node package manager



Replicate couchdb data into leveldb in real time with follow. Must be used with sublevel.


The following example illustrates the simplest use case. It will synchronize couchdb data into a leveldb located at /tmp/level-npm and store the data as (key, value) = (, JSON.stringify(data.doc)), where data is JSON chunks received from the couch.

var levelCouchSync = require('level-couch-sync')
var levelup = require('levelup')
var sublevel = require('level-sublevel')
var db = sublevel(levelup('/tmp/level-npm'))
levelCouchSync('', db, 'registry-sync')

If you provide a map/iterator function you can decide for yourself what kind of data your want to persist. An easy way to accomplish this, is to create more sublevels and shove data into them. This example shows how you can store basic package metadata in a sublevel named 'package':

var levelCouchSync = require('level-couch-sync')
var levelup = require('levelup')
var sublevel = require('level-sublevel')
var db = sublevel(levelup('/tmp/level-npm-advanced'))
var packageDb = db.sublevel('package')
levelCouchSync('', db, 'registry-sync',
    function (data, emit) {
      var doc = data.doc
      emit(, JSON.stringify({
        name        :,
        author      :,
        repository  : doc.repository
      }), packageDb)

Each emit() call adds a (key, value, sublevel) triplet to a batch operation that is executed once the iterator returns, which means you can call emit() many times during each time the iterator is invoked.

levelCouchSync() returns an EventEmitter, which you can attach listeners to. The following example logs package versions and progress to stdout. See the events section for more details.

// .. 
var sync = levelCouchSync(url, db, 'registry-sync')
sync.on('data', function (data) {
  console.log(, data.doc.versions && Object.keys(data.doc.versions))
sync.on('progress', function (ratio) {
  console.log(Math.floor(ratio*10000)/100 + '%')

Run the samples in the example/ folder and try it out! It should work on all systems where levelup can be compiled. If you want to take a closer look at what the data looks like you can use lev, which is an awesome cli tool for viewing any leveldb. All you need is a path to it.


The API is very simple and only contain one function.

require('level-couch-sync')(sourceUrl, db, metaDb[, map])

This function returns an EventEmitter instance and has three mandatory arguments and one optional.

  • sourceUrl is a string pointing out the url to the couch we are getting the updates from
  • db must be a level-sublevel instance and is used to store the data if there is no map iterator provided
  • metaDb must be a level-sublevel instance or a string. If it's a string, a sublevel will be created with that name. metaDb handles metadata of the ongoing transfer and keeps track of the update_seq, which means that if the process crashes, it will automatically continue where it left off
  • map(data, emit) is an iterator function called for each JSON data received from the couch. The first argument data is the JSON received from the couch. emit(key, value, sublevel) is a function you call each time you want to persist some data. It takes the following three arguments:
    • key is a string and is the key used to store the value
    • value is an object that you are free to build as you please
    • sublevel is a level-sublevel instance used to store the key and the value


level-couch-sync emits various events as the leveldb is syncronized with the couch:

  • sync.emit('data', data) emitted for each data object received from the couch
  • sync.emit('progress', ratio) emitted each time data has been written to levelup. The ratio is defined as how much data that has been written from the current update sequence. When there is something to be read from the couch then 0 < ratio < 1.0 and when ratio > 1.0 it means we are syncing live!
  • sync.emit('fail', err) emitted when there is an error fetching the sourceUrl from couchdb before the request will be tried again using fibonacci backoff
  • sync.emit('max', maxSeq) emitted when a request has been made to the source url. maxSeq is the value of the update_seq property in the JSON response

progress bar

you can create a progress bar like used in npmd, just provide a name for the couchdb, and a function that returns a tagline describing the document that was updated.

sync.createProgressBar(name, function (data) {
  return toTagline(data)

by default, name is the url of the couchdb instance, and the tagline will be doc._id+'@'+doc._rev