pear-docs/howto/replicate-and-persist-with-hypercore.md

How to replicate and persist with Hypercore

In the HyperDHT How-to (Connect Two Peers) and the Hyperswarm How-to (Connect Many Peers), peers can exchange chat messages so long as both are online at the same time and directly connected. The application is ephemeral, the messages are not persisted - they will be lost if the recipient is offline. Hypercore provides the persistence.

Hypercore is a secure, distributed append-only log. It is built for sharing enormous datasets and streams of real-time data. It has a secure transport protocol, making it easy to build fast and scalable peer-to-peer applications.

Now extend the ephemeral chat example above but using Hypercore to add many significant new features:

• Persistence: The owner of the Hypercore can add messages at any time, and they'll be persisted to disk. Whenever they come online, readers can replicate these messages over Hyperswarm.
• Many Readers: New messages added to the Hypercore will be broadcast to interested readers. The owner gives each reader a reading capability (core.key) and a corresponding discovery key (core.discoveryKey). The former is used to authorize the reader, ensuring that they have permission to read messages, and the latter is used to discover the owner (and other readers) on the swarm.

The following example consists of two Pear Terminal Applications: reader-app and writer-app. When these two applications are opened, two peers are created and connected to each other. A Hypercore is used to store the data entered into the command line.

The writer-app code stores the data entered into the command line to the Hypercore instance. The Hypercore instance is replicated with other peers using Hyperswarm.

Create the writer-app project with these commands:

mkdir writer-app
cd writer-app
pear init -y -t terminal

Alter the generated writer-app/index.js to the following:

import path from 'path'
import Hyperswarm from 'hyperswarm'
import Hypercore from 'hypercore'
import b4a from 'b4a'

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

const core = new Hypercore(path.join(Pear.config.storage, 'writer-storage'))

// core.key and core.discoveryKey will only be set after core.ready resolves
await core.ready()
console.log('hypercore key:', b4a.toString(core.key, 'hex'))

// Append all stdin data as separate blocks to the core
Pear.stdio.in.on('data', (data) => core.append(data))

// core.discoveryKey is *not* a read capability for the core
// It's only used to discover other peers who *might* have the core
swarm.join(core.discoveryKey)
swarm.on('connection', conn => core.replicate(conn))

The reader-app uses Hyperswarm to connect to the previously initiated peer and synchronize the local Hypercore instance with the Hypercore instance of the writer.

Create the reader-app project with these commands:

mkdir reader-app
cd reader-app
pear init -y -t terminal

Alter the generated reader-app/index.js to the following:

import Hyperswarm from 'hyperswarm'
import Hypercore from 'hypercore'
import goodbye from 'graceful-goodbye'

const swarm = new Hyperswarm()
goodbye(() => swarm.destroy())

const core = new Hypercore('./reader-storage', process.argv[2])
await core.ready()

const foundPeers = core.findingPeers()
swarm.join(core.discoveryKey)
swarm.on('connection', conn => core.replicate(conn))

// swarm.flush() will wait until *all* discoverable peers have been connected to
// It might take a while, so don't await it
// Instead, use core.findingPeers() to mark when the discovery process is completed
swarm.flush().then(() => foundPeers())

// This won't resolve until either
//    a) the first peer is found
// or b) no peers could be found
await core.update()

let position = core.length
console.log(`Skipping ${core.length} earlier blocks...`)
for await (const block of core.createReadStream({ start: core.length, live: true })) {
  console.log(`Block ${position++}: ${block}`)
}

There should now be two folders, writer-app and reader-app.

In one terminal run:

pear dev ./writer-app

If the writer-app is not in the current dir, adjust the path to point to writer-app.

In another terminal run

pear dev ./reader-app

Again if the reader-app is not in the current dir, adjust the path to point to reader-app.

As inputs are made to the terminal running the writer application, outputs should be shown in the terminal running the reader application.