> For the complete documentation index, see [llms.txt](https://docs.constellationnetwork.io/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.constellationnetwork.io/metagraph-development/metagraph-framework/framework-architecture.md).

# Framework Architecture

This section contains information about the Metagraph Framework and its relation to the deployed architecture of a metagraph.

In order to understand how the framework functions, it is important to understand the multi-layered architecture of a metagraph. Make sure you're familiar with [Metagraph Architecture](https://docs.constellationnetwork.io/metagraphs/concepts/architecture) before continuing.

## Code Structure[​](https://docs.constellationnetwork.io/sdk/metagraph-framework/architecture#code-structure) <a href="#code-structure" id="code-structure"></a>

A new metagraph project generated from the [currency template](https://github.com/Constellation-Labs/currency.g8) will have the following module directory structure:

```
- modules/
- - l0/
- - - Main.scala
- - l1/
- - - Main.scala
- - data_l1/
- - - Main.scala
- - shared_data
- - - Main.scala
```

Let's break down each directory and its function.

**L0**[**​**](https://docs.constellationnetwork.io/sdk/metagraph-framework/architecture#l0)

This directory contains a `Main.scala` file with a Main object instance that extends `CurrencyL0App`. `CurrencyL0App` contains overridable functions that allow for customization of the operation of the metagraph L0 layer including validation, snapshot formation, and management of off chain state. It also contains the `rewards` overridable function that allows for minting of new tokens on the network.

**note**

While the class `CurrencyL0App` has the "Currency" in its name, it defines the L0 layer through which both Currency L1 and Data L1 data flows.

**L1**[**​**](https://docs.constellationnetwork.io/sdk/metagraph-framework/architecture#l1)

This directory contains a `Main.scala` file with a Main object instance that extends `CurrencyL1App`. `CurrencyL1App` contains overridable functions relevant to customizing token validation behavior such as `transactionValidator`.

**L1 Data**[**​**](https://docs.constellationnetwork.io/sdk/metagraph-framework/architecture#l1-data)

This directory also contains a `Main.scala` file, with a Main object instance that extends `CurrencyL1App`. In order to have this L1 behave as a DataApplication, the `dataApplication` method should be overridden with your custom configuration. The metagraph examples repo has implemented examples to reference, for example the [NFT example](https://github.com/Constellation-Labs/metagraph-examples/blob/main/examples/nft/modules/data_l1/src/main/scala/com/my/nft/data_l1/Main.scala).

**Shared Data**[**​**](https://docs.constellationnetwork.io/sdk/metagraph-framework/architecture#shared-data)

This directory contains an empty Main.scala file but is provided as a suggestion for application directory structure. Several of the lifecycle functions are run on both Data L1 and on L0. For example, serializers/deserializers, validators, and data types will likely shared between layers. Organizing them in a separate directory makes their use in multiple layers clear.

See the [Data Application Lifecycle](https://docs.constellationnetwork.io/sdk/metagraph-framework/data/lifecycle-functions) for more information.


---

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