level-supports v6.2.0

level-supports

Create a manifest describing the abilities of an abstract-level database. No longer compatible with levelup or abstract-leveldown since version 3.0.0.

Usage

const { supports } = require('level-supports')

db.supports = supports({
  permanence: false,
  encodings: {
    utf8: true
  }
})

Receivers of the db can then use it like so:

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

API

manifest = supports([manifest, ..])

Given zero or more manifest objects, returns a merged and enriched manifest object that has truthy properties for each of the features listed below.

For future extensibility, the properties are truthy rather than strictly typed booleans. Falsy or absent properties are converted to false, other values are allowed:

supports().seek // false
supports({ seek: true }).seek // true
supports({ seek: {} }).seek // {}
supports({ seek: 1 }, { seek: 2 }).seek // 2

For consumers of the manifest this means they should check support like so:

if (db.supports.seek)

Rather than:

if (db.supports.seek === true)

Note: the manifest describes high-level features that typically encompass multiple methods of a db. It is currently not a goal to describe a full API, or versions of it.

Features

implicitSnapshots (boolean)

Does the database read from a snapshot as described in abstract-level? Must be false if any of the following is true:

• Reads don't operate on a snapshot
• Snapshots are created asynchronously.

Aliased as snapshots for backwards compatibility.

explicitSnapshots (boolean)

Does the database implement db.snapshot() and do read methods accept a snapshot option as described in abstract-level?

has (boolean)

Does the database implement has() and hasMany()? Tracked in Level/community#142.

permanence (boolean)

Does data survive after process (or environment) exit? Typically true. False for memory-level.

seek (boolean)

Do iterators support seek(..)?

deferredOpen (boolean)

Can operations like db.put() be called without explicitly opening the db? Like so:

const db = new Level()
await db.put('key', 'value')

Always true since abstract-level@1.

createIfMissing, errorIfExists (boolean)

Does db.open() support these options?

events (object)

Which events does the database emit, as indicated by nested properties? For example:

if (db.supports.events.put) {
  db.on('put', listener)
}

streams (boolean)

Does database have the methods createReadStream, createKeyStream and createValueStream, following the API documented in the legacy levelup module? Always false since the introduction of abstract-level which moved streams to a standalone module called level-read-stream.

encodings (object)

Which encodings (by name) does the database support, as indicated by nested properties? For example:

{ utf8: true, json: true }

As the encodings property cannot be false (anymore, since level-supports v3.0.0) it implies that the database supports keyEncoding and valueEncoding options on all relevant methods, uses a default encoding of utf8 and that hence all of its read operations return strings rather than buffers by default.

This matrix lists which encodings are supported as indicated by e.g. db.supports.encodings.utf8. Encodings that encode to another (like how 'json' encodes to 'utf8') are excluded here, though they are present in db.supports.encodings.

1. Transcoded (which may have a performance impact).
2. Can be controlled via storeEncoding option.
3. Whether it's transcoded depends on environment (Node.js or browser).

additionalMethods (object)

Declares support of additional methods, that are not part of the abstract-level interface. In the form of:

{
  foo: true,
  bar: true
}

Which says the db has two methods, foo and bar. It might be used like so:

if (db.supports.additionalMethods.foo) {
  db.foo()
}

For future extensibility, the properties of additionalMethods should be taken as truthy rather than strictly typed booleans. We may add additional metadata (see #1).

signals (object)

Which methods or method groups take a signal option? At the time of writing there is only one method group: iterators. This includes db.iterator(), db.keys() and db.values(). For example:

if (db.supports.signals.iterators) {
  const ac = new AbortController()
  const iterator = db.keys({ signal: ac.signal })

  ac.abort()
}

Install

With npm do:

npm install level-supports

Contributing

Level/supports 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.

Donate

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

License

MIT