abstract-level

Abstract class for a lexicographically sorted key-value database. The successor to abstract-leveldown with builtin encodings, sublevels, events, promises and support of Uint8Array. If you are upgrading please see UPGRADING.md.

:pushpin: Which module should I use? What happened to levelup? Head over to the FAQ.

![level badge][level-badge]

Table of Contents

Usage

Usage of a typical implementation looks as follows.

// Create a database
const db = new Level('./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
}

All asynchronous methods also support callbacks.

db.put('a', { x: 123 }, function (err) {
  if (err) throw err

  db.get('a', function (err, value) {
    console.log(value) // { x: 123 }
  })
})

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 Level<string, string>.
const db = new Level<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 as well as browsers. Supported runtime environments may differ per implementation. As far as abstract-level goes, the following browsers are supported and continuously tested.

Public API For Consumers

This module has a public API for consumers of a database and a private API for concrete implementations. The public API, as documented in this section, offers a simple yet rich interface that is common between all implementations. Implementations may have additional options or methods. TypeScript type declarations are included (and exported for reuse) only for the public API.

An abstract-level database is at its core a key-value database. A key-value pair is referred to as an entry here and typically returned as an array, comparable to Object.entries().

db = new Constructor(...[, options])

Creating a database is done by calling a class constructor. Implementations export a class that extends the AbstractLevel class and has its own constructor with an implementation-specific signature. All constructors should have an options argument as the last. Typically, constructors take a location as their first argument, pointing to where the data will be stored. That may be a file path, URL, something else or none at all, since not all implementations are disk-based or persistent. Others take another database rather than a location as their first argument.

The optional options object may contain:

• keyEncoding (string or object, default 'utf8'): encoding to use for keys
• valueEncoding (string or object, default 'utf8'): encoding to use for values.

See Encodings for a full description of these options. Other options (except passive) are forwarded to db.open() which is automatically called in a next tick after the constructor returns. Any read & write operations are queued internally until the database has finished opening. If opening fails, those queued operations will yield errors.

db.status

Read-only getter that returns a string reflecting the current state of the database:

• 'opening' - waiting for the database to be opened
• 'open' - successfully opened the database
• 'closing' - waiting for the database to be closed
• 'closed' - successfully closed the database.

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

Open the database. The callback function will be called with no arguments when successfully opened, or with a single error argument if opening failed. If no callback is provided, a promise is returned. Options passed to open() take precedence over options passed to the database constructor. Not all implementations support the createIfMissing and errorIfExists options (notably memory-level and browser-level) and will indicate so via db.supports.createIfMissing and db.supports.errorIfExists.

The optional options object may contain:

• createIfMissing (boolean, default: true): If true, create an empty database if one doesn't already exist. If false and the database doesn't exist, opening will fail.
• errorIfExists (boolean, default: false): If true and the database already exists, opening will fail.
• passive (boolean, default: false): Wait for, but do not initiate, opening of the database.

It's generally not necessary to call open() because it's automatically called by the database constructor. It may however be useful to capture an error from failure to open, that would otherwise not surface until another method like db.get() is called. It's also possible to reopen the database after it has been closed with close(). Once open() has then been called, any read & write operations will again be queued internally until opening has finished.

The open() and close() methods are idempotent. If the database is already open, the callback will be called in a next tick. If opening is already in progress, the callback will be called when that has finished. If closing is in progress, the database will be reopened once closing has finished. Likewise, if close() is called after open(), the database will be closed once opening has finished and the prior open() call will receive an error.

db.close([callback])

Close the database. The callback function will be called with no arguments if closing succeeded or with a single error argument if closing failed. If no callback is provided, a promise is returned.

A database may have associated resources like file handles and locks. When the database is no longer needed (for the remainder of a program) it's recommended to call db.close() to free up resources.

After db.close() has been called, no further read & write operations are allowed unless and until db.open() is called again. For example, db.get(key) will yield an error with code LEVEL_DATABASE_NOT_OPEN. Any unclosed iterators or chained batches will be closed by db.close() and can then no longer be used even when db.open() is called again.

db.supports

A manifest describing the features supported by this database. Might be used like so:

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

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

Get a value from the database by key. The optional options object may contain:

• keyEncoding: custom key encoding for this operation, used to encode the key.
• valueEncoding: custom value encoding for this operation, used to decode the value.

The callback function will be called with an error if the operation failed. If the key was not found, the error will have code LEVEL_NOT_FOUND. If successful the first argument will be null and the second argument will be the value. If no callback is provided, a promise is returned.

If the database indicates support of snapshots via db.supports.snapshots then db.get() should read from a snapshot of the database, created at the time db.get() was called. This means it should not see the data of simultaneous write operations. However, this is currently not verified by the abstract test suite.

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

Get multiple values from the database by an array of keys. The optional options object may contain:

• keyEncoding: custom key encoding for this operation, used to encode the keys.
• valueEncoding: custom value encoding for this operation, used to decode values.

The callback function will be called with an error if the operation failed. 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.

