an approach for incremental ids using levelup.

level-incremental-id.js

var levelup = require("levelup")
var path    = __dirname + "db"
var options = {encoding: "json"}
var db = levelup(path, options)

function id(cb) {
  if (typeof cb != "function") throw new TypeError

  if (!id.queue) {
    id.queue = []
    setImmediate(run)
  }

  id.queue.push(cb)
}

function run() {
  var cb = id.queue.shift()

  if (!cb) return delete id.queue
    
  db.get("last_id", function(err, id) {
    if (err) {
      if (err.name == "NotFoundError") id = 0
      else return cb(err)
    }

    id++

    db.put("last_id", id, function(err) {
      if (err) cb(err)
      else cb(null, id)

      run()
    })
  })
}

module.exports = id