fastify-riotjs-and-lowdb/node_modules/levelup

levelup

Table of Contents

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• Introduction
• Supported Platforms
• Usage
• API
• Promise Support
• Events
• Multi-process Access
• Contributing
• Big Thanks
• Donate
• License

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. If you want to use Promises, you will need a polyfill like pinkie in older browsers like IE.

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.open()
• db.close()
• db.put()
• db.get()
• db.del()
• db.batch() (array form)
• db.batch() (chained form)
• db.isOpen()
• db.isClosed()
• db.createReadStream()
• db.createKeyStream()
• db.createValueStream()
• db.iterator()

Special Notes

• What happened to db.createWriteStream()

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.

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.open([options][, callback])

Opens the underlying store. In general you should never 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(), although this is not generally advised.

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() is the primary method for fetching data from the store. 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
})

options is passed on to the underlying store.

If no callback is passed, 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.

var 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 eventually commit, an atomic batch operation. Depending on how it's used, it is possible to obtain greater performance when using the chained form of batch() over the array form.

db.batch()
  .del('father')
  .put('name', 'Yuri Irsenovich Kim')
  .put('dob', '16 February 1941')
  .put('spouse', 'Kim Young-sook')
  .put('occupation', 'Clown')
  .write(function () { console.log('Done!') })

batch.put(key, value)

Queue a put operation on the current batch, not committed until a write() is called on the batch.

This method may throw a WriteError if there is a problem with your put (such as the value being null or undefined).

batch.del(key)

Queue a del operation on the current batch, not committed until a write() is called on the batch.

This method may throw a WriteError if there is a problem with your delete.

batch.clear()

Clear all queued operations on the current batch, any previous operations will be discarded.

batch.length

The number of queued operations on the current batch.

batch.write([options][, callback])

Commit the queued operations for this batch. All operations not cleared will be written to the underlying store atomically, that is, they will either all succeed or fail with no partial commits.

The optional options object is passed to the .write() operation of the underlying batch object.

If no callback is passed, a promise is returned.

db.isOpen()

A levelup instance can be in one of the following states:

• "new" - newly created, not opened or closed
• "opening" - waiting for the underlying store to be opened
• "open" - successfully opened the store, available for use
• "closing" - waiting for the store to be closed
• "closed" - store has been successfully closed, should not be used

isOpen() will return true only when the state is "open".

db.isClosed()

See isOpen()

isClosed() will return true only when the state is "closing" or "closed", it can be useful for determining if read and write operations are permissible.

db.createReadStream([options])

Returns a Readable Stream of key-value pairs. A pair is an object with key and value properties. By default it will stream all entries in the underlying store from start to end. Use the options described below to control the range, direction and results.

db.createReadStream()
  .on('data', function (data) {
    console.log(data.key, '=', data.value)
  })
  .on('error', function (err) {
    console.log('Oh my!', err)
  })
  .on('close', function () {
    console.log('Stream closed')
  })
  .on('end', function () {
    console.log('Stream ended')
  })

You can supply an options object as the first parameter to createReadStream() with the following properties:

• gt (greater than), gte (greater than or equal) define the lower bound of the range to be streamed. Only entries where the key is greater than (or equal to) this option will be included in the range. When reverse=true the order will be reversed, but the entries streamed will be the same.

• lt (less than), lte (less than or equal) define the higher bound of the range to be streamed. Only entries where the key is less than (or equal to) this option will be included in the range. When reverse=true the order will be reversed, but the entries streamed will be the same.

• reverse (boolean, default: false): stream entries in reverse order. Beware that due to the way that stores like LevelDB work, a reverse seek can be slower than a forward seek.

• limit (number, default: -1): limit the number of entries collected by this stream. This number represents a maximum number of entries and may not be reached if you get to the end of the range first. A value of -1 means there is no limit. When reverse=true the entries with the highest keys will be returned instead of the lowest keys.

• keys (boolean, default: true): whether the results should contain keys. If set to true and values set to false then results will simply be keys, rather than objects with a key property. Used internally by the createKeyStream() method.

• values (boolean, default: true): whether the results should contain values. If set to true and keys set to false then results will simply be values, rather than objects with a value property. Used internally by the createValueStream() method.

Legacy options:

• start: instead use gte

• end: instead use lte

db.createKeyStream([options])

Returns a Readable Stream of keys rather than key-value pairs. Use the same options as described for createReadStream to control the range and direction.

You can also obtain this stream by passing an options object to createReadStream() with keys set to true and values set to false. The result is equivalent; both streams operate in object mode.

db.createKeyStream()
  .on('data', function (data) {
    console.log('key=', data)
  })

// same as:
db.createReadStream({ keys: true, values: false })
  .on('data', function (data) {
    console.log('key=', data)
  })

db.createValueStream([options])

Returns a Readable Stream of values rather than key-value pairs. Use the same options as described for createReadStream to control the range and direction.

You can also obtain this stream by passing an options object to createReadStream() with values set to true and keys set to false. The result is equivalent; both streams operate in object mode.

db.createValueStream()
  .on('data', function (data) {
    console.log('value=', data)
  })

// same as:
db.createReadStream({ keys: false, values: true })
  .on('data', function (data) {
    console.log('value=', data)
  })

db.iterator([options])

Returns an abstract-leveldown iterator, which is what powers the readable streams above. Options are the same as the range options of createReadStream and are passed to the underlying store.

What happened to db.createWriteStream?

db.createWriteStream() has been removed in order to provide a smaller and more maintainable core. It primarily existed to create symmetry with db.createReadStream() but through much discussion, removing it was the best course of action.

The main driver for this was performance. While db.createReadStream() performs well under most use cases, db.createWriteStream() was highly dependent on the application keys and values. Thus we can't provide a standard implementation and encourage more write-stream implementations to be created to solve the broad spectrum of use cases.

Check out the implementations that the community has produced here.

Promise Support

levelup ships with native Promise support out of the box.

Each function accepting a callback returns a promise if the callback is omitted. This applies for:

• db.get(key[, options])
• db.put(key, value[, options])
• db.del(key[, options])
• db.batch(ops[, options])
• db.batch().write()

The only exception is the levelup constructor itself, which if no callback is passed will lazily open the underlying store in the background.

Example:

var db = levelup(leveldown('./my-db'))

db.put('foo', 'bar')
  .then(function () { return db.get('foo') })
  .then(function (value) { console.log(value) })
  .catch(function (err) { console.error(err) })

Or using async/await:

const main = async () => {
  const db = levelup(leveldown('./my-db'))

  await db.put('foo', 'bar')
  console.log(await db.get('foo'))
}

Events

levelup is an EventEmitter and emits the following events.

Event	Description	Arguments
put	Key has been updated	key, value (any)
del	Key has been deleted	key (any)
batch	Batch has executed	operations (array)
opening	Underlying store is opening	-
open	Store has opened	-
ready	Alias of open	-
closing	Store is closing	-
closed	Store has closed.	-

For example you can do:

db.on('put', function (key, value) {
  console.log('inserted', { key, value })
})

Multi-process Access

Stores like LevelDB are thread-safe but they are not suitable for accessing with multiple processes. You should only ever have a store open from a single Node.js process. Node.js clusters are made up of multiple processes so a levelup instance cannot be shared between them either.

See Level/awesome for modules like multileveldown that may help if you require a single store to be shared across processes.

Contributing

Level/levelup is an OPEN Open Source Project. This means that:

Individuals making significant and valuable contributions are given commit-access to the project to contribute as they see fit. This project is more like an open wiki than a standard guarded open source project.

See the Contribution Guide for more details.

Big Thanks

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License

MIT © 2012-present Contributors.