If the database indicates support of snapshots via db.supports.snapshots then db.getMany() should read from a snapshot of the database, created at the time db.getMany() was called. This means it should not see the data of simultaneous write operations. However, this is currently not verified by the abstract test suite.

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

Add a new entry or overwrite an existing entry. The optional options object may contain:

• keyEncoding: custom key encoding for this operation, used to encode the key.
• valueEncoding: custom value encoding for this operation, used to encode the value.

The callback function will be called with no arguments if the operation was successful or with an error if it failed. If no callback is provided, a promise is returned.

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

Delete an entry by key. The optional options object may contain:

• keyEncoding: custom key encoding for this operation, used to encode the key.

The callback function will be called with no arguments if the operation was successful or with an error if it failed. If no callback is provided, a promise is returned.

db.batch(operations[, options][, callback])

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 must be an object with at least a type property set to either 'put' or 'del'. If the type is 'put', the operation must have key and value properties. It may optionally have keyEncoding and / or valueEncoding properties to encode keys or values with a custom encoding for just that operation. If the type is 'del', the operation must have a key property and may optionally have a keyEncoding property.

An operation of either type may also have a sublevel property, to prefix the key of the operation with the prefix of that sublevel. This allows atomically committing data to multiple sublevels. Keys and values will be encoded by the sublevel, to the same effect as a sublevel.batch(..) call. In the following example, the first value will be encoded with 'json' rather than the default encoding of db:

const people = db.sublevel('people', { valueEncoding: 'json' })
const nameIndex = db.sublevel('names')

await db.batch([{
  type: 'put',
  sublevel: people,
  key: '123',
  value: {
    name: 'Alice'
  }
}, {
  type: 'put',
  sublevel: nameIndex,
  key: 'Alice',
  value: '123'
}])

The optional options object may contain:

• keyEncoding: custom key encoding for this batch, used to encode keys.
• valueEncoding: custom value encoding for this batch, used to encode values.

Encoding properties on individual operations take precedence. In the following example, the first value will be encoded with the 'utf8' encoding and the second with 'json'.

await db.batch([
  { type: 'put', key: 'a', value: 'foo' },
  { type: 'put', key: 'b', value: 123, valueEncoding: 'json' }
], { valueEncoding: 'utf8' })

The callback function will be called with no arguments if the batch was successful or with an error if it failed. If no callback is provided, a promise is returned.

chainedBatch = db.batch()

Create a chained batch, when batch() is called with zero arguments. A chained batch can be used to build and eventually commit an atomic batch of operations. Depending on how it's used, it is possible to obtain greater performance with this form of batch(). On several implementations however, it is just sugar.

await db.batch()
  .del('bob')
  .put('alice', 361)
  .put('kim', 220)
  .write()

iterator = db.iterator([options])

Create an iterator. The optional options object may contain the following range options to control the range of entries to be iterated:

• gt (greater than) or gte (greater than or equal): define the lower bound of the range to be iterated. Only entries where the key is greater than (or equal to) this option will be included in the range. When reverse is true the order will be reversed, but the entries iterated will be the same.
• lt (less than) or lte (less than or equal): define the higher bound of the range to be iterated. Only entries where the key is less than (or equal to) this option will be included in the range. When reverse is true the order will be reversed, but the entries iterated will be the same.
• reverse (boolean, default: false): iterate entries in reverse order. Beware that a reverse seek can be slower than a forward seek.
• limit (number, default: Infinity): limit the number of entries yielded. This number represents a maximum number of entries and will not be reached if the end of the range is reached first. A value of Infinity or -1 means there is no limit. When reverse is true the entries with the highest keys will be returned instead of the lowest keys.

The gte and lte range options take precedence over gt and lt respectively. If no range options are provided, the iterator will visit all entries of the database, starting at the lowest key and ending at the highest key (unless reverse is true). In addition to range options, the options object may contain:

• keys (boolean, default: true): whether to return the key of each entry. If set to false, the iterator will yield keys that are undefined. Prefer to use db.keys() instead.
• values (boolean, default: true): whether to return the value of each entry. If set to false, the iterator will yield values that are undefined. Prefer to use db.values() instead.
• keyEncoding: custom key encoding for this iterator, used to encode range options, to encode seek() targets and to decode keys.
• valueEncoding: custom value encoding for this iterator, used to decode values.

Lastly, an implementation is free to add its own options.

:pushpin: To instead consume data using streams, see level-read-stream and level-web-stream.

keyIterator = db.keys([options])

Create a key iterator, having the same interface as db.iterator() except that it yields keys instead of entries. If only keys are needed, using db.keys() may increase performance because values won't have to fetched, copied or decoded. Options are the same as for db.iterator() except that db.keys() does not take keys, values and valueEncoding options.

// Iterate lazily
for await (const key of db.keys({ gt: 'a' })) {
  console.log(key)
}

// Get all at once. Setting a limit is recommended.
const keys = await db.keys({ gt: 'a', limit: 10 }).all()

valueIterator = db.values([options])

Create a value iterator, having the same interface as db.iterator() except that it yields values instead of entries. If only values are needed, using db.values() may increase performance because keys won't have to fetched, copied or decoded. Options are the same as for db.iterator() except that db.values() does not take keys and values options. Note that it does take a keyEncoding option, relevant for the encoding of range options.

