Integrate a light client node

⚠️ Update Notice:

Please read Substrate to Polkadot SDK page first.

As you learned in Light clients in Substrate Connect, light client nodes provide secure and decentralized access to blockchain data with minimal hardware and software requirements.

This tutorial demonstrates how you can connect to any Substrate-based blockchain using a light client. To illustrate this, you'll learn how to connect your application to the Statemint parachain. Statemint is a system parachain that is connected to Polkadot and has a publicly-accessible chain specification file.

Before you begin

Before you begin, verify the following:

You have a code editor set up for working with Javascript and Typescript.
You have the yarn package manager installed.

Tutorial objectives

By completing this tutorial, you will accomplish the following objectives:

Connect to the Polkadot relay chain using the Substrate Connect Javascript library.
Learn how to specify a custom chain specification file for Substrate Connect to use.
Connect to a parachain associated with the custom chain specification.

Connect to a well-known chain

Before the light client can connect to a network, you must have a web application that specifies the network the light client should connect to, the nodes for it to communicate with, and the consensus-critical state it must have at genesis. This information is available in the chain specification file for the network.

Substrate Connect is preconfigured to recognize several chains that are defined in the WellKnownChain enumeration list. These well-known chains are:

Polkadot identified as polkadot
Kusama identified as ksmcc3
Rococo identified as rococo_v2_2
Westend identified as westend2

To connect to one of these chains:

Open a new terminal shell on your computer.
Create a web application to use Substrate Connect by cloning the empty-webapp template with the following command:
```
git clone https://github.com/bernardoaraujor/empty-webapp
```
Change to the empty-webapp directory by running the following command:
```
cd empty-webapp
```
Install Substrate Connect by running the following command:
```
yarn add @substrate/connect
```
Install dependencies from the Polkadot-JS RPC provider by running the following command:
```
yarn add @polkadot/rpc-provider
```
Install dependencies from the Polkadot-JS API by running the following command:
```
yarn add @polkadot/api
```
After you install these dependencies, you can use them in the sample application.
Open the empty-webapp/index.ts file in your preferred code editor.

Copy and paste the following application code to create a Substrate Connect instance with substrate-connect as the provider that connects to the Polkadot relay chain using the polkadot chain specification file.

import { ScProvider } from "@polkadot/rpc-provider/substrate-connect";
import * as Sc from "@substrate/connect";
import { ApiPromise } from "@polkadot/api";

window.onload = () => {
 void (async () => {
   try {
     const provider = new ScProvider(Sc, Sc.WellKnownChain.polkadot);

     await provider.connect();
     const api = await ApiPromise.create({ provider });
     await api.rpc.chain.subscribeNewHeads((lastHeader: { number: unknown; hash: unknown }) => {
       console.log(`💡 New block #${lastHeader.number} has hash ⚡️ ${lastHeader.hash}`);
     });
   } catch (error) {
     console.error(<Error>error);
   }
 })();
};

In this sample application code, creating a Substrate Connect instance is similar to how you create a WebSocket instance using the Polkadot-JS API. With the Polkadot-JS API only, you would create an instance like this:

// Import
import { ApiPromise, WsProvider } from "@polkadot/api";

// Construct
const wsProvider = new WsProvider("wss://rpc.polkadot.io");
const api = await ApiPromise.create({ provider: wsProvider });

For Substrate Connect, you replace the WebSocket (WsProvider) provider with the Substrate Connect (ScProvider) provider, and, instead of a WebSocket URL client address, you specify the chain specification for the network to connect to (WellKnownChain.polkadot).

Install any remaining dependencies by running the following command:
```
yarn
```
Start the web application by running the following command:

yarn dev

If there are compiler errors when you start the local server, you might be missing a dependency not accounted for by the current yarn configuration. If a dependency is missing, you can add the package by running a command similar to the following:

yarn add -D buffer

Verify the browser opens the URL http://localhost:3001/.
Open the browser console for your browser.

How you navigate to and open the browser console varies depending on the browser and operating system you use. For example, on Chrome, select More Tools, Developer Tools, then click Console.

Verify the smoldot process is initialized, followed by the hashes of the incoming blocks from Polkadot.

