# Stream formats in action

Since we have already discussed what stream formats are and how to use them, let's make use of them in our project!
The first thing to do is to define modules responsible for describing the formats used in our pipeline.
We will put them in a separate directory - `lib/formats`. Let's start with the format describing the [packets](../glossary/glossary.md#packet):

**_`lib/formats/packet_format.ex`_**

```elixir
defmodule Basic.Formats.Packet do
 @moduledoc """
 A module describing the format of the packet.
 """
 defstruct type: :custom_packets
end
```

The definition of the module is not complicated, as you can see in the code snippet above - we are only defining a structure within that module, with a `:type` parameter, whose default value is `:custom_packets`.

In our [pipeline](../glossary/glossary.md#pipeline) we will also send another type of data - [frames](../glossary/glossary.md#frame). Let's define a format for them:

**_`lib/formats/frame_format.ex`_**

```elixir
defmodule Basic.Formats.Frame do
 @moduledoc """
 A module describing the format of the frame.
 """
 defstruct encoding: :utf8
end
```

Same as in the case of the previous format - we are defining a structure with a single field, called `:encoding`, and the default value of that field - `:utf8`.

That's it! Format modules are really simple, and using them is not that hard either, as you will see in the subsequent chapters - defining the accepted stream formats and sending stream format actions!

Before advancing you can test the `Source` [element](../glossary/glossary.md/#source), using the tests provided in `/test` directory.

```console
mix test test/elements/source_test.exs
```

In case of errors, you may go back to the [Source chapter](/basic_pipeline/03_Source.md) or take a look how [Source](https://github.com/membraneframework/membrane_basic_pipeline_tutorial/blob/template/end/lib/elements/Source.ex) and [formats](https://github.com/membraneframework/membrane_basic_pipeline_tutorial/tree/template/end/lib/formats) should look like.