// Iterate lazily
for await (const value of db.values({ gt: 'a' })) {
  console.log(value)
}

// Get all at once. Setting a limit is recommended.
const values = await db.values({ gt: 'a', limit: 10 }).all()

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

Delete all entries or a range. Not guaranteed to be atomic. Accepts the following options (with the same rules as on iterators):

• gt (greater than) or 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. When reverse is true the order will be reversed, but the entries deleted will be the same.
• lt (less than) or 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. When reverse is 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 with limit, to delete the last N entries.
• limit (number, default: Infinity): limit the number of entries to be deleted. This number represents a maximum number of entries and will not be reached if the end of the range is reached first. A value of Infinity or -1 means there is no limit. When reverse is true the entries with the highest keys will be deleted instead of the lowest keys.
• keyEncoding: custom key encoding for this operation, used to encode range options.

The gte and lte range options take precedence over gt and lt respectively. 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. If no callback is provided, a promise is returned.

sublevel = db.sublevel(name[, options])

Create a sublevel that has the same interface as db (except for additional, implementation-specific methods) and prefixes the keys of operations before passing them on to db. The name argument is required and must be a string.

const example = db.sublevel('example')

await example.put('hello', 'world')
await db.put('a', '1')

// Prints ['hello', 'world']
for await (const [key, value] of example.iterator()) {
  console.log([key, value])
}

Sublevels effectively separate a database into sections. Think SQL tables, but evented, ranged and realtime! Each sublevel is an AbstractLevel instance with its own keyspace, events and encodings. For example, it's possible to have one sublevel with 'buffer' keys and another with 'utf8' keys. The same goes for values. Like so:

db.sublevel('one', { valueEncoding: 'json' })
db.sublevel('two', { keyEncoding: 'buffer' })

An own keyspace means that sublevel.iterator() only includes entries of that sublevel, sublevel.clear() will only delete entries of that sublevel, and so forth. Range options get prefixed too.

Fully qualified keys (as seen from the parent database) take the form of prefix + key where prefix is separator + name + separator. If name is empty, the effective prefix is two separators. Sublevels can be nested: if db is itself a sublevel then the effective prefix is a combined prefix, e.g. '!one!!two!'. Note that a parent database will see its own keys as well as keys of any nested sublevels:

// Prints ['!example!hello', 'world'] and ['a', '1']
for await (const [key, value] of db.iterator()) {
  console.log([key, value])
}

:pushpin: The key structure is equal to that of subleveldown which offered sublevels before they were built-in to abstract-level. This means that an abstract-level sublevel can read sublevels previously created with (and populated by) subleveldown.

Internally, sublevels operate on keys that are either a string, Buffer or Uint8Array, depending on parent database and choice of encoding. Which is to say: binary keys are fully supported. The name must however always be a string and can only contain ASCII characters.

The optional options object may contain:

• separator (string, default: '!'): Character for separating sublevel names from user keys and each other. Must sort before characters used in name. An error will be thrown if that's not the case.
• keyEncoding (string or object, default 'utf8'): encoding to use for keys
• valueEncoding (string or object, default 'utf8'): encoding to use for values.

The keyEncoding and valueEncoding options are forwarded to the AbstractLevel constructor and work the same, as if a new, separate database was created. They default to 'utf8' regardless of the encodings configured on db. Other options are forwarded too but abstract-level has no relevant options at the time of writing. For example, setting the createIfMissing option will have no effect. Why is that?

Like regular databases, sublevels open themselves but they do not affect the state of the parent database. This means a sublevel can be individually closed and (re)opened. If the sublevel is created while the parent database is opening, it will wait for that to finish. If the parent database is closed, then opening the sublevel will fail and subsequent operations on the sublevel will yield errors with code LEVEL_DATABASE_NOT_OPEN.

encoding = db.keyEncoding([encoding])

Returns the given encoding argument as a normalized encoding object that follows the level-transcoder encoding interface. See Encodings for an introduction. The encoding argument may be:

• A string to select a known encoding by its name
• An object that follows one of the following interfaces: level-transcoder, level-codec, abstract-encoding, multiformats
• A previously normalized encoding, such that keyEncoding(x) equals keyEncoding(keyEncoding(x))
• Omitted, null or undefined, in which case the default keyEncoding of the database is returned.

Other methods that take keyEncoding or valueEncoding options, accept the same as above. Results are cached. If the encoding argument is an object and it has a name then subsequent calls can refer to that encoding by name.

Depending on the encodings supported by a database, this method may return a transcoder encoding that translates the desired encoding from / to an encoding supported by the database. Its encode() and decode() methods will have respectively the same input and output types as a non-transcoded encoding, but its name property will differ.

