classic-level

An abstract-level database backed by LevelDB. The successor to leveldown with builtin encodings, sublevels, events, hooks and support of Uint8Array. If you are upgrading, please see UPGRADING.md.

:pushpin: What is abstract-level? Head on over to Frequently Asked Questions.

Table of Contents

• Usage
• Supported Platforms
• API
  • db = new ClassicLevel(location[, options])
  • Opening
  • Closing
  • Reading
  • Writing
  • Additional Methods
    • db.location
    • db.approximateSize(start, end[, options])
    • db.compactRange(start, end[, options])
    • db.getProperty(property)
    • ClassicLevel.destroy(location)
    • ClassicLevel.repair(location)
• Development
  • Getting Started
  • Contributing
  • Publishing
• Donate
• License

Usage

const { ClassicLevel } = require('classic-level')

// Create a database
const db = new ClassicLevel('./db', { valueEncoding: 'json' })

// Add an entry with key 'a' and value 1
await db.put('a', 1)

// Add multiple entries
await db.batch([{ type: 'put', key: 'b', value: 2 }])

// Get value of key 'a': 1
const value = await db.get('a')

// Iterate entries with keys that are greater than 'a'
for await (const [key, value] of db.iterator({ gt: 'a' })) {
  console.log(value) // 2
}

Usage from TypeScript requires generic type parameters.

// Specify types of keys and values (any, in the case of json).
// The generic type parameters default to ClassicLevel<string, string>.
const db = new ClassicLevel<string, any>('./db', { valueEncoding: 'json' })

// All relevant methods then use those types
await db.put('a', { x: 123 })

// Specify different types when overriding encoding per operation
await db.get<string, string>('a', { valueEncoding: 'utf8' })

// Though in some cases TypeScript can infer them
await db.get('a', { valueEncoding: db.valueEncoding('utf8') })

// It works the same for sublevels
const abc = db.sublevel('abc')
const xyz = db.sublevel<string, any>('xyz', { valueEncoding: 'json' })

Supported Platforms

We aim to support Active LTS and Current Node.js releases, Electron >= 30, as well as any future Node.js and Electron releases thanks to Node-API.

The classic-level npm package ships with prebuilt binaries for popular 64-bit platforms as well as ARM, M1, Android, Alpine (musl), Windows 32-bit, Linux flavors with an old glibc (2.28) and is known to work on:

• Linux, including ARM platforms such as Raspberry Pi and Kindle
• Mac OS (10.7 and later)
• Windows.

When installing classic-level, node-gyp-build will check if a compatible binary exists and fallback to compiling from source 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:

npm install classic-level --build-from-source

If you are working on classic-level itself and want to recompile the C++ code, run npm run rebuild.

API

The API of classic-level follows that of abstract-level with a few additional methods and options that are specific to LevelDB. The documentation below only covers the differences.

db = new ClassicLevel(location[, options])

Create a database or open an existing database. The location argument must be a directory path (relative or absolute) where LevelDB will store its files. If the directory does not yet exist (and options.createIfMissing is true) it will be created recursively. Options are the same as in abstract-level except for the additional options accepted by db.open() and thus by this constructor.

A classic-level database obtains an exclusive lock. If another process or instance has already opened the underlying LevelDB store at the same location then opening will fail with error code LEVEL_LOCKED.

Opening

The db.open([options]) method has additional options:

• multithreading (boolean, default: false): allow multiple threads to access the database. This is only relevant when using worker threads.

It also has the following options for advanced performance tuning, only to be modified if you can prove actual benefit for your particular application.

• compression (boolean, default: true): Unless set to false, all compressible data will be run through the Snappy compression algorithm before being stored. Snappy is very fast 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.

• writeBufferSize (number, default: 4 * 1024 * 1024): 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 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, default: 4096): The approximate size of the blocks that make up the table files. The size relates 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 database 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 size 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): The maximum amount of bytes to write to a file before switching to a new one. From 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.

Closing

The db.close() method has an additional behavior: it waits for any pending operations to finish before closing. For example:

// close() will wait for the put() to finish.
const promise1 = db.put('key', 'value')
const promise2 = db.close()

Reading

The db.get(key[, options]), db.getMany(keys[, options]) and db.iterator([options]) methods have an additional option:

• fillCache (boolean, default: true): unless set to false, LevelDB will fill its in-memory LRU cache with data that was read.

A classic-level database supports snapshots (as indicated by db.supports.snapshots) which means db.get(), db.getMany() and db.iterator() read from a snapshot of the database, created synchronously at the time that db.get(), db.getMany() or db.iterator() was called. This means they will not see the data of simultaneous write operations, commonly referred to as having snapshot guarantees.

