> ## Documentation Index
> Fetch the complete documentation index at: https://docs.twenty.com/llms.txt
> Use this file to discover all available pages before exploring further.
# Overview
> Configure the app itself — its identity, default permissions, and what runs at install time.
A Twenty app's **config layer** is what describes the app *to the platform* — its identity, the permissions it holds, and the code that runs during install or upgrade. These declarations don't add new data shapes or runtime behavior; they tell Twenty *who the app is* and *how to set it up*.
```text theme={null}
┌────────────────────────────────────────────────────────┐
│ Application — identity, default role, variables, │
│ marketplace metadata │
│ │
│ ┌──────────────────────────────────────────────────┐ │
│ │ Role — what the app's logic functions can read │ │
│ │ and write (referenced by Application) │ │
│ └──────────────────────────────────────────────────┘ │
└────────────────────────────────────────────────────────┘
│
▼ (at install / upgrade time)
┌──────────────────────────────────┐
│ Pre-install hook │ before metadata migration
└──────────────────────────────────┘
┌──────────────────────────────────┐
│ Post-install hook │ after metadata migration
└──────────────────────────────────┘
```
## In this section
`defineApplication` — identity, default role, variables, marketplace metadata.
`defineRole` — declare what your app's logic functions can read and write.
`definePreInstallLogicFunction` and `definePostInstallLogicFunction` — back up data, seed defaults, validate upgrades.
## How the pieces relate
* **Application** is the entry point. Every app has exactly one `defineApplication()` call, and it points at one **Role** as its default.
* The **Role** controls what the app's logic functions and front components can read and write. Follow least-privilege: only grant the permissions your code actually needs.
* **Install Hooks** run during install or upgrade — pre-install before the metadata migration (so it can refuse a risky upgrade), post-install after the migration (so it can seed default data against the new schema).
Install hooks share the [logic function](/developers/extend/apps/logic/logic-functions) runtime — same handler signature, same environment variables, same typed API client — but they're declared with their own define functions and live outside the regular trigger model (HTTP, cron, database events).