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level-updown

A LevelDOWN backed by LevelUP so LevelUP can use LevelUP through LevelDOWN

Simple and highly extensible to support creative database hackery

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Why?

LevelDOWN is a "backend" for LevelUP supporting LevelDB. It's also a well-defined API through AbstractLevelDOWN which can be implemented by many storage systems that allow for the basic primitives required by LevelUP.

LevelUP is a storage system that supports all of those primitives so implementing a LevelDOWN that is backed by LevelUP is fairly simple. Why? Because LevelDOWN is a simpler and cleaner API and being able to use it as an extensibility point opens up some creative possibilities that are otherwise difficult when trying to extend LevelUP directly.

Because level-updown builds on AbstractLevelDOWN, it already has checks and fixes for predictable argument types so you need to do significantly less argument checking than if you were extending LevelUP. This includes the assumption that entries passing through are only going to be String or Buffer objects.

level-updown uses externr to expose a large number of potential extension points. Each point can be extended by injecting a wrapper function that can either inspect or adjust the call flow and the arguments that move around. What's more, many different types of extensions can operate on the same instance of level-updown at the same time by simply plugging in their extension functions to the chain. Alternatively, you can layer level-updown by passing an instance of it to LevelUP and passing that new LevelUP into a new instance of level-updown and so on! The possibilities for crazy people are endless.

LevelCEPTION:

------------------------------------------------
| LevelDOWN / level.js / MemDOWN / LMDB / etc. |
------------------------------------------------
                       |
                  -----------
                  | LevelUP |
                  -----------
                       |
                ----------------  -------------
                | level-updown |--| extension |
                ----------------  -------------
                       |
                  -----------
                  | LevelUP |
                  -----------
                       |
 -------------  ----------------  -------------
 | extension |--| level-updown |--| extension |
 -------------  ----------------  -------------
                       |
                  -----------
                  | LevelUP |
                  -----------

Examples

Let's make a level-uppercase that upper-cases all values put into the database.

We only need to intercept the put() and batch() calls to make this work.

var levelup = require('levelup')
  , updown  = require('level-updown')

module.exports = function uppercase (db) {
  return levelup({ db: function () {
    var ud = updown(db)

    ud.extendWith({
        prePut   : prePut
      , preBatch : preBatch
    })

    return ud
  }})
}

function prePut (key, value, options, callback, next) {
  next(key, value.toString().toUpperCase(), options, callback)
}

function preBatch (array, options, callback, next) {
  for (var i = 0; i < array.length; i++) {
    if (array[i].type == 'put')
      array[i].value = array[i].value.toString().toUpperCase()
  }

  next(array, options, callback)
}

Now use it:

var levelup   = require('levelup')
  , uppercase = require('./level-uppercase')

var db   = levelup('uc.db')
  , ucdb = uppercase(db)

// one database, two complete LevelUP "views" to it

db.put('db foo', 'db bar')
ucdb.put('ucdb foo', 'ucdb bar')

// check the contents

db.createReadStream().on('data', console.log)

Gives:

{ key: 'db foo', value: 'db bar' }
{ key: 'ucdb foo', value: 'UCDB BAR' }

No monkey patching!

API

LevelUPDOWN(levelup)

The object exposed on exports is LevelUPDOWN which can be instantiated with new or just by calling it. It requires a levelup argument that is a proper (or fully compatible) LevelUP object. In return you get a fully API compliant LevelDOWN instance backed by your LevelUP.

Extensible using externr, see below.

LevelUPDOWNIterator(updown, [, options])

Exposed on exports as LevelUPDOWNIterator in case you need direct access to the prototype. Normally instantiated by a call to db.iterator() and is a fully compliant LevelDOWN iterator.

Extensible using externr, see below.

factory()

Exposed on exports as factory, a simple utility function if you need something to pass to a new LevelUP constructor as the 'db' property. However, when extending level-updown you would normally create your own factory functions or just instantiate new LevelUP instances yourself.

LevelUPDOWN#extendWith(extensionMap)

The extendWith() method on each level-updown instance is the externr extension injection mechanism. You must pass an Object to it where the properties are the keys of the extension points you wish to use. The extensions take the form of functions with a specific list of arguments for each extension point.

LevelUPDOWN extension points

Each of the extension points below can be passed in to LevelUPDOWN#extendWith() as a property by that name where the value is a function whose signature matches the ones presented below. Each extension function below is presented as a noop version that you can use as a starting-point for creating extensions.preIterator

You can modify and replace any of the arguments before calling the next() function in the asynchronous extension points, you can even opt to not call next() or perhaps call callback directly (the callback function supplied originally to LevelUPDOWN). Common operations would include modifying or replacing keys, values and options before continuing on with the call chain via next(). You must pass the correct number and type of arguments to next() for the asynchronous extension functions.

The last two extension functions below, 'preIterator' and 'postIterator' are synchronous functions, and therefore have a single argument. You must return an argument to match, either a modified version of the original or a new one in its place.

'open'

The open() method in level-updown is a process.nextTick() noop by default. This extension function is executed prior to executing the internal noop.

Noop form:

function open (options, callback, next) {
  next(options, callback)
}

'close'

The close() method in level-updown is a process.nextTick() noop by default. This extension function is executed prior to executing the internal noop.

Noop form:

function close (callback, next) {
  next(callback)
}

'prePut'

This extension function is called as the first step in the put() method.

