You can define background tasks to be run *after* returning a response.

This is useful for operations that need to happen after a request, but that the client doesn't really have to be waiting for the operation to complete before receiving his response.

This includes, for example:

* Email notifications sent after performing an action:
    * As connecting to an email server and sending an email tends to be "slow" (several seconds), you can return the response right away and send the email notification in the background.
* Processing data:
    * For example, let's say you receive a file that must go through a slow process, you can return a response of "Accepted" (HTTP 202) and process it in the background.

## Using `BackgroundTasks`

First, import `BackgroundTasks` and define a parameter in your *path operation function* with a type declaration of `BackgroundTasks`:

```Python hl_lines="1 13"
{!./src/background_tasks/tutorial001.py!}
```

**FastAPI** will create the object of type `BackgroundTasks` for you and pass it as that parameter.

!!! tip
    You declare a parameter of `BackgroundTasks` and use it in a very similar way as to when <a href="/tutorial/using-request-directly/" target="_blank">using the `Request` directly</a>.


## Create a task function

Create a function to be run as the background task.

It is just a standard function that can receive parameters.

It can be an `async def` or normal `def` function, **FastAPI** will know how to handle it correctly.

In this case, the task function will write to a file (simulating sending an email). 

And as the write operation doesn't use `async` and `await`, we define the function with normal `def`:

```Python hl_lines="6 7 8 9"
{!./src/background_tasks/tutorial001.py!}
```

## Add the background task

Inside of your *path operation function*, pass your task function to the *background tasks* object with the method `.add_task()`:

```Python hl_lines="14"
{!./src/background_tasks/tutorial001.py!}
```

`.add_task()` receives as arguments:

* A task function to be run in the background (`write_notification`).
* Any sequence of arguments that should be passed to the task function in order (`email`).
* Any keyword arguments that should be passed to the task function (`message="some notification"`).

## Dependency Injection

Using `BackgroundTasks` also works with the dependency injection system, you can declare a parameter of type `BackgroundTasks` at multiple levels: in a *path operation function*, in a dependency (dependable), in a sub-dependency, etc.

**FastAPI** knows what to do in each case and how to re-use the same object, so that all the background tasks are merged together and are run in the background afterwards:

```Python hl_lines="11 14 20 23"
{!./src/background_tasks/tutorial002.py!}
```

In this example, the messages will be written to the `log.txt` file *after* the response is sent.

If there was a query in the request, it will be written to the log in a background task.

And then another background task generated at the *path operation function* will write a message using the `email` path parameter.

## Technical Details

The class `BackgroundTasks` comes directly from <a href="https://www.starlette.io/background/" target="_blank">`starlette.background`</a>.

It is imported/included directly into FastAPI so that you can import it from `fastapi` and avoid accidentally importing the alternative `BackgroundTask` (without the `s` at the end) from `starlette.background`.

By only using `BackgroundTasks` (and not `BackgroundTask`), it's then possible to use it as a *path operation function* parameter and have **FastAPI** handle the rest for you, just like when using the `Request` object directly.

It's still possible to use `BackgroundTask` alone in FastAPI, but you have to create the object in your code and return a Starlette `Response` including it.

You can see more details in <a href="https://www.starlette.io/background/" target="_blank">Starlette's official docs for Background Tasks</a>.

## Recap

Import and use `BackgroundTasks` with parameters in *path operation functions* and dependencies to add background tasks.