Assume that e.g. db.keyEncoding().encode(key) is safe to call at any time including if the database isn't open, because encodings must be stateless. If the given encoding is not found or supported, a LEVEL_ENCODING_NOT_FOUND or LEVEL_ENCODING_NOT_SUPPORTED error is thrown.

encoding = db.valueEncoding([encoding])

Same as db.keyEncoding([encoding]) except that it returns the default valueEncoding of the database (if the encoding argument is omitted, null or undefined).

key = db.prefixKey(key, keyFormat)

Add sublevel prefix to the given key, which must be already-encoded. If this database is not a sublevel, the given key is returned as-is. The keyFormat must be one of 'utf8', 'buffer', 'view'. If 'utf8' then key must be a string and the return value will be a string. If 'buffer' then Buffer, if 'view' then Uint8Array.

const sublevel = db.sublevel('example')

console.log(db.prefixKey('a', 'utf8')) // 'a'
console.log(sublevel.prefixKey('a', 'utf8')) // '!example!a'

db.defer(fn)

Call the function fn at a later time when db.status changes to 'open' or 'closed'. Used by abstract-level itself to implement "deferred open" which is a feature that makes it possible to call operations like db.put() before the database has finished opening. The defer() method is exposed for implementations and plugins to achieve the same on their custom operations:

db.foo = function (key, callback) {
  if (this.status === 'opening') {
    this.defer(() => this.foo(key, callback))
  } else {
    // ..
  }
}

When deferring a custom operation, do it early: after normalizing optional arguments but before encoding (to avoid double encoding and to emit original input if the operation has events) and before any fast paths (to avoid calling back before the database has finished opening). For example, db.batch([]) has an internal fast path where it skips work if the array of operations is empty. Resources that can be closed on their own (like iterators) should however first check such state before deferring, in order to reject operations after close (including when the database was reopened).

chainedBatch

chainedBatch.put(key, value[, options])

Queue a put operation on this batch, not committed until write() is called. This will throw a LEVEL_INVALID_KEY or LEVEL_INVALID_VALUE error if key or value is invalid. The optional options object may contain:

• keyEncoding: custom key encoding for this operation, used to encode the key.
• valueEncoding: custom value encoding for this operation, used to encode the value.
• sublevel (sublevel instance): act as though the put operation is performed on the given sublevel, to similar effect as sublevel.batch().put(key, value). This allows atomically committing data to multiple sublevels. The key will be prefixed with the prefix of the sublevel, and the key and value will be encoded by the sublevel (using the default encodings of the sublevel unless keyEncoding and / or valueEncoding are provided).

chainedBatch.del(key[, options])

Queue a del operation on this batch, not committed until write() is called. This will throw a LEVEL_INVALID_KEY error if key is invalid. The optional options object may contain:

• keyEncoding: custom key encoding for this operation, used to encode the key.
• sublevel (sublevel instance): act as though the del operation is performed on the given sublevel, to similar effect as sublevel.batch().del(key). This allows atomically committing data to multiple sublevels. The key will be prefixed with the prefix of the sublevel, and the key will be encoded by the sublevel (using the default key encoding of the sublevel unless keyEncoding is provided).

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.

There are no options by default but implementations may add theirs. Note that write() does not take encoding options. Those can only be set on put() and del() because implementations may synchronously forward such calls to an underlying store and thus need keys and values to be encoded at that point.

The callback function will be called with no arguments if the batch was successful or with an error if it failed. If no callback is provided, a promise is returned.

After write() or close() has been called, no further operations are allowed.

chainedBatch.close([callback])

Free up underlying resources. This should be done even if the chained batch has zero queued operations. Automatically called by write() so normally not necessary to call, unless the intent is to discard a chained batch without committing it. The callback function will be called with no arguments. If no callback is provided, a promise is returned. Closing the batch is an idempotent operation, such that calling close() more than once is allowed and makes no difference.

chainedBatch.length

The number of queued operations on the current batch.

chainedBatch.db

A reference to the database that created this chained batch.

iterator

An iterator allows one to lazily read a range of entries stored in the database. The entries will be sorted by keys in lexicographic order (in other words: byte order) which in short means key 'a' comes before 'b' and key '10' comes before '2'.

An iterator reads from a snapshot of the database, created at the time db.iterator() was called. This means the iterator will not see the data of simultaneous write operations. Most but not all implementations can offer this guarantee, as indicated by db.supports.snapshots.

Iterators can be consumed with for await...of and iterator.all(), or by manually calling iterator.next() or nextv() in succession. In the latter case, iterator.close() must always be called. In contrast, finishing, throwing, breaking or returning from a for await...of loop automatically calls iterator.close(), as does iterator.all().

An iterator reaches its natural end in the following situations:

• The end of the database has been reached
• The end of the range has been reached
• The last iterator.seek() was out of range.