The db.iterator([options]) method also accepts:

• highWaterMarkBytes (number, default: 16 * 1024): limit the amount of data that the iterator will hold in memory.

While iterator.nextv(size) is reading entries from LevelDB into memory, it sums up the byte length of those entries. If and when that sum has exceeded highWaterMarkBytes, reading will stop. If nextv(2) would normally yield two entries but the first entry is too large, then only one entry will be yielded. More nextv(size) calls must then be made to get the remaining entries.

If memory usage is less of a concern, increasing highWaterMarkBytes can increase the throughput of nextv(size). If set to 0 then nextv(size) will never yield more than one entry, as highWaterMarkBytes will be exceeded on each call. It can not be set to Infinity. On key- and value iterators (see below) it applies to the byte length of keys or values respectively, rather than the combined byte length of keys and values.

Optimal performance can be achieved by setting highWaterMarkBytes to at least size multiplied by the expected byte length of an entry, ensuring that size is always met. In other words, that nextv(size) will not stop reading before size amount of entries have been read into memory. If the iterator is wrapped in a Node.js stream or Web Stream then the size parameter is dictated by the stream's highWaterMark option. For example:

const { EntryStream } = require('level-read-stream')

// If an entry is 50 bytes on average
const stream = new EntryStream(db, {
  highWaterMark: 1000,
  highWaterMarkBytes: 1000 * 50
})

Side note: the "watermark" analogy makes more sense in Node.js streams because its internal highWaterMark can grow, indicating the highest that the "water" has been. In a classic-level iterator however, highWaterMarkBytes is fixed once set. Getting exceeded does not change it.

The highWaterMarkBytes option is also applied to an internal cache that classic-level employs for next() and for await...of. When next() is called, that cache is populated with at most 1000 entries, or less than that if highWaterMarkBytes is exceeded by the total byte length of entries. To avoid reading too eagerly, the cache is not populated on the first next() call, or the first next() call after a seek(). Only on subsequent next() calls.

Writing

The db.put(key, value[, options]), db.del(key[, options]) and db.batch(operations[, options]) and chainedBatch.write([options]) methods have an additional option:

• sync (boolean, default: false): if set to true, LevelDB will perform a synchronous write of the data although the operation will be asynchronous as far as Node.js or Electron is concerned. Normally, LevelDB passes the data to the operating system for writing and returns immediately. In contrast, a synchronous write will use fsync() or equivalent, so the write will not complete until the data is actually on disk. Synchronous writes are significantly slower than asynchronous writes.

Additional Methods

The following methods and properties are not part of the abstract-level interface.

db.location

Read-only getter that returns the location string that was passed to the constructor (as-is).

db.approximateSize(start, end[, options])

Get the approximate number of bytes of file system space used by the range [start..end). The result might not include recently written data. The optional options object may contain:

• keyEncoding: custom key encoding for this operation, used to encode start and end.

Returns a promise for a number.

db.compactRange(start, end[, options])

Manually trigger a database compaction in the range [start..end]. The optional options object may contain:

• keyEncoding: custom key encoding for this operation, used to encode start and end.

Returns a promise.

db.getProperty(property)

Get internal details from LevelDB. When issued with a valid property string, a string value is returned synchronously. 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.

ClassicLevel.destroy(location)

Completely remove an existing LevelDB database directory. You can use this method in place of a full directory removal 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.

Returns a promise for the completion of the destroy operation.

Before calling destroy(), close a database if it's using the same location:

const db = new ClassicLevel('./db')
await db.close()
await ClassicLevel.destroy('./db')

ClassicLevel.repair(location)

Attempt a restoration of a damaged database. It can also be used to perform a compaction of the LevelDB log into table files. From 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.

Returns a promise for the completion of the repair operation.

You will find information on the repair operation in the LOG file inside the database directory.

Before calling repair(), close a database if it's using the same location.

Development

Getting Started

This repository uses git submodules. Clone it recursively:

git clone --recurse-submodules https://github.com/Level/classic-level.git

Alternatively, initialize submodules inside the working tree:

cd classic-level
git submodule update --init --recursive

Contributing

Level/classic-level 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.

Publishing

1. Increment the version: npm version ..
2. Push to GitHub: git push --follow-tags
3. Wait for CI to complete
4. Download prebuilds into ./prebuilds: npm run download-prebuilds
5. Optionally verify loading a prebuild: npm run test-prebuild
6. Optionally verify which files npm will include: canadian-pub
7. Finally: npm publish

Donate

Support us with a monthly donation on Open Collective and help us continue our work.

License

MIT