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README.md
abstract-leveldown
An abstract prototype matching the
leveldown
API. Useful for extendinglevelup
functionality by providing a replacement toleveldown
.
Table of Contents
Click to expand
Background
This module provides a simple base prototype for a key-value store. It has a public API for consumers and a private API for implementors. To implement a abstract-leveldown
compliant store, extend its prototype and override the private underscore versions of the methods. For example, to implement put()
, override _put()
on your prototype.
Where possible, the default private methods have sensible noop defaults that essentially do nothing. For example, _open(callback)
will invoke callback
on a next tick. Other methods like _clear(..)
have functional defaults. Each method listed below documents whether implementing it is mandatory.
The private methods are always provided with consistent arguments, regardless of what is passed in through the public API. All public methods provide argument checking: if a consumer calls open()
without a callback argument they'll get an Error('open() requires a callback argument')
.
Where optional arguments are involved, private methods receive sensible defaults: a get(key, callback)
call translates to _get(key, options, callback)
where the options
argument is an empty object. These arguments are documented below.
If you are upgrading: please see UPGRADING.md.
Example
Let's implement a simplistic in-memory leveldown
replacement:
var AbstractLevelDOWN = require('abstract-leveldown').AbstractLevelDOWN
var util = require('util')
// Constructor
function FakeLevelDOWN () {
AbstractLevelDOWN.call(this)
}
// Our new prototype inherits from AbstractLevelDOWN
util.inherits(FakeLevelDOWN, AbstractLevelDOWN)
FakeLevelDOWN.prototype._open = function (options, callback) {
// Initialize a memory storage object
this._store = {}
// Use nextTick to be a nice async citizen
process.nextTick(callback)
}
FakeLevelDOWN.prototype._serializeKey = function (key) {
// As an example, prefix all input keys with an exclamation mark.
// Below methods will receive serialized keys in their arguments.
return '!' + key
}
FakeLevelDOWN.prototype._put = function (key, value, options, callback) {
this._store[key] = value
process.nextTick(callback)
}
FakeLevelDOWN.prototype._get = function (key, options, callback) {
var value = this._store[key]
if (value === undefined) {
// 'NotFound' error, consistent with LevelDOWN API
return process.nextTick(callback, new Error('NotFound'))
}
process.nextTick(callback, null, value)
}
FakeLevelDOWN.prototype._del = function (key, options, callback) {
delete this._store[key]
process.nextTick(callback)
}
Now we can use our implementation with levelup
:
var levelup = require('levelup')
var db = levelup(new FakeLevelDOWN())
db.put('foo', 'bar', function (err) {
if (err) throw err
db.get('foo', function (err, value) {
if (err) throw err
console.log(value) // 'bar'
})
})
See memdown
if you are looking for a complete in-memory replacement for leveldown
.
Browser Support
Public API For Consumers
db = constructor(..)
Constructors typically take a location
argument pointing to a location on disk where the data will be stored. Since not all implementations are disk-based and some are non-persistent, implementors are free to take zero or more arguments in their constructor.
db.status
A read-only property. An abstract-leveldown
compliant store can be in one of the following states:
'new'
- newly created, not opened or closed'opening'
- waiting for the store to be opened'open'
- successfully opened the store, available for use'closing'
- waiting for the store to be closed'closed'
- store has been successfully closed, should not be used.
db.supports
A read-only manifest. Might be used like so:
if (!db.supports.permanence) {
throw new Error('Persistent storage is required')
}
if (db.supports.bufferKeys && db.supports.promises) {
await db.put(Buffer.from('key'), 'value')
}
db.open([options, ]callback)
Open the store. The callback
function will be called with no arguments when the store has been successfully opened, or with a single error argument if the open operation failed for any reason.
The optional options
argument may contain:
createIfMissing
(boolean, default:true
): Iftrue
and the store doesn't exist it will be created. Iffalse
and the store doesn't exist,callback
will receive an error.errorIfExists
(boolean, default:false
): Iftrue
and the store exists,callback
will receive an error.
Not all implementations support the above options.
db.close(callback)
Close the store. The callback
function will be called with no arguments if the operation is successful or with a single error
argument if closing failed for any reason.
db.get(key[, options], callback)
Get a value from the store by key
. The optional options
object may contain:
asBuffer
(boolean, default:true
): Whether to return thevalue
as a Buffer. Iffalse
, the returned type depends on the implementation.