An iterator keeps track of calls that are in progress. It doesn't allow concurrent next(), nextv() or all() calls (including a combination thereof) and will throw an error with code LEVEL_ITERATOR_BUSY if that happens:

// Not awaited and no callback provided
iterator.next()

try {
  // Which means next() is still in progress here
  iterator.all()
} catch (err) {
  console.log(err.code) // 'LEVEL_ITERATOR_BUSY'
}

for await...of iterator

Yields entries, which are arrays containing a key and value. The type of key and value depends on the options passed to db.iterator().

try {
  for await (const [key, value] of db.iterator()) {
    console.log(key)
  }
} catch (err) {
  console.error(err)
}

Note for implementors: this uses iterator.next() and iterator.close() under the hood so no further method implementations are needed to support for await...of.

iterator.next([callback])

Advance to the next entry and yield that entry. If an error occurs, the callback function will be called with an error. Otherwise, the callback receives null, a key and a value. The type of key and value depends on the options passed to db.iterator(). If the iterator has reached its natural end, both key and value will be undefined.

If no callback is provided, a promise is returned for either an entry array (containing a key and value) or undefined if the iterator reached its natural end.

Note: iterator.close() must always be called once there's no intention to call next() or nextv() again. Even if such calls yielded an error and even if the iterator reached its natural end. Not closing the iterator will result in memory leaks and may also affect performance of other operations if many iterators are unclosed and each is holding a snapshot of the database.

iterator.nextv(size[, options][, callback])

Advance repeatedly and get at most size amount of entries in a single call. Can be faster than repeated next() calls. The size argument must be an integer and has a soft minimum of 1. There are no options by default but implementations may add theirs.

If an error occurs, the callback function will be called with an error. Otherwise, the callback receives null and an array of entries, where each entry is an array containing a key and value. The natural end of the iterator will be signaled by yielding an empty array. If no callback is provided, a promise is returned.

const iterator = db.iterator()

while (true) {
  const entries = await iterator.nextv(100)

  if (entries.length === 0) {
    break
  }

  for (const [key, value] of entries) {
    // ..
  }
}

await iterator.close()

iterator.all([options][, callback])

Advance repeatedly and get all (remaining) entries as an array, automatically closing the iterator. Assumes that those entries fit in memory. If that's not the case, instead use next(), nextv() or for await...of. There are no options by default but implementations may add theirs. If an error occurs, the callback function will be called with an error. Otherwise, the callback receives null and an array of entries, where each entry is an array containing a key and value. If no callback is provided, a promise is returned.

const entries = await db.iterator({ limit: 100 }).all()

for (const [key, value] of entries) {
  // ..
}

iterator.seek(target[, options])

Seek to the key closest to target. Subsequent calls to iterator.next(), nextv() or all() (including implicit calls in a for await...of loop) will yield entries with keys equal to or larger than target, or equal to or smaller than target if the reverse option passed to db.iterator() was true.

The optional options object may contain:

• keyEncoding: custom key encoding, used to encode the target. By default the keyEncoding option of the iterator is used or (if that wasn't set) the keyEncoding of the database.

If range options like gt were passed to db.iterator() and target does not fall within that range, the iterator will reach its natural end.

Note: Not all implementations support seek(). Consult db.supports.seek or the support matrix.

iterator.close([callback])

Free up underlying resources. The callback function will be called with no arguments. If no callback is provided, a promise is returned. Closing the iterator is an idempotent operation, such that calling close() more than once is allowed and makes no difference.

If a next() ,nextv() or all() call is in progress, closing will wait for that to finish. After close() has been called, further calls to next() ,nextv() or all() will yield an error with code LEVEL_ITERATOR_NOT_OPEN.

iterator.db

A reference to the database that created this iterator.

iterator.count

Read-only getter that indicates how many keys have been yielded so far (by any method) excluding calls that errored or yielded undefined.

iterator.limit

Read-only getter that reflects the limit that was set in options. Greater than or equal to zero. Equals Infinity if no limit, which allows for easy math:

const hasMore = iterator.count < iterator.limit
const remaining = iterator.limit - iterator.count

keyIterator

A key iterator has the same interface as iterator except that its methods yield keys instead of entries. For the keyIterator.next(callback) method, this means that the callback will receive two arguments (an error and key) instead of three. Usage is otherwise the same.

valueIterator

A value iterator has the same interface as iterator except that its methods yield values instead of entries. For the valueIterator.next(callback) method, this means that the callback will receive two arguments (an error and value) instead of three. Usage is otherwise the same.

sublevel

A sublevel is an instance of the AbstractSublevel class, which extends AbstractLevel and thus has the same API as documented above. Sublevels have a few additional properties.

sublevel.prefix

Prefix of the sublevel. A read-only string property.

const example = db.sublevel('example')
const nested = example.sublevel('nested')

console.log(example.prefix) // '!example!'
console.log(nested.prefix) // '!example!!nested!'

sublevel.db

Parent database. A read-only property.

const example = db.sublevel('example')
const nested = example.sublevel('nested')

console.log(example.db === db) // true
console.log(nested.db === db) // true

Encodings

Any method that takes a key argument, value argument or range options like gte, hereby jointly referred to as data, runs that data through an encoding. This means to encode input data and decode output data.

