# Advance Handlers

## Overview

Advance handlers are async function callbacks that receive the `metadata` and `payload` of an input, and must return either `"accept"` or `"reject"`.
The following example defines a request handler that accepts all requests, but just logs the input metadata.

```ts twoslash
import { createApp } from "@deroll/app";
import { AdvanceRequestHandler } from "@deroll/core";

const handler: AdvanceRequestHandler = async ({ metadata, payload }) => {
    console.log(metadata);
// @noErrors
    return "
//          ^|
}

// create application
const app = createApp();
app.addAdvanceHandler(handler);
```

## Handlers Execution

The application created with `createApp` holds a list of advance handlers that are executed in the order they were added.

By default, when a handler returns `"accept"` the execution chain stops.

```ts twoslash
import { createApp } from "@deroll/app";

// create application
const app = createApp();

// ---cut---
app.addAdvanceHandler(async (data) => {
    console.log(data);
    return "accept";
});

app.addAdvanceHandler(async (data) => {
    // this code is never executed, because the first handler always returns "accept" // [!code hl]
    return "accept";
});
```

If a handler returns `"reject"` or **raises an exception**, the next handler in the list is executed.

```ts twoslash
import { createApp } from "@deroll/app";

// create application
const app = createApp();

// ---cut---
app.addAdvanceHandler(async (data) => {
    console.log(data);
    return "reject";
});

app.addAdvanceHandler(async (data) => {
    // this code will execute // [!code hl]
    return "accept";
});
```

If no handler returns `"accept"` the input is rejected and the machine is reverted to the initial state by the Cartesi node.

## Broadcast Mode

If the application is created with the `broadcastAdvanceRequests` option set to `true`, then all handlers are executed regardless the return value of each handler.

```ts twoslash
import { createApp } from "@deroll/app";

// create application
const app = createApp({
    broadcastAdvanceRequests: true
//  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
});

app.addAdvanceHandler(async (data) => {
    console.log(data);
    return "accept";
});

app.addAdvanceHandler(async (data) => {
    // this code executes, even when the first handler returns "accept" // [!code hl]
    return "accept";
});
```