# External hosting

> Route specific capabilities to your own backend.

Source: https://docs.coline.app/sdk/hosting

Most apps never need this. With `hosting: { default: "coline" }` everything runs on Coline. Use external hosting, per capability, when:

* a tool needs a private database or a service only your network can reach
* work runs longer than a job attempt allows, or needs your own compute
* you already run the logic and want Kairo to call it

## The handler

`createColineAppHandler` checks signatures, timestamps and replays, then dispatches tools, renders, schedules, events, webhooks and jobs to the same code you'd run on Coline. One endpoint serves all of them.

```ts
import { createColineAppHandler } from "@colineapp/sdk/v2";
import app from "./app.config";

// Set COLINE_DELIVERY_SECRET once as a developer secret in the developer
// console, and use the same value here.
const handler = createColineAppHandler({
  app,
  secret: process.env.COLINE_DELIVERY_SECRET!,
});

// Next.js: app/coline/app/route.ts
export const POST = handler;
```

Then point the capabilities you want at it:

```ts
hosting: {
  default: "coline",
  overrides: { "todos.sync_crm": "external" },
  external: { baseUrl: "https://your-app.example.com" },
},
```

Everything not listed in `overrides` keeps running on Coline.

## Testing deliveries

The workspace API can send a signed test delivery to your backend and replay stored ones. See [apps in a workspace](/api/endpoints#apps-in-a-workspace).