registry.js

'use strict'

var dnsEqual = require('dns-equal')
var flatten = require('array-flatten')
var Service = require('./service')

var REANNOUNCE_MAX_MS = 60 * 60 * 1000
var REANNOUNCE_FACTOR = 3

module.exports = Registry

function Registry (server) {
  this._server = server
  this._services = []
}

Registry.prototype.publish = function (opts) {
  var service = new Service(opts)
  service.start = start.bind(null, this, service)
  service.stop = stop.bind(null, this, service)
  service.start({ probe: opts.probe !== false })
  return service
}

Registry.prototype.unpublishAll = function (cb) {
  teardown(this._server, this._services, cb)
  this._services = []
}

Registry.prototype.destroy = function () {
  this._services.forEach(function (service) {
    service._destroyed = true
  })
}

function start (registry, service, opts) {
  if (service._activated) return
  service._activated = true

  registry._services.push(service)

  if (opts.probe) {
    probe(registry._server.mdns, service, function (exists) {
      if (exists) {
        service.stop()
        service.emit('error', new Error('Service name is already in use on the network'))
        return
      }
      announce(registry._server, service)
    })
  } else {
    announce(registry._server, service)
  }
}

function stop (registry, service, cb) {
  if (!service._activated) return // TODO: What about the callback?

  teardown(registry._server, service, cb)

  var index = registry._services.indexOf(service)
  if (index !== -1) registry._services.splice(index, 1)
}

/**
 * Check if a service name is already in use on the network.
 *
 * Used before announcing the new service.
 *
 * To guard against race conditions where multiple services are started
 * simultaneously on the network, wait a random amount of time (between
 * 0 and 250 ms) before probing.
 *
 * TODO: Add support for Simultaneous Probe Tiebreaking:
 * https://tools.ietf.org/html/rfc6762#section-8.2
 */
function probe (mdns, service, cb) {
  var sent = false
  var retries = 0
  var timer

  mdns.on('response', onresponse)
  setTimeout(send, Math.random() * 250)

  function send () {
    // abort if the service have or is being stopped in the meantime
    if (!service._activated || service._destroyed) return

    mdns.query(service.fqdn, 'ANY', function () {
      // This function will optionally be called with an error object. We'll
      // just silently ignore it and retry as we normally would
      sent = true
      timer = setTimeout(++retries < 3 ? send : done, 250)
      timer.unref()
    })
  }

  function onresponse (packet) {
    // Apparently conflicting Multicast DNS responses received *before*
    // the first probe packet is sent MUST be silently ignored (see
    // discussion of stale probe packets in RFC 6762 Section 8.2,
    // "Simultaneous Probe Tiebreaking" at
    // https://tools.ietf.org/html/rfc6762#section-8.2
    if (!sent) return

    if (packet.answers.some(matchRR) || packet.additionals.some(matchRR)) done(true)
  }

  function matchRR (rr) {
    return dnsEqual(rr.name, service.fqdn)
  }

  function done (exists) {
    mdns.removeListener('response', onresponse)
    clearTimeout(timer)
    cb(!!exists)
  }
}

/**
 * Initial service announcement
 *
 * Used to announce new services when they are first registered.
 *
 * Broadcasts right away, then after 3 seconds, 9 seconds, 27 seconds,
 * and so on, up to a maximum interval of one hour.
 */
function announce (server, service) {
  var delay = 1000
  var packet = service._records()

  server.register(packet)

  ;(function broadcast () {
    // abort if the service have or is being stopped in the meantime
    if (!service._activated || service._destroyed) return

    server.mdns.respond(packet, function () {
      // This function will optionally be called with an error object. We'll
      // just silently ignore it and retry as we normally would
      if (!service.published) {
        service._activated = true
        service.published = true
        service.emit('up')
      }
      delay = delay * REANNOUNCE_FACTOR
      if (delay < REANNOUNCE_MAX_MS && !service._destroyed) {
        setTimeout(broadcast, delay).unref()
      }
    })
  })()
}

/**
 * Stop the given services
 *
 * Besides removing a service from the mDNS registry, a "goodbye"
 * message is sent for each service to let the network know about the
 * shutdown.
 */
function teardown (server, services, cb) {
  if (!Array.isArray(services)) services = [services]

  services = services.filter(function (service) {
    return service._activated // ignore services not currently starting or started
  })

  var records = flatten.depth(services.map(function (service) {
    service._activated = false
    var records = service._records()
    records.forEach(function (record) {
      record.ttl = 0 // prepare goodbye message
    })
    return records
  }), 1)

  if (records.length === 0) return cb && cb()

  server.unregister(records)

  // send goodbye message
  server.mdns.respond(records, function () {
    services.forEach(function (service) {
      service.published = false
    })
    if (cb) cb.apply(null, arguments)
  })
}