Several encodings are builtin courtesy of level-transcoder and can be selected by a short name like 'utf8' or 'json'. The default encoding is 'utf8' which ensures you'll always get back a string. Encodings can be specified for keys and values independently with keyEncoding and valueEncoding options, either in the database constructor or per method to apply an encoding selectively. For example:

const db = level('./db', {
  keyEncoding: 'view',
  valueEncoding: 'json'
})

// Use binary keys
const key = Uint8Array.from([1, 2])

// Encode the value with JSON
await db.put(key, { x: 2 })

// Decode the value with JSON. Yields { x: 2 }
const obj = await db.get(key)

// Decode the value with utf8. Yields '{"x":2}'
const str = await db.get(key, { valueEncoding: 'utf8' })

The keyEncoding and valueEncoding options accept a string to select a known encoding by its name, or an object to use a custom encoding like charwise. See keyEncoding() for details. If a custom encoding is passed to the database constructor, subsequent method calls can refer to that encoding by name. Supported encodings are exposed in the db.supports manifest:

const db = level('./db', {
  keyEncoding: require('charwise'),
  valueEncoding: 'json'
})

// Includes builtin and custom encodings
console.log(db.supports.encodings.utf8) // true
console.log(db.supports.encodings.charwise) // true

An encoding can both widen and limit the range of data types. The default 'utf8' encoding can only store strings. Other types, though accepted, are irreversibly stringified before storage. That includes JavaScript primitives which are converted with String(x), Buffer which is converted with x.toString('utf8') and Uint8Array converted with TextDecoder#decode(x). Use other encodings for a richer set of data types, as well as binary data without a conversion cost - or loss of non-unicode bytes.

For binary data two builtin encodings are available: 'buffer' and 'view'. They use a Buffer or Uint8Array respectively. To some extent these encodings are interchangeable, as the 'buffer' encoding also accepts Uint8Array as input data (and will convert that to a Buffer without copying the underlying ArrayBuffer), the 'view' encoding also accepts Buffer as input data and so forth. Output data will be either a Buffer or Uint8Array respectively and can also be converted:

const db = level('./db', { valueEncoding: 'view' })
const buffer = await db.get('example', { valueEncoding: 'buffer' })

In browser environments it may be preferable to only use 'view'. When bundling JavaScript with Webpack, Browserify or other, you can choose not to use the 'buffer' encoding and (through configuration of the bundler) exclude the buffer shim in order to reduce bundle size.

Regardless of the choice of encoding, a key or value may not be null or undefined due to preexisting significance in iterators and streams. No such restriction exists on range options because null and undefined are significant types in encodings like charwise as well as some underlying stores like IndexedDB. Consumers of an abstract-level implementation must assume that range options like { gt: undefined } are not the same as {}. The abstract test suite does not test these types. Whether they are supported or how they sort may differ per implementation. An implementation can choose to:

• Encode these types to make them meaningful
• Have no defined behavior (moving the concern to a higher level)
• Delegate to an underlying database (moving the concern to a lower level).

Lastly, one way or another, every implementation must support data of type String and should support data of type Buffer or Uint8Array.

Events

An abstract-level database is an EventEmitter and emits the following events.

For example you can do:

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

Any keys, values and range options in these events are the original arguments passed to the relevant operation that triggered the event, before having encoded them.

Errors

Errors thrown or yielded from the methods above will have a code property that is an uppercase string. Error codes will not change between major versions, but error messages will. Messages may also differ between implementations; they are free and encouraged to tune messages.

A database may also throw TypeError errors (or other core error constructors in JavaScript) without a code and without any guarantee on the stability of error properties - because these errors indicate invalid arguments and other programming mistakes that should not be catched much less have associated logic.

Error codes will be one of the following.

LEVEL_NOT_FOUND

When a key was not found.

LEVEL_DATABASE_NOT_OPEN

When an operation was made on a database while it was closing or closed. Or when a database failed to open() including when close() was called in the mean time, thus changing the eventual status. The error may have a cause property that explains a failure to open:

try {
  await db.open()
} catch (err) {
  console.error(err.code) // 'LEVEL_DATABASE_NOT_OPEN'

  if (err.cause && err.cause.code === 'LEVEL_LOCKED') {
    // Another process or instance has opened the database
  }
}

LEVEL_DATABASE_NOT_CLOSED

When a database failed to close(). Including when open() was called in the mean time, thus changing the eventual status. The error may have a cause property that explains a failure to close.

LEVEL_ITERATOR_NOT_OPEN

When an operation was made on an iterator while it was closing or closed, which may also be the result of the database being closed.

LEVEL_ITERATOR_BUSY

When iterator.next() or seek() was called while a previous next() call was still in progress.

LEVEL_BATCH_NOT_OPEN

When an operation was made on a chained batch while it was closing or closed, which may also be the result of the database being closed or that write() was called on the chained batch.

LEVEL_ENCODING_NOT_FOUND

When a keyEncoding or valueEncoding option specified a named encoding that does not exist.

LEVEL_ENCODING_NOT_SUPPORTED

When a keyEncoding or valueEncoding option specified an encoding that isn't supported by the database.

