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# Simplify MPP integrations with relays \[Delegate payment validation and settlement]

`mppx` now supports relays. A relay is a networked interface that abstracts payment-method settlement away from your application.

## What a relay does

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

* `validate` checks whether a Credential can be accepted without consuming it or changing payment state.
* `broadcast` revalidates the Credential, submits the payment, and returns the information needed to create a Receipt.

This gives 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`. `mppx` sends Credential validation and broadcast requests to Tempo API:

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

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

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

### Any payment method

For any 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 }) {
    return relay.broadcast({ credential, request })
  },
  async validate({ credential, request }) {
    return relay.validate({ credential, request })
  },
})
```

## Author a relay

MPP does not require a relay or standardize its API. Relay authors can choose their payment methods, policy, and transport.

Implement a non-mutating `validate` operation and a terminal `broadcast` operation. Because conditions can change after validation, `broadcast` must repeat the relevant checks before it settles or accepts payment.

## Learn more

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