replic8

kappa-db/replic8

Replication manager for hypercore & hypercoreprotocol compatible data-structures.

API Poposal 0.7.0

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This is an working alpha, feedback and testing is highly appreciated!

• Dynamic feed exchange (live + oneshot)
• Track peer-connections and feeds
• Implement middleware interface
• Realtime feed forwards
• Provide backwards compatibility with multifeed (patch available!)
• Provide corestore support through middleware wrapper
• Solve expectedFeeds issue to renable composite-core support. (substreams!)
• Test and document unshare operation
• Update API-docs outdated!
• Provide connection statistics (transfer rate / transfered bytes / latency)
• Expose peer-substreams through API to applications/middleware.

Installation

A pre-release is now available on npm.

npm i replic8

Usage

const middleware1 = require('...')
const replic8 = require('replic8')
const hyperswarm = require('hyperswarm')
 
// Communication and exchange encryption
const swarmKey = Buffer.alloc(32)
swarmKey.write('passwords and secrets')
 
// Initialize a new manger
const mgr = replic8(swarmKey, { live: true })
 
mgr.on('error', console.error) // Subscribe to all errors
 
// register some filters or decorators
mgr.use(middleware1)
 
// lastly register your core storage
mgr.use(aCoreStorage)
 
const swarm = hyperswarm.join('hash the swarmkey')
swarm.on('connection', mgr.handleConnection)

Middleware Interface

Is up to date v0.7.0 !

All callbacks are optional, a middleware can for instance implement only the describe callback.

const app = {
 
  // Share available cores
  share (next) {
    next(null, [feed1, feed2, key4]) // Accepts cores or keys (buffer/hexstring)
  },
 
  // Attach custom meta-data that will be transmitted
  // during core exchange
  describe({ key, meta, resolve }, next) {
 
    // resolve provides the feed if your middleware requires it.
    resolve((err, feed) => {
      if (err) return next(err) // recover from middleware errors
 
      next(null, { length: feed.length, timestamp: new Date() })
    })
  },
 
  // Custom application logic to filter what to accept.
  accept({ key, meta, resolve }, next) {
    const select = meta.type === 'hyperdrive'
    next(null, select)
  },
 
  // provide core instance via key to other
  // middleware and replication
  resolve(key, next) {
    const feed = findFeedByKeySomehow(key)
    next(null, feed)
  },
 
  // hook that will be invoked when
  // this middleware gets appended to a replication stack
  mounted(manager, namespace) {
    // exposes possiblity to attach
    // internal/nested middleware
    manager.use(namespace, this.multifeed)
    manager.use(namespace, MyStaleFeedsFilter)
 
    // Initiate a side-channel replicating bulk resources
    manager.use(namespace + '/attachments', this.drivesMultifeed)
    manager.use(namespace + '/attachments', require('./examples/type-decorator'))
  },
 
  // Invoked when replication manager is closing
  close () {
    this.multifeed.close()
  }
}
 
mgr.use(app)

Examples

Replication filter

// Given an application that decorates announcement with `lastActivity` timestamp
// Filter stale feeds from replication.
const aWeek = 60*60*24*7*1000
const timeFilter = {
  accept ({key, meta}, next) {
    if (new Date().getTime() - meta.lastActivity < aWeek) {
      next(null, key)
    } else {
      next()
    }
  }
}
 
mgr.use(timeFilter)

More examples

• Array storage
• CoreType decorator
• corestore-replic8 adapter

Backwards compatibility

Replic8 is my continued work from multifeed's internal replication management.

Any application currently using multifeed should have access to the middleware api.

// Multifeed accept an external replic8 instance
const multi = multifeed(ram, aKey, { replicate: mgr})
 
// -- or --
 
const multi = multifeed(ram, aKey)
multi.use(mgr)
 
// Multifeed initializes a new replic8 instance internally if no
// replication manager is present when multi.replicate() or multi.use() is invoked.

A note on stack-order

TLDR;

resolve and accept = First to Last

share and decorate = Last to First

Middleware traversal order depends on the direction of communication.

When sending data from local to remote, middleware stack is traversed in LIFO order.

And when receiving data from remote to local, middleware stack is traversed in FIFO order.

This is to make it easier writing useful middleware, Filters should have their share invoked last to process a complete list of locally available feeds, and should receive first priority on accept.

Stores should have their share invoked first since they provide the lists of available feeds, and their accept last so that any feeds that reach it must have passed the filters, also they must honor the rule:

``last accept callback in the stack instantiates the feed locally if desired and missing.''

