# Lifespan Events { #lifespan-events }

You can define logic (code) that should be executed before the application **starts up**. This means that this code will be executed **once**, **before** the application **starts receiving requests**.

The same way, you can define logic (code) that should be executed when the application is **shutting down**. In this case, this code will be executed **once**, **after** having handled possibly **many requests**.

And of course, you can use the same model configured to use base64 to handle both input (*validate*) with `val_json_bytes` and output (*serialize*) with `ser_json_bytes` when receiving and sending JSON data.

## A Custom `PNGStreamingResponse` { #a-custom-pngstreamingresponse }

In the examples above, the data bytes were streamed, but the response didn't have a `Content-Type` header, so the client didn't know what type of data it was receiving.

You can create a custom sub-class of `StreamingResponse` that sets the `Content-Type` header to the type of data you're streaming.

The contents are **encrypted**, even though they are being sent with the **HTTP protocol**.

It is a common practice to have **one program/HTTP server** running on the server (the machine, host, etc.) and **managing all the HTTPS parts**: receiving the **encrypted HTTPS requests**, sending the **decrypted HTTP requests** to the actual HTTP application running in the same server (the **FastAPI** application, in this case), take the **HTTP response** from the application, **encrypt it** using...

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 the response.

This includes, for example:

* Email notifications sent after performing an action:

{* ../../docs_src/path_params/tutorial003_py310.py hl[6,11] *}

Otherwise, the path for `/users/{user_id}` would match also for `/users/me`, "thinking" that it's receiving a parameter `user_id` with a value of `"me"`.

Similarly, you cannot redefine a path operation:

{* ../../docs_src/path_params/tutorial003b_py310.py hl[6,11] *}

* `upgrade`: Upgrades

For `await` to work, it has to be inside a function that supports this asynchronicity. To do that, you just declare it with `async def`:

```Python hl_lines="1"
async def get_burgers(number: int):

	}
	// By default, ColumnType's Unique is not affected by UniqueIndex, so we don't care about UniqueIndex.
	return m.RunWithValue(value, func(stmt *gorm.Statement) error {
		// We're currently only receiving boolean values on `Unique` tag,
		// so the UniqueConstraint name is fixed
		constraint := m.DB.NamingStrategy.UniqueName(stmt.Table, field.DBName)
		if unique && !field.Unique {

 * behavior:
 *
 * {@snippet :
 * BaseEncoding.base16().lowerCase().decode("deadbeef");
 * }
 *
 * <p>Warning: BaseEncoding instances are immutable. Invoking a configuration method has no effect
 * on the receiving instance; you must store and use the new encoding instance it returns, instead.
 *
 * {@snippet :
 * // Do NOT do this
 * BaseEncoding hex = BaseEncoding.base16();
 * hex.lowerCase(); // does nothing!