​
Start a background task worker

To start a worker to run background tasks for a single function, define a task and call .serve() on it. 
from prefect import task

@task(log_prints=True)
def add(a: int, b: int):
    print(f"{a} + {b} = {a + b}")

add.serve()

​
Trigger a background task

To trigger a background task, call .delay() on the same task in a different terminal. 
add.delay(1, 2)
.delay() has the same signature as the @task decorated function.

​
Start a worker to run background tasks for multiple functions

To start a worker to run background tasks for multiple functions, call serve() with the tasks you want to run. 
from prefect import task
from prefect.task_worker import serve

@task(log_prints=True)
def add(a: int, b: int):
    print(f"{a} + {b} = {a + b}")

@task(log_prints=True)
def multiply(a: int, b: int):
    print(f"{a} * {b} = {a * b}")

# Both these invocations will run once the worker is started
add.delay(1, 2)
multiply.delay(1, 4)

serve(add, multiply) # start a task worker listening for both `add` and `multiply`

​
Submit multiple background tasks

To submit multiple background tasks, call .map() on the task with deferred=True. 
from prefect import task

@task(log_prints=True)
def add(a: int, b: int):
    print(f"{a} + {b} = {a + b}")

A = [1, 2, 3]
B = [4, 5, 6]

add.map(A, B, deferred=True)

add.serve()
.map() accepts Iterable[P.args], Iterable[P.kwargs] or unmapped inputs.
If you want to keep one argument mapped and the other unmapped, you can do so by passing unmapped to the mapped argument. 
from prefect import task, unmapped

@task(log_prints=True)
def add(a: int, b: int):
    print(f"{a} + {b} = {a + b}")

A = [1, 2, 3]

add.map(A, unmapped(4), deferred=True)

add.serve()

​
Further reading