> ## Documentation Index
> Fetch the complete documentation index at: https://docs.prefect.io/llms.txt
> Use this file to discover all available pages before exploring further.

# How to run tasks background tasks

### 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.

```python theme={null}
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.

```python theme={null}
add.delay(1, 2)
```

<Tip>
  `.delay()` has the same signature as the `@task` decorated function.
</Tip>

### 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.

```python theme={null}
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`.

```python theme={null}
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()
```

<Tip>
  `.map()` accepts `Iterable[P.args]`, `Iterable[P.kwargs]` or `unmapped` inputs.
</Tip>

If you want to keep one argument mapped and the other unmapped, you can do so by passing `unmapped` to the mapped argument.

```python theme={null}
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

* [How to deploy a web application with Prefect background tasks](/v3/advanced/background-tasks)
* [Prefect Background Task Examples](https://github.com/PrefectHQ/prefect-background-task-examples) repository for reference implementations of applications leveraging background tasks.