Connect to many peers by topic with Hyperswarm

In Connect two peers by key with HyperDHT, two peers connected directly using the first peer's public key. Hyperswarm helps to discover peers swarming a common topic, and connect to as many of them as possible. This will become clearer in Replicate and persist with Hypercore, but it's the best way to distribute peer-to-peer data structures.

The Hyperswarm module provides a higher-level interface over the underlying HyperDHT, abstracting away the mechanics of establishing and maintaining connections. Instead, 'join' topics, and the swarm discovers peers automatically. It also handles reconnections in the event of failures.

In Connect two peers by key with HyperDHT, we needed to explicitly indicate which peer was the server and which was the client. By using Hyperswarm, we create two peers, have them join a common topic, and let the swarm deal with connections.

This How-to consists of a single application, peer-app.

Create the peer-app project

Create the peer-app project with the following commands:

mkdir peer-app
cd peer-app
pear init -y -t terminal
npm install hyperswarm hypercore-crypto b4a bare-process

Alter the peer-app/index.js file to the following:

import Hyperswarm from 'hyperswarm'
import crypto from 'hypercore-crypto'
import b4a from 'b4a'
import process from 'bare-process'

const swarm = new Hyperswarm()
Pear.teardown(() => swarm.destroy())

// Keep track of all connections and console.log incoming data
const conns = []
swarm.on('connection', conn => {
  const name = b4a.toString(conn.remotePublicKey, 'hex')
  console.log('* got a connection from:', name, '*')
  conns.push(conn)
  conn.once('close', () => conns.splice(conns.indexOf(conn), 1))
  conn.on('data', data => console.log(`${name}: ${data}`))
  conn.on('error', e => console.log(`Connection error: ${e}`))
})

// Broadcast stdin to all connections
process.stdin.on('data', d => {
  for (const conn of conns) {
    conn.write(d)
  }
})

// Join a common topic
const topic = Pear.config.args[0] ? b4a.from(Pear.config.args[0], 'hex') : crypto.randomBytes(32)
const discovery = swarm.join(topic, { client: true, server: true })

// The flushed promise will resolve when the topic has been fully announced to the DHT
discovery.flushed().then(() => {
  console.log('joined topic:', b4a.toString(topic, 'hex'))
})

Run the swarm

In one terminal, run peer-app with bare:

bare peer-app

This will display the topic. Copy/paste that topic into as many additional terminals as desired with bare peer-app <SUPPLY TOPIC HERE>. Each peer will log information about the other connected peers.

Start typing into any terminal, and it will be broadcast to all connected peers.

Connections established here are ephemeral — once both peers go offline the conversation is lost.

See also

• Replicate and persist with Hypercore — persist messages so a reader catches up after the writer disconnects.
• Connect two peers by key with HyperDHT — direct one-to-one connection by public key.
• Apply recommended practices — the one-Hyperswarm-per-app guidance in context.