      ( TRANSMITTER )                                ( RECEIVER )
    -----------------------> ---------------> -------------> ------->  FIFO
   ^   _____________                                 _____________   |
0  |  [ FilterA     ]  <- DESCRIBE  |    ACCEPT ->  [ Filter B    ]  |  0
   |   -------------                |                -------------   v
1  |  [ Application ]  <- (any)     |    ACCEPT ->  [ Filter A    ]  |  1
   ^   -------------                                 -------------   |
2  |  [ Decorator   ]  <- DESCRIBE  |    ACCEPT ->  [ Store       ]  |  2
   |   -------------                |                -------------   |
3  |  [ Store       ]  <- SHARE     |                                v
   |   -------------
  LIFO

API

const mgr = replic8(encryptionKey, opts)

encryptionKey pre-shared-key Buffer(32), used for exchange & meta message encryption opts hypercore-protocol opts

opts.noforward the manager keeps track of which keys have been exchanged to which peers, if a new key is encountered then by default the manager initiates a new announce exchange with all active peers that have not been offered that key yet. This flags turns off that behaviour.

mgr.use(namespace, middleware)

Assembles an application stack where each middleware will be invoked in order of registration.

namespace (optional) creates a virtual sub-exchange channel that helps prevent a core ending up in the wrong store or being instantiated with wrong class.

middleware should be an object that optionally implements methods:

share, describe, accept, resolve, mounted, close

middleware share: function(next)

Share a list of cores: next(null, [...])

middleware describe: function(context, next)

TODO: inaccurate

Invoked during connection initialization directly after a successful handshake.

const { key, meta, resolve } = context

share(key, headers) - function, takes two arrays, where keys is required to contain only feed-keys and headers is expected to contain serializable Objects. The length of both arrays is expected to be equal.

middleware accept: function(context, next)

Invoked when remote end has advertised a list of cores

// Reject/filter a core
next(null, false)
 
// Let a core pass through to next middleware
next()
 
// Accept core by returning an instance (ends stack traversal)
const core = hypercore(storage, context.key)
core.ready(() => {
  next(null, core)
})

middleware resolve: function(key, next)

key - hex-string

next - Function function(err, core)

If middleware.resolve callback is present, it will be invoked right before replication starts. It expects you to map any of the requested keys to cores and then invoke the next function either with an error or with an array of cores "Objects that respond to key and replicate()"

If a key has not been resolved by the time all middleware in the stack has been queried. An error event containing a UnresolvedCoreError will be emitted on the manager instance and the peer-connection will be dropped.⁴

middleware mounted: function(manager, namespace)

Invoked when middleware is added to stack. Can be used to initialize and add additional middleware.

middleware close: function()

Invoked when replication manager is closing

mgr.connections

List of active PeerConnections

mgr.middleware

The current middleware stack

mgr.key

Exchange channel encryption key

mgr.replicate(opts)

Creates a PeerConnection returns it's stream (compatibility)

returns stream

mgr.handleConnection(stream)

The preffered way to add peer-connections to the manager as opposite to mgr.replicate().

returns PeerConnection

mgr.close([err,] cb)

Closes all active connections and invokes close on all middleware.

event 'connected', PeerConnection

Emitted when a new peer connection is added to manager

event 'disconnect', err, PeerConnection

Emitted whenever a peer connection is dropped

event 'error'

PeerConnection

getter conn.state

returns current connection state: init|active|dead

There's alot missing from this section, please see source

License

This project is based on the research from kappa-db and is licensed accordingly under ISC.

---

^1. exchange swarm - A forum where public keys are published and exchanged - as opposite to a resource swarm where only pre-shared keys are trusted and replicated.

^2. This pattern comes with a drawback that it potentially reduces the availability & amount of peers on the conventional resource.discoveryKey topic.

^3. opts object already supports {extensions: []} key. I'm unsure if it's worth designing yet another layer of abstraction on top of that. multifeed/replic8 is in itself built on extension, maybe it's worth reducing the extension use to a single exchange type message that we can apply for a dat-dep recognition.

^4. The reason for mercilessly disconnecting a peer if a core is not resolved is because once announce and select messages have been exchanged both peers are trusted to replicate the negotiated set of cores;

Unless I'm mistaken, if only one of the two peers replicate a core then that peer gets a dangling counter that never gets cleared and the events end and finish will not be fired properly on the stream.

Therefore it's nicer to drop the connection instead of leaving the other peer "hanging".