Compose a database factory from
abstract-leveldownandleveluplayers with predefined defaults per layer.
Click to expand
const compose = require('level-compose')
const leveldown = require('leveldown')
const encode = require('encoding-down')
const levelup = require('levelup')In its simplest form:
const factory = compose(leveldown, encode, levelup)
const db = factory('./db')In node the above is functionally equivalent to require('level')('./db'). If you like a more expressive API the above can be written as:
const factory = compose()
.use(leveldown)
.use(encode)
.use(levelup)Note that compose(...x) is the same as compose().use(...x), just shorter if we have a single input. Let's define some defaults:
const factory = compose()
.use(leveldown)
.use(encode, { valueEncoding: 'json' })
.use(levelup)We can also pass arrays (useful when layers are defined elsewhere):
const factory = compose([
leveldown,
[encode, { valueEncoding: 'json' }],
levelup
])Options objects are given to the layer before it, or multiple layers if the argument preceding the options object is an array:
const factory = compose([leveldown, encode, levelup], {
valueEncoding: 'json'
})We can also use these mechanisms to make a "preset":
const preset = [encode, { valueEncoding: 'json' }]
const factory = compose(leveldown, preset)Similarly, you could create a preset for leveldown with certain cache options. This doesn't work yet because leveldown options must be passed to its open() method and not its constructor. Should we change that?
const cacheSize = 16 << 20 // 16 MB
const preset = [leveldown, { cacheSize }]
const factory = compose(preset, levelup)The first layer can also be a function that merely returns a location:
const tempy = require('tempy')
const location = (loc) => loc || tempy.directory()
const factory = compose(location, leveldown)
factory() // Returns a db in a temporary directory
factory('./db') // Returns a db in ./dbWhat about level-packager?
compose(down, encode, levelup) is functionally equivalent to packager(down). If you need exactly that, stick with level-packager, which is a "preset" meant for the most common use case: creating a database with encodings and deferred open. That preset is simple but cannot be changed. With level-compose you can create your own "presets" and specify default options per layer. Unlike level-packager however, level-compose does not include any layers of its own. You must install them separately.
This project is also meant as an exploration, to find differences between implementations and gaps in their composability. It asserts that if all abstract-leveldown implementations behave the same, it should be possible to chain them together in a generic way.
To level-compose a layer is just a function. That takes 1) an optional location or a db to be wrapped, 2) options and 3) an optional open-callback (which is currently only supported by levelup). The function should return a db object that has either an abstract-leveldown or levelup interface.
The behavior of a composed database depends on whether levelup is included in the layers. If it is, it must be the last and will make the database automatically open itself. If levelup is not included, you must open the database yourself and wait until it is before calling operations like db.put(). This will change somewhere in the future.
As a transitional utility, passing a callback into a composed database that doesn't use levelup will auto-open it:
const factory = compose(leveldown)
factory('./db', function (err, db) {
if (err) throw err // Failed to open
})This also works for implementations that don't have a location:
const memdown = require('memdown')
const factory = compose(memdown)
factory(function (err, db) {
if (err) throw err // Failed to open
})Yes, absolutely. You don't have to use level-compose if you have no need to reuse its input or return value elsewhere.
Not yet. Down the line it'd be nice if we could do something like:
const subdown = require('subleveldown')
const storage = leveldown('./db')
const factory = compose()
.use(storage)
.use(subdown, { separator: '!' })
.use(encode)
const db1 = factory({ prefix: '1', valueEncoding: 'utf8' })
const db2 = factory({ prefix: '2', valueEncoding: 'json' })To get there, we plan to move functionality like deferred-open from levelup into abstract-leveldown. In addition level-compose would have to detect whether an argument is a db instance or an options object (or require it to be wrapped like () => storage).
Yet to document.
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