The callback
function will be called with an Error
if the operation failed for any reason. If successful the first argument will be null
and the second argument will be the value.
db.put(key, value[, options], callback)
Store a new entry or overwrite an existing entry. There are no options
by default but implementations may add theirs. The callback
function will be called with no arguments if the operation is successful or with an Error
if putting failed for any reason.
db.del(key[, options], callback)
Delete an entry. There are no options
by default but implementations may add theirs. The callback
function will be called with no arguments if the operation is successful or with an Error
if deletion failed for any reason.
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 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.
There are no options
by default but implementations may add theirs. 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()
Returns a chainedBatch
.
db.iterator([options])
Returns an iterator
. Accepts the following range options:
gt
(greater than),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. Whenreverse=true
the order will be reversed, but the entries iterated will be the same.lt
(less than),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. Whenreverse=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:-1
): limit the number of entries collected by this iterator. 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 returned instead of the lowest keys.
Legacy options:
start
: instead usegte
end
: instead uselte
.
Note Zero-length strings, buffers and arrays as well as null
and undefined
are invalid as keys, yet valid as range options. These types are significant in encodings like bytewise
and charwise
as well as some underlying stores like IndexedDB. Consumers of an implementation should assume that { gt: undefined }
is not the same as {}
. An implementation can choose to:
- Serialize or encode these types to make them meaningful
- Have no defined behavior (moving the concern to a higher level)
- Delegate to an underlying store (moving the concern to a lower level).
If you are an implementor, a final note: the abstract test suite does not test these types. Whether they are supported or how they sort is up to you; add custom tests accordingly.
In addition to range options, iterator()
takes the following options:
keys
(boolean, default:true
): whether to return the key of each entry. If set tofalse
, calls toiterator.next(callback)
will yield keys with a value ofundefined
.values
(boolean, default:true
): whether to return the value of each entry. If set tofalse
, calls toiterator.next(callback)
will yield values with a value ofundefined
.keyAsBuffer
(boolean, default:true
): Whether to return the key of each entry as a Buffer. Iffalse
, the returned type depends on the implementation.valueAsBuffer
(boolean, default:true
): Whether to return the value of each entry as a Buffer.
Lastly, an implementation is free to add its own options.
db.clear([options, ]callback)
This method is experimental. Not all implementations support it yet.
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.
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.
There are no options
by default but implementations may add theirs. 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
An iterator allows you to iterate the entire store or a range. It operates on a snapshot of the store, created at the time db.iterator()
was called. This means reads on the iterator are unaffected by simultaneous writes. Most but not all implementations can offer this guarantee.
An iterator keeps track of when a next()
is in progress and when an end()
has been called so it doesn't allow concurrent next()
calls, it does allow end()
while a next()
is in progress and it doesn't allow either next()
or end()
after end()
has been called.
iterator.next(callback)
Advance the iterator and yield the entry at that key. 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 end, both key
and value
will be undefined
. This happens in the following situations:
- The end of the store has been reached
- The end of the range has been reached
- The last
iterator.seek()
was out of range.
Note: Don't forget to call iterator.end()
, even if you received an error.
iterator.seek(target)
Seek the iterator to a given key or the closest key. Subsequent calls to iterator.next()
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.
If range options like gt
were passed to db.iterator()
and target
does not fall within that range, the iterator will reach its end.
Note: At the time of writing, leveldown
is the only known implementation to support seek()
. In other implementations, it is a noop.
iterator.end(callback)
End iteration and free up underlying resources. The callback
function will be called with no arguments on success or with an Error
if ending failed for any reason.
iterator.db
A reference to the db
that created this iterator.
Type Support
The following applies to any method above that takes a key
argument or option: all implementations must support a key
of type String and should support a key
of type Buffer. A key
may not be null
, undefined
, a zero-length Buffer, zero-length string or zero-length array.
The following applies to any method above that takes a value
argument or option: all implementations must support a value
of type String or Buffer. A value
may not be null
or undefined
due to preexisting significance in streams and iterators.
Support of other key and value types depends on the implementation as well as its underlying storage. See also db._serializeKey
and db._serializeValue
.
Private API For Implementors
Each of these methods will receive exactly the number and order of arguments described. Optional arguments will receive sensible defaults. All callbacks are error-first and must be asynchronous.
If an operation within your implementation is synchronous, be sure to invoke the callback on a next tick using process.nextTick
or some other means of microtask scheduling. For convenience, the prototypes of AbstractLevelDOWN
, AbstractIterator
and AbstractChainedBatch
include a _nextTick
method that is compatible with node and browsers.
db = AbstractLevelDOWN([manifest])
The constructor. Sets the .status
to 'new'
. Optionally takes a manifest object which abstract-leveldown
will enrich:
AbstractLevelDOWN.call(this, {
bufferKeys: true,
snapshots: true,
// ..
