# App

The `@deroll/app` module provides the entry point of every deroll application: the `App`.

An `App` owns the main loop of a Cartesi application. It fetches [advance](../advance-handlers) and [inspect](../inspect-handlers) requests from the Cartesi Machine, dispatches them to the handlers you register, and exposes methods to produce the outputs a backend can emit.

You create one with [`createApp`](./create-app) and start its loop with `app.start()`.

```ts twoslash
/// <reference types="node" />
// ---cut---
import { createApp } from "@deroll/app";

// create application
const app = createApp();

// handle advance requests (inputs)
app.addAdvanceHandler(async ({ metadata, payload }) => {
    console.log({ metadata, payload });
    return "accept";
});

// handle inspect requests (read-only queries)
app.addInspectHandler(async ({ payload }) => {
    console.log(payload);
});

// start the main loop
app.start().catch(() => process.exit(1));
```

## Registering handlers

* [addAdvanceHandler](./add-advance-handler) — process state-changing inputs; handlers return `"accept"` or `"reject"`.
* [addInspectHandler](./add-inspect-handler) — answer read-only queries; handlers can only produce reports.

## Producing outputs

While handling a request, an `App` can emit four kinds of output:

* [createNotice](./create-notice) — a provable event log.
* [createReport](./create-report) — a stateless, non-provable log (the only output available to inspect handlers).
* [createVoucher](./create-voucher) — an executable on-chain call (`CALL`), for example an asset withdrawal.
* [createDelegateCallVoucher](./create-delegate-call-voucher) — an executable on-chain call run with `DELEGATECALL`, in the application contract's own context.

Notices and vouchers are only available while processing an advance request, because they can change the application state. Reports can be produced from both advance and inspect handlers.