Local fulfillment

The Local Home SDK enhances your smart home integration with Google Assistant by adding a local fulfillment path to route smart home intents.

The SDK lets you write a local fulfillment app, using TypeScript or JavaScript, that contains your smart home business logic. Google Home or Google Nest devices can load and run your app on-device. Your app communicates directly with your existing smart devices over Wi-Fi on a local area network (LAN) to fulfill user commands, over existing protocols.

Integration of the SDK offers performance improvements to your smart home Action, including lower latency and higher reliability. Local fulfillment is supported for all device types and device traits, except those that use secondary user verification.

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Understand how it works

After getting a SYNC response from your cloud fulfillment, the Local Home platform scans the user’s local area network using mDNS, UDP broadcast, or UPnP to discover smart devices connected to Assistant.

The platform sends an IDENTIFY intent to determine if the device is locally controllable, by comparing the device ID in the IDENTIFY response to those returned by the earlier SYNC response. If the detected device is a hub or bridge, the platform sends a REACHABLE_DEVICES intent and treats the hub as the proxy device for communicating locally.

When it receives a response confirming a local device, the platform establishes a local fulfillment path to the user’s Google Home or Google Nest device, and subsequently routes user commands for local fulfillment.

When a user triggers a smart home Action that has a local fulfillment path, Assistant sends the EXECUTE intent or QUERY intent to the Google Home or Google Nest device rather than the cloud fulfillment. The device then runs the local fulfillment app to process the intent.

This figure shows the execution flow for local fulfillment. The
execution path captures a user's intent from a phone with
Google Assistant, then the user intent is processed by
the Google Cloud, then it is executed locally on the Google Home
device and the command is issued directly to the device hub or
directly to the device. The developer cloud is available as a
cloud fallback. — **Figure 1.** Adding a local fulfillment path for your smart home Action.

The Local Home platform is non-deterministic in choosing which Google Home or Google Nest device to trigger the command. The EXECUTE and QUERY intents may come through any Google Home or Google Nest device defined in the same Home Graph structure as the target device.

Local fulfillment app

The local fulfillment app contains the business logic to process the intents sent by the Local Home platform and access your smart device via the local area network. No modification of your hardware is expected to integrate local fulfillment. The app fulfills smart home Action requests by sending control commands to the smart device through application-layer protocols, including HTTP, TCP, or UDP. In the event that the local fulfillment path fails, your cloud fulfilment is used as a fallback execution path.

To support building the app, the Local Home SDK provides these two main classes:

DeviceManager: Provides methods to communicate with smart devices using TCP, UDP sockets, or HTTP requests.
App: Provides methods to attach handlers for intents that Google sends after discovering locally-controllable devices (IDENTIFY, REACHABLE_DEVICES). This class also fulfills user commands (EXECUTE), and answers user queries on current device state (QUERY).

During development, you can build and test the app by loading it on your own hosting server or local development machine. In production, Google hosts your app in a secure JavaScript sandbox environment on the user’s Google Home or Google Nest device.

The Report State API is not currently supported for local fulfillment. Google relies on your cloud fulfillment to process these requests.

Application lifecycle

Your local fulfillment app is loaded on demand when the Local Home platform discovers new local devices matching the project scan configuration, or has pending intents to deliver related to a previously identified device.

Google Home or Google Nest devices are memory constrained and your local fulfillment app may be terminated at any time due to memory pressure in the system. This can happen if your app starts to consume too much memory, or if the system needs to make room for another app. The Local Home platform restarts your app only when there are new intents to deliver and sufficient memory resources for the app to run.

The Local Home platform unloads your app after an idle timeout when the user unlinks their account or there are no longer devices that support local fulfillment associated with the user's agentUserId.

Supported devices

The Local Home platform executes your local fulfillment app on supported Google Home or Google Nest devices. The following table describes the supported devices, and the runtime used on each device. To learn more about runtime requirements, see Execution environment.

Device	Type	Environment
Google Home	Speaker	Chrome
Google Home Mini	Speaker	Chrome
Google Home Max	Speaker	Chrome
Nest Mini	Speaker	Chrome
Nest Hub	Display	Chrome
Nest Hub Max	Display	Chrome
Nest Wifi	Router	Node.js
Nest Wifi	Point	Chrome

Execution environment

The execution environment for your local fulfillment app depends on your device. The Local Home platform supports the following runtime environments:

Chrome: Your local fulfillment app is executed within the context of a Chrome browser window running Chrome M80 or later with support for ECMAScript version ES2018.
Node.js: Your local fulfillment app is executed as a script within a Node.js process running Node v10.x LTS or later with support for ECMAScript version ES2018.

Source code structure

We recommend bundling your dependencies into a single JavaScript file using the bundler configurations provided by the Local Home SDK and packaging your source code as an Immediately-invoked Function Expression (IIFE).

Implementation path

To utilize the Local Home SDK for your smart home integration, you need to perform these tasks:

1	Set up the scan config	Configure the Actions console with the necessary parameters for the Assistant to discover locally controllable devices.
2	Update the SYNC response in your cloud fulfillment	In your cloud fulfillment, modify the `SYNC` request handler to support the `otherDeviceIds` field that the platform uses to establish a local fulfillment path. In that field, specify the IDs of devices that can be locally controlled.
3	Implement the local fulfillment app	Use the Local Home SDK to create a JavaScript app to handle the `IDENTIFY`, `EXECUTE`, and `QUERY` intents. For hub or bridge proxy devices, you should also handle the `REACHABLE_DEVICES` intent.
4	Test and debug your app	Test your integration (or self-certify) by using the test suite for smart home.

To start using local fulfillment, see Add local fulfillment to your smart home Action.