leveldown
Table of Contents
Click to expand
Introduction
This module was originally part of levelup
but was later extracted 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
.
Supported Platforms
We aim to support at least Active LTS and Current Node.js releases, Electron 4.0.0, as well as any future Node.js and Electron releases thanks to N-API. The minimum node version for leveldown
is 8.6.0
. Conversely, for node >= 12, the minimum leveldown
version is 5.0.0
.
The leveldown
npm package ships with prebuilt binaries for popular 64-bit platforms as well as ARM, Android and Alpine (musl) and is known to work on:
- Linux (including ARM platforms such as Raspberry Pi and Kindle)
- Mac OS
- Solaris (SmartOS & Nodejitsu)
- FreeBSD
- Windows
When installing leveldown
, node-gyp-build
will check if a compatible binary exists and fallback to a compile step if it doesn't. In that case you'll need a valid node-gyp
installation.
If you don't want to use the prebuilt binary for the platform you are installing on, specify the --build-from-source
flag when you install. One of:
npm install --build-from-source
npm install leveldown --build-from-source
If you are working on leveldown
itself and want to re-compile the C++ code, run npm run rebuild
.
Notes
- If you get compilation errors on Node.js 12, please ensure you have
leveldown
>= 5. This can be checked by runningnpm ls leveldown
. - On Linux flavors with an old glibc (Debian 8, Ubuntu 14.04, RHEL 7, CentOS 7) you must either update
leveldown
to >= 5.3.0 or use--build-from-source
. - On Alpine 3 it was previously necessary to use
--build-from-source
. This is no longer the case. - The Android prebuilds are made for and built against Node.js core rather than the
nodejs-mobile
fork.
API
If you are upgrading: please see UPGRADING.md
.
leveldown()
db.open()
db.close()
db.put()
db.get()
db.del()
db.batch()
(array form)db.batch()
(chained form)db.approximateSize()
db.compactRange()
db.getProperty()
db.iterator()
db.clear()
chainedBatch
iterator
leveldown.destroy()
leveldown.repair()
db = leveldown(location)
leveldown()
returns a new leveldown
instance. location
is a String pointing to the LevelDB location to be opened.
db.open([options, ]callback)
open()
is an instance method on an existing database object.
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.
options
The optional options
argument may contain:
-
createIfMissing
(boolean, default:true
): Iftrue
, will initialise an empty database at the specified location if one doesn't already exist. Iffalse
and a database doesn't exist you will receive an error in youropen()
callback and your database won't open. -
errorIfExists
(boolean, default:false
): Iftrue
, you will receive an error in youropen()
callback if the database exists at the specified location. -
compression
(boolean, default:true
): Iftrue
, 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
= 8MB): The size (in bytes) of the in-memory LRU cache with frequently used uncompressed block contents.
Advanced options
The following options are for advanced performance tuning. Modify them only if you can prove actual benefit for your particular application.
writeBufferSize
(number, default:4 * 1024 * 1024
= 4MB): The maximum size (in bytes) of the log (in memory and stored in the .log file on disk). Beyond this size, LevelDB will convert the log data to the first level of sorted table files. From the LevelDB documentation:
Larger values increase performance, especially during bulk loads. Up to two write buffers may be held in memory at the same time, so you may wish to adjust this parameter to control memory usage. Also, a larger write buffer will result in a longer recovery time the next time the database is opened.
-
blockSize
(number, default4096
= 4K): The approximate size of the blocks that make up the table files. The size related to uncompressed data (hence "approximate"). Blocks are indexed in the table file and entry-lookups involve reading an entire block and parsing to discover the required entry. -
maxOpenFiles
(number, default:1000
): The maximum number of files that LevelDB is allowed to have open at a time. If your data store is likely to have a large working set, you may increase this value to prevent file descriptor churn. To calculate the number of files required for your working set, divide your total data by'maxFileSize'
. -
blockRestartInterval
(number, default:16
): The number of entries before restarting the "delta encoding" of keys within blocks. Each "restart" point stores the full key for the entry, between restarts, the common prefix of the keys for those entries is omitted. Restarts are similar to the concept of keyframes in video encoding and are used to minimise the amount of space required to store keys. This is particularly helpful when using deep namespacing / prefixing in your keys. -
maxFileSize
(number, default:2* 1024 * 1024
= 2MB): The maximum amount of bytes to write to a file before switching to a new one. From the LevelDB documentation:
... if your filesystem is more efficient with larger files, you could consider increasing the value. The downside will be longer compactions and hence longer latency/performance hiccups. Another reason to increase this parameter might be when you are initially populating a large database.
db.close(callback)
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.
leveldown
waits for any pending operations to finish before closing. For example:
db.put('key', 'value', function (err) {
// This happens first
})
db.close(function (err) {
// This happens second
})
db.put(key, value[, options], callback)
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 Buffers. Other object types are converted to strings with the toString()
method. Keys may not be null
or undefined
and objects converted with toString()
should not result in an empty-string. Values may not be null
or undefined
. Values of ''
, []
and Buffer.alloc(0)
(and any object resulting in a toString()
of one of these) will be stored as a zero-length character array and will therefore be retrieved as either ''
or Buffer.alloc(0)
depending on the type requested.
