Abstract LevelDOWN

An abstract prototype matching the LevelDOWN API. Useful for extending LevelUP functionality by providing a replacement to LevelDOWN.

Abstract LevelDOWN provides a simple, operational noop base prototype that's ready for extending. By default, all operations have sensible "noops" (operations that essentially do nothing). For example, simple operations such as .open(callback) and .close(callback) will simply invoke the callback (on a next tick). More complex operations perform sensible actions, for example: .get(key, callback) will always return a 'NotFound' Error on the callback.

You add functionality by implementing the underscore versions of the operations. For example, to implement a put() operation you add a _put() method to your object. Each of these underscore methods override the default noop operations and are always provided with consistent arguments, regardless of what is passed in by the client.

Additionally, all methods provide argument checking and sensible defaults for optional arguments. All bad-argument errors are compatible with LevelDOWN (they pass the LevelDOWN method arguments tests). For example, if you call .open() without a callback argument you'll get an Error('open() requires a callback argument'). Where optional arguments are involved, your underscore methods will receive sensible defaults. A .get(key, callback) will pass through to a ._get(key, options, callback) where the options argument is an empty object.

Example

A simplistic in-memory LevelDOWN replacement

var util = require('util')
  , AbstractLevelDOWN = require('./').AbstractLevelDOWN

// constructor, passes through the 'location' argument to the AbstractLevelDOWN constructor
function FakeLevelDOWN (location) {
  AbstractLevelDOWN.call(this, location)
}

// our new prototype inherits from AbstractLevelDOWN
util.inherits(FakeLevelDOWN, AbstractLevelDOWN)

// implement some methods

FakeLevelDOWN.prototype._open = function (options, callback) {
  // initialise a memory storage object
  this._store = {}
  // optional use of nextTick to be a nice async citizen
  process.nextTick(function () { callback(null, this) }.bind(this))
}

FakeLevelDOWN.prototype._put = function (key, value, options, callback) {
  key = '$' + key // safety, to avoid key='__proto__'-type skullduggery 
  this._store[key] = value
  process.nextTick(callback)
}

FakeLevelDOWN.prototype._get = function (key, options, callback) {
  var value = this._store['$' + key]
  if (value === undefined) {
    // 'NotFound' error, consistent with LevelDOWN API
    return process.nextTick(function () { callback(new Error('NotFound')) })
  }
  process.nextTick(function () {
    callback(null, value)
  })
}

FakeLevelDOWN.prototype._del = function (key, options, callback) {
  delete this._store['$' + key]
  process.nextTick(callback)
}

// now use it in LevelUP

var levelup = require('levelup')

var db = levelup('/who/cares/', {
  // the 'db' option replaces LevelDOWN
  db: function (location) { return new FakeLevelDOWN(location) }
})

db.put('foo', 'bar', function (err) {
  if (err) throw err
  db.get('foo', function (err, value) {
    if (err) throw err
    console.log('Got foo =', value)
  })
})

Extensible API

Remember that each of these methods, if you implement them, will receive exactly the number and order of arguments described. Optional arguments will be converted to sensible defaults.

AbstractLevelDOWN(location)

AbstractLevelDOWN#_open(options, callback)

AbstractLevelDOWN#_close(callback)

AbstractLevelDOWN#_get(key, options, callback)

AbstractLevelDOWN#_put(key, value, options, callback)

AbstractLevelDOWN#_del(key, options, callback)

AbstractLevelDOWN#_batch(array, options, callback)

Note: At the time of writing, the LevelDOWN batch() API is in flux, see the 2.0-wip branch. If batch() is called without argument or with only an options object then it should return a Batch object with chainable methods. Otherwise it will invoke a classic batch operation.

AbstractLevelDOWN#_approximateSize(start, end, callback)

AbstractLevelDOWN#_iterator(options)

By default an iterator() operation returns a blank AbstractIterator object. The prototype is available on the main exports for you to extend. If you want to implement iterator operations then you should extend the AbstractIterator and return your object in the _iterator(options) method.

AbstractIterator implements the basic state management found in LevelDOWN. It keeps track of when a next() is in progress and when an end() has been called so it doesn't allow concurrent next() calls, it does it allow end() while a next() is in progress and it doesn't allow either next() or end() after end() has been called.

AbstractIterator(db)

Provided with the current instance of AbstractLevelDOWN by default.

AbstractIterator#next(callback)

AbstractIterator#end(callback)

Licence

Abstract LevelDOWN is Copyright (c) 2013 Rod Vagg @rvagg and licensed under the MIT licence. All rights not explicitly granted in the MIT license are reserved. See the included LICENSE file for more details.