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# Simplify MPP integrations with relays \[Delegate payment validation and settlement to an external service]

`mppx` now supports relays. A relay is a networked interface that abstracts payment-method settlement away from your application, reducing integration complexity and giving you new capabilities.

## What a relay does

A relay sits between your application and the payment settlement layer. It handles two lifecycle operations:

* Validation checks whether a payment Credential is valid and can be accepted by the underlying payment rail.
* `broadcast` revalidates the payment Credential, finalizes or accepts payment, and returns the information needed to create a Receipt.

Relays give applications one interface for payment finalization while relay operators handle the details of each payment method.

## Add a relay

### Tempo charges

Pass a Tempo API key to `tempo.charge`. Your `mppx` integration automatically uses the Tempo relay.

```ts twoslash [server.ts]
import { Mppx, tempo } from 'mppx/server'

const mppx = Mppx.create({
  methods: [
    // [!code hl:start]
    tempo.charge({
      relay: {
        apiKey: process.env.TEMPO_API_KEY!,
      },
    }),
    // [!code hl:end]
  ],
})
```

You can point the same integration at a Tempo API-compatible service with `relay.apiBaseUrl`.

### Other payment methods

For any other payment method, connect your relay to the `validate` and `broadcast` hooks with `Method.toServer`:

```ts [method.server.ts]
import { Method } from 'mppx'
import { charge } from './method'

export const chargeServer = Method.toServer(charge, {
  async broadcast({ credential, request }) { // [!code hl]
    return relay.broadcast({ credential, request })
  },
  async validate({ credential, request }) { // [!code hl]
    return relay.validate({ credential, request })
  },
})
```

## Author a relay

MPP does not require relays or require them to conform to a single interface. Relay authors can define their own API shape, payment method support, and underlying functionality—as long as they preserve the core MPP control flow.

## Learn more

* [Relay integration guide](/advanced/relays)