8: Multiple peers in one app!

In review so far we know how to:

1. create a secure append-only log, that a single user can write to
2. read messages from this hypercore
3. replicate one of these hypercores to another, remote hypercore

There is a major gap in our code though if we'd like to make a chat program: with hypercore, there can only be one writer!

Well, what if each user had their own hypercore? Then we could view their contents together to see all of the chat history as other people are writing.

This was the motivation behind multifeed, a module that lets you operate on a set of hypercores, and create a replication stream, in the same way that you replicated hypercores in the last exercise.

Exercise

Make a copy of single-chat.js called multichat.js, and replace usage of hypercore with multifeed. Here is how to create a multifeed, a new writer, and write to that feed:

const hypercore = require('hypercore')
const multifeed = require('multifeed')

const multi = multifeed('./multichat', {valueEncoding:'json'})

multi.writer('local', function (err, feed) {
  // TODO: join swarm

  // 'feed' is a hypercore, just like before
  // except now we can manage many of them together!
  feed.append({
    type: 'chat-message',
    nickname: 'cat-lover',
    text: 'hello world',
    timestamp: '2018-11-05T14:26:000Z' // new Date().toISOString()
  }, function (err, seq) {
    if (err) throw err
    console.log('Data was appended as entry #' + seq)
  })
})

See if you can also add peer discovery and replication to this. Just like hypercore, multifeed has a multi.replicate() API that lets two peers sync their data over a duplex stream. Look back at how you set you peer discovery for hypercore for ideas on how to do this.

Tricky bits

• replication will need to use multi.replicate instead of the old feed.replicate: multifeed will handle replicating all feeds for you.
• wait for multi.writer's callback to be called before starting the hyperswarm (it needs all feeds to be set up before replicating)
• choose a new, arbitrary key for swarming:

const topic = crypto.createHash('sha256')
            .update('foobar-123')
            .digest()

swarm.join(topic)

• if running node multichat.js in two separate windows, each process must be using a different database. A good trick is to do something like this during multifeed creation:

const suffix = process.argv[2]

const multi = multifeed('./multichat-' + suffix, { valueEncoding:'json' })

This way, you can run node multichat.js 1 in one window, and node multichat.js 2 in the other window. If you try to run two processes that open the same multifeed database, things won't work!

Tips

• It's important to use a different database name -multichat, because hypercore and multifeed databases are not compatible.
• Look at the multifeed README example. There is an API called writer(name, cb) that lets you create a new hypercore inside the multifeed.

Once you solve this exercise continue to exercise 9