LEVEL_DECODE_ERROR

When decoding of keys or values failed. The error may have a cause property containing an original error. For example, it might be a SyntaxError from an internal JSON.parse() call:

await db.put('key', 'invalid json', { valueEncoding: 'utf8' })

try {
  const value = await db.get('key', { valueEncoding: 'json' })
} catch (err) {
  console.log(err.code) // 'LEVEL_DECODE_ERROR'
  console.log(err.cause) // 'SyntaxError: Unexpected token i in JSON at position 0'
}

LEVEL_INVALID_KEY

When a key is null, undefined or (if an implementation deems it so) otherwise invalid.

LEVEL_INVALID_VALUE

When a value is null, undefined or (if an implementation deems it so) otherwise invalid.

LEVEL_CORRUPTION

Data could not be read (from an underlying store) due to a corruption.

LEVEL_IO_ERROR

Data could not be read (from an underlying store) due to an input/output error, for example from the filesystem.

LEVEL_INVALID_PREFIX

When a sublevel prefix contains characters outside of the supported byte range.

LEVEL_NOT_SUPPORTED

When a module needs a certain feature, typically as indicated by db.supports, but that feature is not available on a database argument or other. For example, some kind of plugin may depend on seek():

const ModuleError = require('module-error')

module.exports = function plugin (db) {
  if (!db.supports.seek) {
    throw new ModuleError('Database must support seeking', {
      code: 'LEVEL_NOT_SUPPORTED'
    })
  }

  // ..
}

LEVEL_LEGACY

When a method, option or other property was used that has been removed from the API.

LEVEL_LOCKED

When an attempt was made to open a database that is already open in another process or instance. Used by classic-level and other implementations of abstract-level that use exclusive locks.

LEVEL_READONLY

When an attempt was made to write data to a read-only database. Used by many-level.

LEVEL_CONNECTION_LOST

When a database relies on a connection to a remote party and that connection has been lost. Used by many-level.

LEVEL_REMOTE_ERROR

When a remote party encountered an unexpected condition that it can't reflect with a more specific code. Used by many-level.

Shared Access

Unless documented otherwise, implementations of abstract-level do not support accessing a database from multiple processes running in parallel. That includes Node.js clusters and Electron renderer processes.

See Level/awesome for modules like multileveldown and level-party that allow a database to be shared across processes and/or machines. Note: at the time of writing these modules are not yet ported to abstract-level and thus incompatible.

Private API For Implementors

To implement an abstract-level database, extend the AbstractLevel class and override the private underscored versions of its methods. For example, to implement the public put() method, override the private _put() method. The same goes for other classes (some of which are optional to override). All classes can be found on the main export of the npm package:

const {
  AbstractLevel,
  AbstractSublevel,
  AbstractIterator,
  AbstractKeyIterator,
  AbstractValueIterator,
  AbstractChainedBatch
} = require('abstract-level')

Naming-wise, implementations should use a class name in the form of *Level (suffixed, for example MemoryLevel) and an npm package name in the form of *-level (for example memory-level). While utilities and plugins should use a package name in the form of level-* (prefixed).

Each of the private methods listed below will receive exactly the number and types of arguments described, regardless of what is passed in through the public API. Public methods provide type checking: if a consumer calls db.batch(123) they'll get an error that the first argument must be an array. Optional arguments get sensible defaults: a db.get(key) call translates to a db._get(key, options, callback) call.

All callbacks are error-first and must be asynchronous. If an operation within your implementation is synchronous, invoke the callback on a next tick using microtask scheduling. For convenience, instances of AbstractLevel, AbstractIterator and other classes include a nextTick(fn, ...args) utility method that uses process.nextTick() in Node.js and queueMicrotask() in browsers. It's recommended to exclusively use this utility (including in unit tests) as it guarantees a consistent order of operations.

Where possible, the default private methods are sensible noops that do nothing. For example, db._open(callback) will merely invoke callback on a next tick. Other methods have functional defaults. Each method documents whether implementing it is mandatory.

When throwing or yielding an error, prefer using a known error code. If new codes are required for your implementation and you wish to use the LEVEL_ prefix for consistency, feel free to open an issue to discuss. We'll likely want to document those codes here.

Example

Let's implement a basic in-memory database:

const { AbstractLevel } = require('abstract-level')
const ModuleError = require('module-error')

class ExampleLevel extends AbstractLevel {
  // This in-memory example doesn't have a location argument
  constructor (options) {
    // Declare supported encodings
    const encodings = { utf8: true }

    // Call AbstractLevel constructor
    super({ encodings }, options)

    // Create a map to store entries
    this._entries = new Map()
  }

  _open (options, callback) {
    // Here you would open any necessary resources.
    // Use nextTick to be a nice async citizen
    this.nextTick(callback)
  }

  _put (key, value, options, callback) {
    this._entries.set(key, value)
    this.nextTick(callback)
  }

  _get (key, options, callback) {
    const value = this._entries.get(key)

    if (value === undefined) {
      return this.nextTick(callback, new ModuleError(`Key ${key} was not found`, {
        code: 'LEVEL_NOT_FOUND'
      }))
    }

    this.nextTick(callback, null, value)
  }

  _del (key, options, callback) {
    this._entries.delete(key)
    this.nextTick(callback)
  }
}