For example, the console should display log entries similar to the following:

[substrate-connect-extension] [smoldot] Smoldot v0.7.7
[substrate-connect-extension] [smoldot] Chain initialization complete for polkadot. Name: "Polkadot". Genesis hash: 0x91b1…90c3. State root hash: 0x29d0d972cd27cbc511e9589fcb7a4506d5eb6a9e8df205f00472e5ab354a4e17. Network identity: 12D3KooWRse9u6Z9ukP4C92YCCH2gXziNm8ThRch2owaaFh9H6D1. Chain specification or database starting at: 0x7f52…8902 (#14614672)
...
💡 New block #14614893 has hash ⚡️ 0x18f8086952aa5f8f1f8a36ea05af462f6bb26615b481145f7c5daa24ebc0c4cd
💡 New block #14614894 has hash ⚡️ 0x92ca6fd51bc7a2fc5991441e9736bcccf3be45cee6fc88d40d145fc4211ba477
💡 New block #14614894 has hash ⚡️ 0x2353ce49f06206c6dd9882200666fa7d51fc43c1cc6a61cca81ce9fa543409cb

This simple web application only connects to Polkadot to retrieve block hashes. The primary purpose of this application is to demonstrate connecting to the chain without using a centralized entry point to the network, such as the URL for a specific RPC node. However, you could extend this application to do a lot more, because—after you replace WsProvider with ScProvider—you can write code for your application simply by using the existing Polkadot-JS API.

Stop the smoldot light client node by pressing Control-c.

Connect to a custom chain specification

Connecting to a custom chain specification or a publicly-accessible parachain is similar to connecting to one of the well-known chains. The primary difference in the code is that you must explicitly identify the chain specification for Substrate Connect to use.

To connect to Statemint:

Download the custom chain specification file from the cumulus repository.
Copy the downloaded chain specification to the empty-webapp directory you created in Connect to a well-known chain.
Open the index.ts file in your code editor.

Replace the contents with the following code.

import { ApiPromise } from "@polkadot/api";
import { ScProvider } from "@polkadot/rpc-provider/substrate-connect";
import * as Sc from "@substrate/connect";
import jsonParachainSpec from "./statemint.json";

window.onload = () => {
 void (async () => {
   try {
     const relayProvider = new ScProvider(Sc, Sc.WellKnownChain.polkadot);
     const parachainSpec = JSON.stringify(jsonParachainSpec);
     const provider = new ScProvider(Sc, parachainSpec, relayProvider);

     await provider.connect();
     const api = await ApiPromise.create({ provider });
     await api.rpc.chain.subscribeNewHeads((lastHeader: { number: unknown; hash: unknown }) => {
       console.log(`💡 New block #${lastHeader.number} has hash ⚡️ ${lastHeader.hash}`);
     });
   } catch (error) {
     console.error(<Error>error);
   }
 })();
};

As you can see, this code has a few important differences.

The statemint.json chain specification file is imported into the jsonParachainSpec object.
The chain specification is converted to a JSON-encoded string and stored in the parachainSpec variable, so that it can be exchanged with the web server.
The ScProvider provider is created for the polkadot relay chain but is used as a parameter for creating and connecting to the parachain provider.

Substrate Connect requires this information to determine the relay chain that the parachain communicates with.

Start the web application by running the following command:
```
yarn dev
```
Verify the browser opens the URL http://localhost:3001/.
Open the browser console for your browser.

Verify the smoldot process is initialized, followed by the hashes of the incoming blocks from Polkadot.

For example, the console should display log entries similar to the following:

[substrate-connect-extension] [smoldot] Parachain initialization complete for statemint. Name: "Statemint". Genesis hash: 0x68d5…de2f. State root hash: 0xc1ef26b567de07159e4ecd415fbbb0340c56a09c4d72c82516d0f3bc2b782c80. Network identity: 12D3KooWNicu1ZCX6ZNUC96B4TQSiet2NkoQc7SfitxWWE4fQgpK. Relay chain: polkadot (id: 1000)
...
💡 New block #3393027 has hash ⚡️ 0x2401313496be4b1792d704f83b684be6bd2188a618581d30b3addb3648c4ad3a
[substrate-connect-extension] [smoldot] Smoldot v0.7.7. Current memory usage: 222 MiB. Average download: 63.1 kiB/s. Average upload: 735 B/s.
💡 New block #3393028 has hash ⚡️ 0x512af8ad5577f509f3f5123916ff2da6ca0f86df8099eafbc0bc001febec62dd

Congratulations, you've created a simple web application that connects to Statemint and Polkadot using an in-browser light client. Have a look at this demo to learn how to add connections to more chains from the same application.