A richer set of data-types is catered for in levelup
.
options
The 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.
db.get(key[, options], callback)
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 Buffer and cannot be undefined
or null
. Other object types are converted to strings with the toString()
method and the resulting string may not be a zero-length. A richer set of data-types is catered for in levelup
.
Values fetched via get()
that are stored as zero-length character arrays (null
, undefined
, ''
, []
, Buffer.alloc(0)
) will return as empty-String
(''
) or Buffer.alloc(0)
when fetched with asBuffer: true
(see below).
options
The 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 settingfillCache
tofalse
. -
asBuffer
(boolean, default:true
): Used to determine whether to return thevalue
of the entry as a string or a Buffer. Note that converting from a Buffer to a string incurs a cost so if you need a string (and thevalue
can legitimately become a UTF8 string) then you should fetch it as one with{ asBuffer: false }
and you'll avoid this conversion cost.
The callback
function will be called with a single error
if the operation failed for any reason. If successful the first argument will be null
and the second argument will be the value
as a string or Buffer depending on the asBuffer
option.
db.del(key[, options], callback)
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 Buffer and cannot be undefined
or null
. Other object types are converted to strings with the toString()
method and the resulting string may not be a zero-length. A richer set of data-types is catered for in levelup
.
options
The 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.
db.batch(operations[, options], callback)
(array form)
Perform multiple put and/or del operations in bulk. The operations
argument must be an Array
containing a list of operations to be executed sequentially, although as a whole they are performed as an atomic operation.
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 where the key
or value
(in the case of 'put'
) is null
or undefined
will cause an error to be returned on the callback
. Any entries where the type
is 'put'
that have a value
of []
, ''
or Buffer.alloc(0)
will be stored as a zero-length character array and therefore be fetched during reads as either ''
or Buffer.alloc(0)
depending on how they are requested. See levelup
for full documentation on how this works in practice.
The optional options
argument may contain:
sync
(boolean, default:false
). Seedb.put()
for details about this option.
The callback
function will be called with no arguments if the batch is successful or with an Error
if the batch failed for any reason.
db.batch()
(chained form)
Returns a new chainedBatch
instance.
db.approximateSize(start, end, callback)
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 strings or Buffers representing keys in the LevelDB store.
The callback
function will be called with a single error
if the operation failed for any reason. If successful the first argument will be null
and the second argument will be the approximate size as a Number.
db.compactRange(start, end, callback)
compactRange()
is an instance method on an existing database object. Used to manually trigger a database compaction in the range [start..end)
.
The start
and end
parameters may be strings or Buffers 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.
db.getProperty(property)
getProperty
can be used to get internal details from LevelDB. When issued with a valid property string, a readable string will be returned (this method is synchronous).
Currently, the only valid properties are:
-
'leveldb.num-files-at-levelN'
: return the number of files at level N, where N is an integer representing a valid level (e.g. "0"). -
'leveldb.stats'
: returns a multi-line string describing statistics about LevelDB's internal operation. -
'leveldb.sstables'
: returns a multi-line string describing all of the sstables that make up contents of the current database.
db.iterator([options])
Returns a new iterator
instance. The optional options
object may contain:
-
gt
(greater than),gte
(greater than or equal) define the lower bound of the values to be fetched and will determine the starting point wherereverse
is nottrue
. Only records where the key is greater than (or equal to) this option will be included in the range. Whenreverse
istrue
the order will be reversed, but the records returned will be the same. -
lt
(less than),lte
(less than or equal) define the higher bound of the range to be fetched and will determine the starting point wherereverse
is nottrue
. Only records where the key is less than (or equal to) this option will be included in the range. Whenreverse
istrue
the order will be reversed, but the records returned will be the same. -
start, end
legacy ranges - instead usegte, lte
-
reverse
(boolean, default:false
): a boolean, set totrue
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 thenext()
method should receive a non-nullkey
. 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 thenext()
method should receive a non-nullvalue
. 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 yourend
value first. A value of-1
means there is no limit. -
fillCache
(boolean, default:false
): whether LevelDB's LRU-cache should be filled with data read. -
keyAsBuffer
(boolean, default:true
): Used to determine whether to return thekey
of each entry as a string or a Buffer. Note that converting from a Buffer to a string incurs a cost so if you need a string (and thevalue
can legitimately become a UTF8 string) then you should fetch it as one. -
valueAsBuffer
(boolean, default:true
): Used to determine whether to return thevalue
of each entry as a string or a Buffer.
db.clear([options, ]callback)
Delete all entries or a range. Not guaranteed to be atomic. Accepts the following range options (with the same rules as on iterators):
gt
(greater than),gte
(greater than or equal) define the lower bound of the range to be deleted. Only entries where the key is greater than (or equal to) this option will be included in the range. Whenreverse=true
the order will be reversed, but the entries deleted will be the same.lt
(less than),lte
(less than or equal) define the higher bound of the range to be deleted. Only entries where the key is less than (or equal to) this option will be included in the range. Whenreverse=true
the order will be reversed, but the entries deleted will be the same.reverse
(boolean, default:false
): delete entries in reverse order. Only effective in combination withlimit
, to remove the last N records.limit
(number, default:-1
): limit the number of entries to be deleted. 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. Whenreverse=true
the entries with the highest keys will be deleted instead of the lowest keys.
