leveldown
Advanced tools
LevelDOWN is a Node.js binding for LevelDB, a fast key-value storage library written at Google that provides an ordered mapping from string keys to string values. It is used as a backend for the LevelUP module, which provides a more user-friendly API.
Open a Database
This feature allows you to open a LevelDB database. The code sample demonstrates how to open a database located at './mydb'.
const leveldown = require('leveldown');
const db = leveldown('./mydb');
db.open(function (err) {
if (err) throw err;
console.log('Database opened');
});Put Data
This feature allows you to store a key-value pair in the database. The code sample shows how to store the value 'value' under the key 'key'.
db.put('key', 'value', function (err) {
if (err) throw err;
console.log('Data written');
});Get Data
This feature allows you to retrieve a value by its key from the database. The code sample demonstrates how to get the value associated with the key 'key'.
db.get('key', function (err, value) {
if (err) throw err;
console.log('Data retrieved:', value);
});Delete Data
This feature allows you to delete a key-value pair from the database. The code sample shows how to delete the key 'key' from the database.
db.del('key', function (err) {
if (err) throw err;
console.log('Data deleted');
});Batch Operations
This feature allows you to perform multiple operations in a single batch. The code sample demonstrates how to perform a batch operation that puts and deletes multiple key-value pairs.
db.batch()
.put('key1', 'value1')
.del('key2')
.put('key3', 'value3')
.write(function (err) {
if (err) throw err;
console.log('Batch operations completed');
});LevelUP is a higher-level module that provides a more user-friendly API for LevelDB. It uses LevelDOWN as its default backend. Compared to LevelDOWN, LevelUP offers a more convenient and feature-rich interface for interacting with LevelDB.
RocksDB is a high-performance key-value store developed by Facebook. It is similar to LevelDB but offers better performance and more features, such as column families and transactions. The 'rocksdb' npm package provides Node.js bindings for RocksDB.
LMDB (Lightning Memory-Mapped Database) is a fast memory-mapped key-value store. The 'lmdb' npm package provides Node.js bindings for LMDB. Compared to LevelDOWN, LMDB offers better performance for read-heavy workloads and supports ACID transactions.
LevelDOWN was extracted from LevelUP and now serves as a stand-alone binding for LevelDB.
It is strongly recommended that you use LevelUP in preference to LevelDOWN unless you have measurable performance reasons to do so. LevelUP is optimised for usability and safety. Although we are working to improve the safety of the LevelDOWN interface it is still easy to crash your Node process if you don't do things in just the right way.
See the section on safety below for details of known unsafe operations with LevelDOWN.
leveldown()leveldown#open()leveldown#close()leveldown#put()leveldown#get()leveldown#del()leveldown#batch()leveldown#approximateSize()leveldown#iterator()iterator#next()iterator#end()leveldown() returns a new LevelDOWN instance. location is a String pointing to the LevelDB location to be opened.
open() is an instance method on an existing database object.
optionsThe optional options argument may contain:
'createIfMissing' (boolean, default: true): If true, will initialise an empty database at the specified location if one doesn't already exist. If false and a database doesn't exist you will receive an error in your open() callback and your database won't open.
'errorIfExists' (boolean, default: false): If true, you will receive an error in your open() callback if the database exists at the specified location.
'compression' (boolean, default: true): If true, all compressible data will be run through the Snappy compression algorithm before being stored. Snappy is very fast and shouldn't gain much speed by disabling so leave this on unless you have good reason to turn it off.
'cacheSize' (number, default: 8 * 1024 * 1024): The size (in bytes) of the in-memory LRU cache with frequently used uncompressed block contents.
The callback function will be called with no arguments when the database has been successfully opened, or with a single error argument if the open operation failed for any reason.
close() is an instance method on an existing database object. The underlying LevelDB database will be closed and the callback function will be called with no arguments if the operation is successful or with a single error argument if the operation failed for any reason.
put() is an instance method on an existing database object, used to store new entries, or overwrite existing entries in the LevelDB store.
The key and value objects may either be Strings or Node.js Buffer objects and cannot be undefined or null. Other object types are converted to JavaScript Strings with the toString() method and the resulting String may not be a zero-length. A richer set of data-types are catered for in LevelUP.
optionsThe only property currently available on the options object is 'sync' (boolean, default: false). If you provide a 'sync' value of true in your options object, LevelDB will perform a synchronous write of the data; although the operation will be asynchronous as far as Node is concerned. Normally, LevelDB passes the data to the operating system for writing and returns immediately, however a synchronous write will use fsync() or equivalent so your callback won't be triggered until the data is actually on disk. Synchronous filesystem writes are significantly slower than asynchronous writes but if you want to be absolutely sure that the data is flushed then you can use 'sync': true.
The callback function will be called with no arguments if the operation is successful or with a single error argument if the operation failed for any reason.
get() is an instance method on an existing database object, used to fetch individual entries from the LevelDB store.
The key object may either be a String or a Node.js Buffer object and cannot be undefined or null. Other object types are converted to JavaScript Strings with the toString() method and the resulting String may not be a zero-length. A richer set of data-types are catered for in LevelUP.
optionsThe optional options object may contain:
'fillCache' (boolean, default: true): LevelDB will by default fill the in-memory LRU Cache with data from a call to get. Disabling this is done by setting fillCache to false.
'asBuffer' (boolean, default: true): Used to determine whether to return the value of the entry as a String or a Node.js Buffer object. Note that converting from a Buffer to a String incurs a cost so if you need a String (and the value can legitimately become a UFT8 string) then you should fetch it as one with asBuffer: true and you'll avoid this conversion cost.
The callback function will be called with no arguments if the operation is successful or with a single error argument if the operation failed for any reason.
del() is an instance method on an existing database object, used to delete entries from the LevelDB store.
