levelup

Superseded by abstract-level. Please see Frequently Asked Questions.

Table of Contents

<details><summary>Click to expand</summary>

• Introduction
• Supported Platforms
• Usage
• API
  • levelup(db[, options[, callback]])
  • db.supports
  • db.open([options][, callback])
  • db.close([callback])
  • db.put(key, value[, options][, callback])
  • db.get(key[, options][, callback])
  • db.getMany(keys[, options][, callback])
  • db.del(key[, options][, callback])
  • db.batch(array[, options][, callback]) (array form)
  • db.batch() (chained form)
  • db.status
  • db.isOperational()
  • db.createReadStream([options])
  • db.createKeyStream([options])
  • db.createValueStream([options])
  • db.iterator([options])
  • db.clear([options][, callback])
• What happened to db.createWriteStream?
• Promise Support
• Events
• Multi-process Access
• Contributing
• Big Thanks
• Donate
• License

</details>

Introduction

Fast and simple storage. A Node.js wrapper for abstract-leveldown compliant stores, which follow the characteristics of LevelDB.

LevelDB is a simple key-value store built by Google. It's used in Google Chrome and many other products. LevelDB supports arbitrary byte arrays as both keys and values, singular get, put and delete operations, batched put and delete, bi-directional iterators and simple compression using the very fast Snappy algorithm.

LevelDB stores entries sorted lexicographically by keys. This makes the streaming interface of levelup - which exposes LevelDB iterators as Readable Streams - a very powerful query mechanism.

The most common store is leveldown which provides a pure C++ binding to LevelDB. Many alternative stores are available such as level.js in the browser or memdown for an in-memory store. They typically support strings and Buffers for both keys and values. For a richer set of data types you can wrap the store with encoding-down.

The level package is the recommended way to get started. It conveniently bundles levelup, leveldown and encoding-down. Its main export is levelup - i.e. you can do var db = require('level').

Supported Platforms

We aim to support Active LTS and Current Node.js releases as well as browsers. For support of the underlying store, please see the respective documentation.

Usage

If you are upgrading: please see UPGRADING.md.

First you need to install levelup! No stores are included so you must also install leveldown (for example).

$ npm install levelup leveldown

All operations are asynchronous. If you do not provide a callback, a Promise is returned.

var levelup = require('levelup')
var leveldown = require('leveldown')

// 1) Create our store
var db = levelup(leveldown('./mydb'))

// 2) Put a key & value
db.put('name', 'levelup', function (err) {
  if (err) return console.log('Ooops!', err) // some kind of I/O error

  // 3) Fetch by key
  db.get('name', function (err, value) {
    if (err) return console.log('Ooops!', err) // likely the key was not found

    // Ta da!
    console.log('name=' + value)
  })
})

API

levelup(db[, options[, callback]])

The main entry point for creating a new levelup instance.

• db must be an abstract-leveldown compliant store.
• options is passed on to the underlying store when opened and is specific to the type of store being used

Calling levelup(db) will also open the underlying store. This is an asynchronous operation which will trigger your callback if you provide one. The callback should take the form function (err, db) {} where db is the levelup instance. If you don't provide a callback, any read & write operations are simply queued internally until the store is fully opened, unless it fails to open, in which case an error event will be emitted.

This leads to two alternative ways of managing a levelup instance:

levelup(leveldown(location), options, function (err, db) {
  if (err) throw err

  db.get('foo', function (err, value) {
    if (err) return console.log('foo does not exist')
    console.log('got foo =', value)
  })
})

Versus the equivalent:

// Will throw if an error occurs
var db = levelup(leveldown(location), options)

db.get('foo', function (err, value) {
  if (err) return console.log('foo does not exist')
  console.log('got foo =', value)
})

db.supports

A read-only manifest. Might be used like so:

if (!db.supports.permanence) {
  throw new Error('Persistent storage is required')
}

if (db.supports.bufferKeys && db.supports.promises) {
  await db.put(Buffer.from('key'), 'value')
}

db.open([options][, callback])

Opens the underlying store. In general you shouldn't need to call this method directly as it's automatically called by levelup(). However, it is possible to reopen the store after it has been closed with close().

If no callback is passed, a promise is returned.

db.close([callback])

close() closes the underlying store. The callback will receive any error encountered during closing as the first argument.

You should always clean up your levelup instance by calling close() when you no longer need it to free up resources. A store cannot be opened by multiple instances of levelup simultaneously.

If no callback is passed, a promise is returned.

db.put(key, value[, options][, callback])

put() is the primary method for inserting data into the store. Both key and value can be of any type as far as levelup is concerned.

options is passed on to the underlying store.

If no callback is passed, a promise is returned.

db.get(key[, options][, callback])

Get a value from the store by key. The key can be of any type. If it doesn't exist in the store then the callback or promise will receive an error. A not-found err object will be of type 'NotFoundError' so you can err.type == 'NotFoundError' or you can perform a truthy test on the property err.notFound.

db.get('foo', function (err, value) {
  if (err) {
    if (err.notFound) {
      // handle a 'NotFoundError' here
      return
    }
    // I/O or other error, pass it up the callback chain
    return callback(err)
  }

  // .. handle `value` here
})

The optional options object is passed on to the underlying store.

If no callback is passed, a promise is returned.

db.getMany(keys[, options][, callback])

Get multiple values from the store by an array of keys. The optional options object is passed on to the underlying store.

The callback function will be called with an Error if the operation failed for any reason. If successful the first argument will be null and the second argument will be an array of values with the same order as keys. If a key was not found, the relevant value will be undefined.

If no callback is provided, a promise is returned.

db.del(key[, options][, callback])

del() is the primary method for removing data from the store.

db.del('foo', function (err) {
  if (err)
    // handle I/O or other error
});

options is passed on to the underlying store.

If no callback is passed, a promise is returned.

db.batch(array[, options][, callback]) (array form)

batch() can be used for very fast bulk-write operations (both put and delete). The array argument should contain a list of operations to be executed sequentially, although as a whole they are performed as an atomic operation inside the underlying store.

Each operation is contained in an object having the following properties: type, key, value, where the type is either 'put' or 'del'. In the case of 'del' the value property is ignored. Any entries with a key of null or undefined will cause an error to be returned on the callback and any type: 'put' entry with a value of null or undefined will return an error.

const ops = [
  { type: 'del', key: 'father' },
  { type: 'put', key: 'name', value: 'Yuri Irsenovich Kim' },
  { type: 'put', key: 'dob', value: '16 February 1941' },
  { type: 'put', key: 'spouse', value: 'Kim Young-sook' },
  { type: 'put', key: 'occupation', value: 'Clown' }
]

db.batch(ops, function (err) {
  if (err) return console.log('Ooops!', err)
  console.log('Great success dear leader!')
})

options is passed on to the underlying store.

If no callback is passed, a promise is returned.

db.batch() (chained form)

batch(), when called with no arguments will return a Batch object which can be used to build, and e