Noop form:

function prePut (key, value, options, callback, next) {
  next(key, value, options, callback)
}

'postPut'

This extension function is called after the internal LevelUP put() has been executed and we have a possible err object. It is called just prior to calling the user-supplied callback() with the err argument.

Noop form:

function postPut (key, value, options, err, callback, next) {
  next(key, value, options, err, callback)
}

'preGet'

This extension function is called as the first step in the get() method.

Noop form:

function preGet (key, options, callback, next) {
  next(key, options, callback)
}

'postGet'

This extension function is called after the internal LevelUP get() has been executed and we have a possible err object or a value. It is called just prior to calling the user-supplied callback() with the err and value arguments.

Noop form:

function postGet (key, options, err, value, callback, next) {
  next(key, options, err, value, callback)
}

'preDel'

This extension function is called as the first step in the del() method.

Noop form:

function preDel (key, options, callback, next) {
  next(key, options, callback)
}

'postDel'

This extension function is called after the internal LevelUP del() has been executed and we have a possible err object. It is called just prior to calling the user-supplied callback() with the err argument.

Noop form:

function postDel (key, options, err, callback, next) {
  next(key, options, err, callback)
}

'preBatch'

This extension function is called as the first step in the batch() method. It should be safe to assume that the array object is an Array and has appropriate entries in it.

Also note that level-updown does not implement a custom form of the chained-batch supported by LevelUP and LevelDOWN. Therefore it uses the default chained-batch implementation by AbstractLevelDOWN which simply collects an array of operations and passes them to batch(), so we only have to worry about this extension point for both array and chained forms of batch().

Noop form:

function preBatch (array, options, callback, next) {
  next(array, options, callback, callback)
}

'postBatch'

This extension function is called after the internal LevelUP batch() has been executed and we have a possible err object. It is called just prior to calling the user-supplied callback() with the err argument.

Noop form:

function postBatch (array, options, err, callback, next) {
  next(array, options, err, callback)
}

'preIterator'

It is important that iterator-creation happen within the same tick as the call to iterator() (and createReadStream() in LevelUP) so that a LevelDB snapshot can be made for the iterator straight away. The synchronous form of iterator() is the only supported means of creating an iterator from LevelDOWN for this reason (and because it doesn't involve any I/O).

Therefore, the extension functions for 'preIterator' and 'postIterator' take the form of function (arg) { return arg }. You are given an argument and you can either modify or replace it before returning it and this cannot occur asynchronously.

'preIterator' is called prior to creating a new LevelUPDOWNIterator object so you are given the opportunity to modify or replace the options, or replace the iterator-creation process and return something completely different.

It is important to note that the LevelUP createReadStream() mechanism is bypassed completely and a new iterator is created with levelup.db.iterator(), i.e. by calling iterator() on the LevelDOWN object being used by the LevelUP instance you are using. This should be safe assuming that the LevelDOWN in use is an AbstractLevelDOWN implementing and/or tested version.

You are provided with an object with two properties, a 'options' property containing the original options object and an 'iteratorFactory' function that takes an options argument that will be called internally to create a new iterator. The default 'iteratorFactory' creates a new LevelUPDOWNIterator and calls the 'preIterator' extension function(s) (below).

Noop form:

function preIterator (pre) {
  // `pre` contains:
  //  - pre.options: options argument passed to the new iterator
  //  - pre.iteratorFactory: a function used to create a new iterator
  return pre
}

'postIterator'

(See note above about the reasons for this extension function being synchronous)

This extension function is called just prior to a new LevelUPDOWNIterator (or other) iterator being created. It is passed the new iterator object and the returned value is then passed back to the caller of the original iterator() call.

A common use for this function is to have the opportunity to extend the new iterator with iterator.extendWith() (see below) prior to handing it off. Because each iterator instance is new for each call to iterator() (or createReadStream() in LevelUP) you must extend each one as it is created.

Noop form:

function postIterator (iterator) {
  return iterator
}

LevelUPDOWNIterator#extendWith(extensionMap)

The extendWith() method on each level-updown iterator instance is the externr extension injection mechanism. You must pass an Object to it where the properties are the keys of the extension points you wish to use. The extensions take the form of functions with a specific list of arguments for each extension point.

LevelUPDOWNIterator extension points

'preNext'

This extension function is called as the first step in the next() method.

Noop form:

function preNext (callback, next) {
  next(callback)
}

'postNext'

This extension function is called after the internal next() has been executed and we have possible err, key and value objects. It is called just prior to calling the user-supplied callback() with the err, key and value arguments.

Note that null values err, key and value are used by LevelDOWN to indicate the end of the iterator.

Noop form:

function postNext (err, key, value, callback, next) {
  next(err, key, value, callback)
}

'preEnd'

This extension function is called as the first step in the end() method.

Noop form:

function preEnd (callback, next) {
  next(callback)
}

'postEnd'

This extension function is called after the internal end() has been executed and we have a possible err object. It is called just prior to calling the user-supplied callback() with the err argument.

Noop form:

function postEnd (err, callback, next) {
  next(err, callback)
}

License

level-updown is Copyright (c) 2014 Rod Vagg @rvagg and licensed under the MIT licence. All rights not explicitly granted in the MIT license are reserved. See the included LICENSE.md file for more details.