})
db._open(options, callback)
Open the store. 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 store. If closing failed, call the callback
function with an Error
. Otherwise call callback
without any arguments.
The default _close()
is a sensible noop and invokes callback
on a next tick.
db._serializeKey(key)
Convert a key
to a type supported by the underlying storage. All methods below that take a key
argument or option - including db._iterator()
with its range options and iterator._seek()
with its target
argument - will receive serialized keys. For example, if _serializeKey
is implemented as:
FakeLevelDOWN.prototype._serializeKey = function (key) {
return Buffer.isBuffer(key) ? key : String(key)
}
Then db.get(2, callback)
translates into db._get('2', options, callback)
. Similarly, db.iterator({ gt: 2 })
translates into db._iterator({ gt: '2', ... })
and iterator.seek(2)
translates into iterator._seek('2')
.
If the underlying storage supports any JavaScript type or if your implementation wraps another implementation, it is recommended to make _serializeKey
an identity function (returning the key as-is). Serialization is irreversible, unlike encoding as performed by implementations like encoding-down
. This also applies to _serializeValue
.
The default _serializeKey()
is an identity function.
db._serializeValue(value)
Convert a value
to a type supported by the underlying storage. All methods below that take a value
argument or option will receive serialized values. For example, if _serializeValue
is implemented as:
FakeLevelDOWN.prototype._serializeValue = function (value) {
return Buffer.isBuffer(value) ? value : String(value)
}
Then db.put(key, 2, callback)
translates into db._put(key, '2', options, callback)
.
The default _serializeValue()
is an identity function.
db._get(key, options, callback)
Get a value by key
. The options
object will always have the following properties: asBuffer
. If the key does not exist, call the callback
function with a new Error('NotFound')
. 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 NotFound
error. It must be overridden.
db._put(key, value, options, callback)
Store a new entry or overwrite an existing entry. There are no default options but options
will always be an object. If putting failed, call the callback
function with an Error
. Otherwise call callback
without any arguments.
The default _put()
invokes callback
on a next tick. It must be overridden.
db._del(key, options, callback)
Delete an entry. There are no default options but options
will always be an object. If deletion failed, call the callback
function with an Error
. Otherwise call callback
without any arguments.
The default _del()
invokes callback
on a next tick. It must be overridden.
db._batch(operations, options, callback)
Perform multiple put and/or del operations in bulk. The operations
argument is always an Array
containing a list of operations to be executed sequentially, although as a whole they should be performed as an atomic operation. Each operation is guaranteed to have at least type
and key
properties. There are no default options but options
will always be an object. If the batch failed, call the callback
function with an Error
. Otherwise call callback
without any arguments.
The default _batch()
invokes callback
on a next tick. It must be overridden.
db._chainedBatch()
The default _chainedBatch()
returns a functional AbstractChainedBatch
instance that uses db._batch(array, options, callback)
under the hood. The prototype is available on the main exports for you to extend. If you want to implement chainable batch operations in a different manner then you should extend AbstractChainedBatch
and return an instance of this prototype in the _chainedBatch()
method:
var AbstractChainedBatch = require('abstract-leveldown').AbstractChainedBatch
var inherits = require('util').inherits
function ChainedBatch (db) {
AbstractChainedBatch.call(this, db)
}
inherits(ChainedBatch, AbstractChainedBatch)
FakeLevelDOWN.prototype._chainedBatch = function () {
return new ChainedBatch(this)
}
db._iterator(options)
The default _iterator()
returns a noop AbstractIterator
instance. It must be overridden, by extending AbstractIterator
(available on the main module exports) and returning an instance of this prototype in the _iterator(options)
method.
The options
object will always have the following properties: reverse
, keys
, values
, limit
, keyAsBuffer
and valueAsBuffer
.
db._clear(options, callback)
This method is experimental and optional for the time being. To enable its tests, set the clear
option of the test suite to true
.
Delete all entries or a range. Does not have to be atomic. It is recommended (and possibly mandatory in the future) to operate on a snapshot so that writes scheduled after a call to clear()
will not be affected.
The default _clear()
uses _iterator()
and _del()
to provide a reasonable fallback, but requires binary key support. It is recommended to implement _clear()
with more performant primitives than _iterator()
and _del()
if the underlying storage has such primitives. Implementations that don't support binary keys must implement their own _clear()
.
Implementations that wrap another db
can typically forward the _clear()
call to that db
, having transformed range options if necessary.
