---
title: "Coordinate apps on one machine"
description: "Let MainSDK apps on the same host share attack and pressure signals over a local bus, and ask a declared peer for its status."
section: "guides"
status: "Building for 0.4"
spec: "neighbours"
canonical: "https://mainsdk.dev/docs/guides/coordinate-apps-on-one-machine"
updated: "2026-10-28"
---

# Coordinate apps on one machine

> Let MainSDK apps on the same host share attack and pressure signals over a local bus, and ask a declared peer for its status.

Status: Building for 0.4 · Canonical: https://mainsdk.dev/docs/guides/coordinate-apps-on-one-machine · Updated: 2026-10-28

You will turn on the host bus for an app, choose what it hears and shares, and check it from the command line. It takes about 10 minutes. The bus is local to one machine and one OS user.

## You need

- Two or more MainSDK apps on the same machine, with `apps.registry: on`.

## 1. Join the bus

```yaml title="main.yaml"
apps:
  registry: on
neighbours:
  host: on
  listen: [attack.*, pressure.*]
  share: [attack.suspected, pressure.high]
  priority: 5
```

Anything not in `share` stays inside the app (`MAIN-NEIGHBOUR-0007`).

## 2. Hear the neighbours

```ts title="app/signals.ts"
import { signals } from "mainsdk";

signals.on("attack.*", (s) => console.log(s.name, s.source.project, s.trust), { from: ["host"] });
```

Host signals reach handlers only when you ask for them with `from`.

## 3. Check the bus

```bash
npx mainsdk neighbours
```

It prints the host bus settings, this app's socket name and the last host signals. To change the shared key, run `npx mainsdk neighbours rotate-key`; running apps re-read it within 60 seconds.

## 4. Ask a peer

List the peer under `apps.peers` with `apps.link: on`, accept the verb under `neighbours.asks.accept`, then:

```ts
import { neighbours } from "mainsdk";

const status = await neighbours.ask("prj_01JD8X", "status");
```

## Check it worked

`mainsdk neighbours` shows the socket and, after a neighbour shares a signal, that signal under the last host signals.

## Troubleshooting

- `MAIN-NEIGHBOUR-0001`: a bad value, or `host: on` with `apps.registry: off`.
- `MAIN-NEIGHBOUR-0002`: the host key or folder is unsafe. The bus stays off and the app keeps serving. It stays off on Windows.
- `MAIN-NEIGHBOUR-0004`: an ask was refused. Check `asks.accept` and the peer lists.

See the [neighbours interface](https://mainsdk.dev/docs/interfaces/neighbours) and [signals](https://mainsdk.dev/docs/guides/react-to-signals).