing" or "failed".