The options
object will always have the following properties: reverse
and limit
.
iterator = AbstractIterator(db)
The first argument to this constructor must be an instance of your AbstractLevelDOWN
implementation. The constructor will set iterator.db
which is used to access db._serialize*
and ensures that db
will not be garbage collected in case there are no other references to it.
iterator._next(callback)
Advance the iterator and yield the entry at that key. If nexting failed, call the callback
function with an Error
. Otherwise, call callback
with null
, a key
and a value
.
The default _next()
invokes callback
on a next tick. It must be overridden.
iterator._seek(target)
Seek the iterator to a given key or the closest key. This method is optional.
iterator._end(callback)
Free up underlying resources. This method is guaranteed to only be called once. If ending failed, call the callback
function with an Error
. Otherwise call callback
without any arguments.
The default _end()
invokes callback
on a next tick. Overriding is optional.
chainedBatch = AbstractChainedBatch(db)
The first argument to this constructor must be an instance of your AbstractLevelDOWN
implementation. The constructor will set chainedBatch.db
which is used to access db._serialize*
and ensures that db
will not be garbage collected in case there are no other references to it.
chainedBatch._put(key, value)
Queue a put
operation on this batch.
chainedBatch._del(key)
Queue a del
operation on this batch.
chainedBatch._clear()
Clear all queued operations on this batch.
chainedBatch._write(options, callback)
The default _write
method uses db._batch
. If the _write
method is overridden it must atomically commit the queued operations. There are no default options but options
will always be an object. If committing fails, call the callback
function with an Error
. Otherwise call callback
without any arguments.
Test Suite
To prove that your implementation is abstract-leveldown
compliant, include the abstract test suite in your test.js
(or similar):
const test = require('tape')
const suite = require('abstract-leveldown/test')
const YourDOWN = require('.')
suite({
test: test,
factory: function () {
return new YourDOWN()
}
})
This is the most minimal setup. The test
option must be a function that is API-compatible with tape
. The factory
option must be a function that returns a unique and isolated database instance. The factory will be called many times by the test suite.
If your implementation is disk-based we recommend using tempy
(or similar) to create unique temporary directories. Your setup could look something like:
const test = require('tape')
const tempy = require('tempy')
const suite = require('abstract-leveldown/test')
const YourDOWN = require('.')
suite({
test: test,
factory: function () {
return new YourDOWN(tempy.directory())
}
})
Excluding tests
As not every implementation can be fully compliant due to limitations of its underlying storage, some tests may be skipped. For example, to skip snapshot tests:
suite({
// ..
snapshots: false
})
This also serves as a signal to users of your implementation. The following options are available:
bufferKeys
: set tofalse
if binary keys are not supported by the underlying storageseek
: set tofalse
if youriterator
does not implement_seek
clear
: defaults tofalse
until a next major release. Set totrue
if your implementation either implements_clear()
itself or is suitable to use the default implementation of_clear()
(which requires binary key support).snapshots
: set tofalse
if any of the following is true:- Reads don't operate on a snapshot
- Snapshots are created asynchronously
createIfMissing
anderrorIfExists
: set tofalse
ifdb._open()
does not support these options.
This metadata will be moved to manifests (db.supports
) in the future.
Setup and teardown
To perform (a)synchronous work before or after each test, you may define setUp
and tearDown
functions:
suite({
// ..
setUp: function (t) {
t.end()
},
tearDown: function (t) {
t.end()
}
})
Reusing testCommon
The input to the test suite is a testCommon
object. Should you need to reuse testCommon
for your own (additional) tests, use the included utility to create a testCommon
with defaults:
const test = require('tape')
const suite = require('abstract-leveldown/test')
const YourDOWN = require('.')
const testCommon = suite.common({
test: test,
factory: function () {
return new YourDOWN()
}
})
suite(testCommon)
The testCommon
object will have all the properties describe above: test
, factory
, setUp
, tearDown
and the skip options. You might use it like so:
test('setUp', testCommon.setUp)
test('custom test', function (t) {
var db = testCommon.factory()
// ..
})
test('another custom test', function (t) {
var db = testCommon.factory()
// ..
})
test('tearDown', testCommon.tearDown)
Spread The Word
If you'd like to share your awesome implementation with the world, here's what you might want to do:
- Add an awesome badge to your
README
:![level badge](https://leveljs.org/img/badge.svg)
- Publish your awesome module to npm
- Send a Pull Request to Level/awesome to advertise your work!
Install
With npm do:
npm install abstract-leveldown
Contributing
Level/abstract-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.
Big Thanks
Cross-browser Testing Platform and Open Source ♥ Provided by Sauce Labs.
Donate
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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. 💖
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License
MIT © 2013-present Rod Vagg and Contributors.