Hypermerge

Hypermerge is a Node.js library for building p2p collaborative applications without any server infrastructure. It combines Automerge, a CRDT, with hypercore, a distributed append-only log.

If successful this project would provide a way to have apps data sets that are conflict free and offline first (thanks to CRDT's) and serverless (thanks to hypercore/DAT).

While the DAT community has done a lot of work to secure their tool set, zero effort has been made with hypermerge itself to deal with security and privacy concerns. Due to the secure nature of the tools its built upon a properly audited and secure version of this library would be possible in the future.

How it works

Concepts

The base object you make with hypermerge is a Repo. A repo is responsible for managing documents and replicating to peers.

Basic Setup (Serverless or with a Server)

import { Repo } from "hypermerge"

const storage = require("random-access-file")
const path = ".data"

const repo = new Repo({ path, storage })

// DAT's discovery swarm or truly serverless discovery
const DiscoverySwarm = require("discovery-swarm");
const defaults = require('dat-swarm-defaults')
const discovery = new DiscoverySwarm(defaults({stream: repo.stream, id: repo.id }));

repo.replicate(discovery)

Create / Edit / View / Delete a document

  const url = repo.create({ hello: "world" })

  repo.doc<any>(url, (doc) => {
    console.log(doc) // { hello: "world" }
  })

  // this is an automerge change function - see automerge for more info
  // basically you get to treat the state as a plain old javacript object
  // operations you make will be added to an internal append only log and
  // replicated to peers

  repo.change(url, (state:any) => {
    state.foo = "bar"
  })

  repo.doc<any>(url, (doc) => {
    console.log(doc) // { hello: "world", foo: "bar" }
  })

  // to watch a document that changes over time ...
  const handle = repo.watch(url, (doc:any) => {
    console.log(doc)
    if (doc.foo === "bar") {
      handle.close()
    }
  })

NOTE: If you're familiar with Automerge: the change function in Hypermerge is asynchronous, while the Automerge.change function is synchronous. What this means is that although Automerge.change returns an object representing the new state of your document, repo.change (or handle.change) does NOT. So:

// ok in Automerge!
doc1 = Automerge.change(doc1, "msg", (doc) => {
  doc.foo = "bar"
})

// NOT ok in Hypermerge!
doc1 = repo.change(url1, (doc) => {
  doc.foo = "bar"
})

Instead, you should expect to get document state updates via repo.watch (or handle.subscribe) as shown in the example above.

Two repos on different machines

const docUrl = repoA.create({ numbers: [ 2,3,4 ]})
// this will block until the state has replicated to machine B

repoA.watch<MyDoc>(docUrl, state => {
  console.log("RepoA", state)
  // { numbers: [2,3,4] } 
  // { numbers: [2,3,4,5], foo: "bar" }
  // { numbers: [2,3,4,5], foo: "bar" } // (local changes repeat)
  // { numbers: [1,2,3,4,5], foo: "bar", bar: "foo" }
})

repoB.watch<MyDoc>(docUrl, state => {
  console.log("RepoB", state)
  // { numbers: [1,2,3,4,5], foo: "bar", bar: "foo" }
})


repoA.change<MyDoc>(docUrl, (state) => {
  state.numbers.push(5)
  state.foo = "bar"
})

repoB.change<MyDoc>(docUrl, (state) => {
  state.numbers.unshift(1)
  state.bar = "foo"
})

Accessing Files

Hypermerge supports a special kind of core called a hyperfile. Hyperfiles are unchanging, written-once hypercores that store binary data.

Here's a simple example of reading and writing hyperfile buffers.

export function writeBuffer(buffer, callback) {
  const hyperfileUrl = repo.writeFile(buffer, 'application/octet-stream') // TODO: mime type
  callback(null, hyperfileUrl)
}

export function fetch(hyperfileId, callback) {
  repo.readFile(hyperfileId, (error, data) => {
    if (error) {
      callback(error)
      return
    }

    callback(null, data)
  })
}

Note that hyperfiles are conveniently well-suited to treatment as a native protocol for Electron applications. This allows you to refer to them throughout your application directly as though they were regular files for images and other assets without any special treatment. Here's how to register that:

  protocol.registerBufferProtocol('hyperfile', (request, callback) => {
    try {
      Hyperfile.fetch(request.url, (data) => {
        callback({ data })
      })
    } catch (e) {
      log(e)
    }
  }, (error) => {
    if (error) {
      log('Failed to register protocol')
    }
  })

Splitting the Front-end and Back-end

Both Hypermerge and Automerge supports separating the front-end (where materialized documents live and changes are made) from the backend (where CRDT computations are handled as well as networking and compression.) This is useful for maintaining application performance by moving expensive computation operations off of the render thread to another location where they don't block user input.

The communication between front-end and back-end is all done via simple Javascript objects and can be serialized/deserialized through JSON if required.

  // establish a back-end
  const back = new RepoBackend({ storage: raf, path: HYPERMERGE_PATH, port: 0 })
  const discovery = new DiscoverySwarm({ /* your config here */ })
  back.replicate(discovery)

  // elsewhere, create a front-end (you'll probably want to do this in different threads)
  const front = new RepoFrontend()

  // the `subscribe` method sends a message stream, the `receive` receives it
  // for demonstration here we simply output JSON and parse it back in the same location
  // note that front-end and back-end must each subscribe to each other's streams
  back.subscribe((msg) => front.receive(JSON.parse(JSON.stringify(msg))))
  front.subscribe((msg) => back.receive(JSON.parse(JSON.stringify(msg))))
  
}

Note: each back-end only supports a single front-end today.

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