Now we can use our implementation (with either callbacks or promises):

const db = new ExampleLevel()

await db.put('foo', 'bar')
const value = await db.get('foo')

console.log(value) // 'bar'

Although our basic implementation only supports 'utf8' strings internally, we do get to use encodings that encode to that. For example, the 'json' encoding which encodes to 'utf8':

const db = new ExampleLevel({ valueEncoding: 'json' })
await db.put('foo', { a: 123 })
const value = await db.get('foo')

console.log(value) // { a: 123 }

See memory-level if you are looking for a complete in-memory implementation. The example above notably lacks iterator support and would not pass the abstract test suite.

db = AbstractLevel(manifest[, options])

The database constructor. Sets the status to 'opening'. Takes a manifest object that the constructor will enrich with defaults. At minimum, the manifest must declare which encodings are supported in the private API. For example:

class ExampleLevel extends AbstractLevel {
  constructor (location, options) {
    const manifest = {
      encodings: { buffer: true }
    }

    // Call AbstractLevel constructor.
    // Location is not handled by AbstractLevel.
    super(manifest, options)
  }
}

Both the public and private API of abstract-level are encoding-aware. This means that private methods receive keyEncoding and valueEncoding options too. Implementations don't need to perform encoding or decoding themselves. Rather, the keyEncoding and valueEncoding options are lower-level encodings that indicate the type of already-encoded input data or the expected type of yet-to-be-decoded output data. They're one of 'buffer', 'view', 'utf8' and always strings in the private API.

If the manifest declared support of 'buffer', then keyEncoding and valueEncoding will always be 'buffer'. If the manifest declared support of 'utf8' then keyEncoding and valueEncoding will be 'utf8'.

For example: a call like await db.put(key, { x: 2 }, { valueEncoding: 'json' }) will encode the { x: 2 } value and might forward it to the private API as db._put(key, '{"x":2}', { valueEncoding: 'utf8' }, callback). Same for the key (omitted for brevity).

The public API will coerce user input as necessary. If the manifest declared support of 'utf8' then await db.get(24) will forward that number key as a string: db._get('24', { keyEncoding: 'utf8', ... }, callback). However, this is not true for output: a private API call like db._get(key, { keyEncoding: 'utf8', valueEncoding: 'utf8' }, callback) must yield a string value to the callback.

All private methods below that take a key argument, value argument or range option, will receive that data in encoded form. That includes iterator._seek() with its target argument. So if the manifest declared support of 'buffer' then db.iterator({ gt: 2 }) translates into db._iterator({ gt: Buffer.from('2'), ...options }) and iterator.seek(128) translates into iterator._seek(Buffer.from('128'), options).

The AbstractLevel constructor will add other supported encodings to the public manifest. If the private API only supports 'buffer', the resulting db.supports.encodings will nevertheless be as follows because all other encodings can be transcoded to 'buffer':

{ buffer: true, view: true, utf8: true, json: true, ... }

Implementations can also declare support of multiple encodings. Keys and values will then be encoded and decoded via the most optimal path. For example, in classic-level (previously leveldown) it's:

super({ encodings: { buffer: true, utf8: true } }, options, callback)

This has the benefit that user input needs less conversion steps: if the input is a string then classic-level can pass that to its LevelDB binding as-is. Vice versa for output.

db._open(options, callback)

Open the database. The options object will always have the following properties: createIfMissing, errorIfExists. If opening failed, call the callback function with an error. Otherwise call callback without any arguments.

The default _open() is a sensible noop and invokes callback on a next tick.

db._close(callback)

Close the database. When this is called, db.status will be 'closing'. If closing failed, call the callback function with an error, which resets the status to 'open'. Otherwise call callback without any arguments, which sets status to 'closed'. Make an effort to avoid failing, or if it does happen that it is subsequently safe to keep using the database. If the database was never opened or failed to open then _close() will not be called.

The default _close() is a sensible noop and invokes callback on a next tick. In native implementations (native addons written in C++ or other) it's recommended to delay closing if any operations are in flight. See classic-level (previously leveldown) for an example of this behavior. The JavaScript side in abstract-level will prevent new operations before the database is reopened (as explained in constructor documentation above) while the C++ side should prevent closing the database before existing operations have completed.

db._get(key, options, callback)

Get a value by key. The options object will always have the following properties: keyEncoding and valueEncoding. If the key does not exist, call the callback function with an error that has code LEVEL_NOT_FOUND. Otherwise call callback with null as the first argument and the value as the second.

The default _get() invokes callback on a next tick with a LEVEL_NOT_FOUND error. It must be overridden.

db._getMany(keys, options, callback)

Get multiple values by an array of keys. The options object will always have the following properties: keyEncoding and valueEncoding. If an error occurs, call the callback function with an error. Otherwise call callback with null as the first argument and an array of values as the second. If a key does not exist, set the relevant value to undefined.

The defaul

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