If no options are provided, all entries will be deleted. The callback
function will be called with no arguments if the operation was successful or with an Error
if it failed for any reason.
chainedBatch
chainedBatch.put(key, value)
Queue a put
operation on this batch. This may throw if key
or value
is invalid, following the same rules as the array form of db.batch()
.
chainedBatch.del(key)
Queue a del
operation on this batch. This may throw if key
is invalid.
chainedBatch.clear()
Clear all queued operations on this batch.
chainedBatch.write([options, ]callback)
Commit the queued operations for this batch. All operations will be written atomically, that is, they will either all succeed or fail with no partial commits.
The optional options
argument may contain:
sync
(boolean, default:false
). Seedb.put()
for details about this option.
The callback
function will be called with no arguments if the batch is successful or with an Error
if the batch failed for any reason. After write
has been called, no further operations are allowed.
chainedBatch.db
A reference to the db
that created this chained batch.
iterator
iterator.next(callback)
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:
- the iterator comes to the end of the store
- the
end
key has been reached; or - the
limit
has been reached; or - the last
seek()
was out of range
Otherwise, 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 Buffer depending on thekeyAsBuffer
argument when theiterator()
was called.value
- either a string or a Buffer depending on thevalueAsBuffer
argument when theiterator()
was called.
iterator.seek(key)
seek()
is an instance method on an existing iterator object, used to seek the underlying LevelDB iterator to a given key.
By calling seek(key)
, subsequent calls to next(cb)
will return key/values larger or smaller than key
, based on your reverse
setting in the iterator constructor.
iterator.end(callback)
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.
iterator.db
A reference to the db
that created this iterator.
leveldown.destroy(location, callback)
destroy()
is used to completely remove an existing LevelDB database directory. You can use this function in place of a full directory rm if you want to be sure to only remove LevelDB-related files. If the directory only contains LevelDB files, the directory itself will be removed as well. If there are additional, non-LevelDB files in the directory, those files, and the directory, will be left alone.
The callback will be called when the destroy operation is complete, with a possible error
argument.
leveldown.repair(location, callback)
repair()
can be used to attempt a restoration of a damaged LevelDB store. From the LevelDB documentation:
If a DB cannot be opened, you may attempt to call this method to resurrect as much of the contents of the database as possible. Some data may be lost, so be careful when calling this function on a database that contains important information.
You will find information on the repair operation in the LOG file inside the store directory.
A repair()
can also be used to perform a compaction of the LevelDB log into table files.
The callback will be called when the repair operation is complete, with a possible error
argument.
Safety
Database State
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 for yourself.
Snapshots
leveldown
exposes a feature of LevelDB called snapshots. This means that when you do e.g. createReadStream
and createWriteStream
at the same time, any data modified by the write stream will not affect data emitted from the read stream. In other words, a LevelDB Snapshot captures the latest state at the time the snapshot was created, enabling the snapshot to iterate or read the data without seeing any subsequent writes. Any read not performed on a snapshot will implicitly use the latest state.
Getting Support
There are multiple ways you can find help in using LevelDB in Node.js:
- IRC: you'll find an active group of
levelup
users in the ##leveldb channel on Freenode, including most of the contributors to this project. - Mailing list: there is an active Node.js LevelDB Google Group.
- GitHub: you're welcome to open an issue here on this GitHub repository if you have a question.
Contributing
Level/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 Contribution Guide for more details.
Git Submodules
This project uses Git Submodules. This means that you should clone it recursively if you're planning on working on it:
$ git clone --recurse-submodules https://github.com/Level/leveldown.git
Alternatively, you can initialize submodules inside the cloned folder:
$ git submodule update --init --recursive
Publishing
- Increment the version:
npm version ..
- Push to GitHub:
git push --follow-tags
- Wait for Travis and AppVeyor builds to complete
- Download prebuilds into
./prebuilds
:npm run download-prebuilds
- Optionally verify loading a prebuild:
npm run test-prebuild
- Optionally verify which files npm will include:
irish-pub
- Add changelog to the GitHub release
- Finally:
npm publish
Donate
To sustain Level
and its activities, become a backer or sponsor on Open Collective. Your logo or avatar will be displayed on our 28+ GitHub repositories and npm packages. 💖
Backers
Sponsors
License
MIT © 2012-present Rod Vagg and Contributors.
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 License. A large portion of leveldown
Windows support comes from the Windows LevelDB port (archived) by Krzysztof Kowalczyk (@kjk
). If you're using leveldown
on Windows, you should give him your thanks!