The key object may either be a String or a Node.js Buffer object and cannot be undefined or null. Other object types are converted to JavaScript Strings with the toString() method and the resulting String may not be a zero-length. A richer set of data-types are catered for in LevelUP.
optionsThe only property currently available on the options object is 'sync' (boolean, default: false). See leveldown#put() for details about this option.
The callback function will be called with no arguments if the operation is successful or with a single error argument if the operation failed for any reason.
batch() is an instance method on an existing database object. Used for very fast bulk-write operations (both put and delete). The operations argument should be an Array containing a list of operations to be executed sequentially. 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. See LevelUP for full documentation on how this works in practice.
optionsThe only property currently available on the options object is 'sync' (boolean, default: false). See leveldown#put() for details about this option.
The callback function will be called with no arguments if the operation is successful or with a single error argument if the operation failed for any reason.
approximateSize() is an instance method on an existing database object. Used to get the approximate number of bytes of file system space used by the range [start..end). The result may not include recently written data.
The start and end parameters may be either String or Node.js Buffer objects representing keys in the LevelDB store.
The callback function will be called with no arguments if the operation is successful or with a single error argument if the operation failed for any reason.
iterator() is an instance method on an existing database object. It returns a new Iterator instance.
optionsThe optional options object may contain:
'start': the key you wish to start the read at. By default it will start at the beginning of the store. Note that the start doesn't have to be an actual key that exists, LevelDB will simply find the next key, greater than the key you provide.
'end': the key you wish to end the read on. By default it will continue until the end of the store. Again, the end doesn't have to be an actual key as an (inclusive) <=-type operation is performed to detect the end. You can also use the destroy() method instead of supplying an 'end' parameter to achieve the same effect.
'reverse' (boolean, default: false): a boolean, set to true if you want the stream to go in reverse order. Beware that due to the way LevelDB works, a reverse seek will be slower than a forward seek.
'keys' (boolean, default: true): whether the callback to the next() method should receive a non-null key. There is a small efficiency gain if you ultimately don't care what the keys are as they don't need to be converted and copied into JavaScript.
'values' (boolean, default: true): whether the callback to the next() method should receive a non-null value. There is a small efficiency gain if you ultimately don't care what the values are as they don't need to be converted and copied into JavaScript.
'limit' (number, default: -1): limit the number of results collected by this iterator. This number represents a maximum number of results and may not be reached if you get to the end of the store or your 'end' value first. A value of -1 means there is no limit.
'fillCache' (boolean, default: false): wheather LevelDB's LRU-cache should be filled with data read.
'keyAsBuffer' (boolean, default: true): Used to determine whether to return the key of each entry as a String or a Node.js Buffer object. Note that converting from a Buffer to a String incurs a cost so if you need a String (and the value can legitimately become a UFT8 string) then you should fetch it as one.
'valueAsBuffer' (boolean, default: true): Used to determine whether to return the value of each entry as a String or a Node.js Buffer object.
next() is an instance method on an existing iterator object, used to increment the underlying LevelDB iterator and return the entry at that location.
the callback function will be called with no arguments in any of the following situations:
end key has been reached; orlimit has been reachedOtherwise, the callback function will be called with the following 3 arguments:
error - any error that occurs while incrementing the iterator.key - either a String or a Node.js Buffer object depending on the keyAsBuffer argument when the createIterator() was called.value - either a String or a Node.js Buffer object depending on the valueAsBuffer argument when the createIterator() was called.end() is an instance method on an existing iterator object. The underlying LevelDB iterator will be deleted and the callback function will be called with no arguments if the operation is successful or with a single error argument if the operation failed for any reason.
Currently LevelDOWN does not track the state of the underlying LevelDB instance. This means that calling open() on an already open database may result in an error. Likewise, calling any other operation on a non-open database may result in an error.
LevelUP currently tracks and manages state and will prevent out-of-state operations from being send to LevelDOWN. If you use LevelDOWN directly then you must track and manage state.
You should make sure to always end() your LevelDOWN Iterators, it will not be done for you, even on a close() of the database your Iterator belongs to. If you don't end() then the underlying resources aren't cleaned up and this will result in a Node process death.
LevelDOWN 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 CONTRIBUTING.md file for more details.
LevelDOWN is only possible due to the excellent work of the following contributors:
| Rod Vagg | GitHub/rvagg | Twitter/@rvagg |
|---|---|---|
| John Chesley | GitHub/chesles | Twitter/@chesles |
| Jake Verbaten | GitHub/raynos | Twitter/@raynos2 |
| Dominic Tarr | GitHub/dominictarr | Twitter/@dominictarr |
| Max Ogden | GitHub/maxogden | Twitter/@maxogden |
| Lars-Magnus Skog | GitHub/ralphtheninja | Twitter/@ralphtheninja |
| David Björklund | GitHub/kesla | Twitter/@david_bjorklund |
| Julian Gruber | GitHub/juliangruber | Twitter/@juliangruber |
| Paolo Fragomeni | GitHub/hij1nx | Twitter/@hij1nx |
Copyright (c) 2012-2013 LevelDOWN contributors (listed above).
LevelDOWN is licensed under an MIT +no-false-attribs license. All rights not explicitly granted in the MIT license are reserved. See the included LICENSE file for more details.
LevelDOWN builds on the excellent work of the LevelDB and Snappy teams from Google and additional contributors. LevelDB and Snappy are both issued under the New BSD Licence.
FAQs
A low-level Node.js LevelDB binding
The npm package leveldown receives a total of 548,632 weekly downloads. As such, leveldown popularity was classified as popular.
We found that leveldown demonstrated a not healthy version release cadence and project activity because the last version was released a year ago. It has 3 open source maintainers collaborating on the project.
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