delta-crdts

Delta state-based CRDTs in Javascript.

• (See original paper)
• Demo video

Install

$ npm install delta-crdts

Import

const CRDTs = require('delta-crdts')

Instantiate a type

const type = 'rga' // or any of the other supported CRDT types
const Type = CRDT(type)

Create a replica

To create a replica you need pass in a unique node id.

const replica = Type('node id')

Mutate that replica

const deltas = []
deltas.push(replica.push('some value'))
deltas.push(replica.insertAt(0, 'some other value'))

Create a second replica

const replica2 = Type('node id 2')

Apply the deltas

deltas.forEach((delta) => replica2.apply(delta))

Query the value

replica2.value() // ['some value', 'some other value']

Initialize a replica from the entire state

const replica3 = Type('node id 3')
replica3.apply(replica2.state())

Conflict management

You can do concurrent edits on both replicas:

// create 2 replicas
const replicas = [Type('id1'), Type('id2')]

// create concurrent deltas
const deltas = [[], []]

deltas[0].push(replicas[0].push('a'))
deltas[0].push(replicas[0].push('b'))

deltas[1].push(replicas[1].push('c'))
deltas[1].push(replicas[1].push('d'))

deltas[0].forEach((delta) => replicas[1].apply(delta))
deltas[1].forEach((delta) => replicas[0].apply(delta))

assert.deepEqual(replicas[0].value(), replicas[1].value())

Extend

You can extend the types, creating your own CRDT.

Example:

const Zero = {
  initial: () => 0,
  join: (s1, s2) => 0,
  value: (state) => state,
  mutators: {
    doSomething (id, state, arg1) => {
      // example mutator, returning a delta
      return 0
    }
  }
}

CRDT.define('zero', Zero)

// now you can use it

const replica = CRDT('zero')('node id')

Support for incremental value computation

It's possible to allow types to have incremental value computation. If a type supports that, the value is incrementally computed on each delta that is applied.

To add support for incremental value computation to a CRDT, the type definition should support the following function:

Type.incrementalValue = function (beforeState, newState, delta, cache = { value: <some initial value>, ... }) {
  // ...
}

As an example you can get inspiration from the RGA implementation.

Types

The following types are built-in:

(* means that the type is causal and can be embedded in an ORMap)

Counters

Name	Identifier	Mutators	Value Type
Increment-only Counter	gcounter	.inc()	int
PN-Counter	pncounter	.inc(),.dec()	int
Lex-Counter	lexcounter	.inc(),.dec()	int
Causal Counter *	ccounter	.inc(),.dec()	int

Flags

Name	Identifier	Mutators	Value Type
Enable-Wins Flag *	ewflag	.enable(), .disable()	Boolean
Disable-Wins Flag *	dwflag	.enable(), .disable()	Boolean

Sets

Name	Identifier	Mutators	Value Type
Grow-Only Set	gset	.add(element)	Set
Two-Phase Set	2pset	.add(element), .remove(element)	Set
Add-Wins-Observed-Remove Set *	aworset	.add(element), .remove(element)	Set
Remove-Wins-Observed-Remove Set *	rworset	.add(element), .remove(element)	Set
Remove-Wins-Last-Write-Wins Set	rwlwwset	.add(element), .remove(element)	Set

Arrays

Name	Identifier	Mutators	Value Type
Replicable Growable Array	rga	.push(element), .insertAt(pos, element), .removeAt(pos), updateAt(pos, element), insertAllAt(pos, elements)	Array

Registers

Name	Identifier	Mutators	Value Type
Last-Write-Wins Register	lwwreg	.write(value)	Value
Multi-Value Register *	mvreg	.write(value)	Set of concurrent values

Maps

Name	Identifier	Mutators	Value Type
Observed-Remove Map *	ormap	.remove(key), applySub(key, crdt_name, mutator_name, ...args)	Object

Embedding CRDTs in ORMaps

OR-Maps support embedding of other causal CRDTs. Example:

const ORMap = CRDT('ormap')
const m = ORMap('id1')
const delta = m.applySub('a', 'mvreg', 'write', 'A')
console.log(m.value()) // => {a: new Set(['A'])}

Of this collection, causal CRDTs are:

• AWORSet
• CCounter
• DWFlag
• EWFlag
• MVReg
• ORMap
• RWORSet

Sets, uniqueness and object id

For testing uniqueness in a way that is safe when replicas are distributed, for objects we calculate the hash using the hast-it package.

If you want, you can override it by providing a hashCode attribute in your object.

For all objects where typeof object !== 'object', we use the value itself as comparison.

Static methods

You may get the static definition of a type by doing

const type = CRDT.type(typeName)

Each type has a series of static methods may need to use:

Type.initial()

Returns the initial state for the type. Example:

const GCounter = CRDT.type('gcounter')
const initial = GCounter.initial()

Type.value(state)

Returns the view value of a given state.

Type.join(s1, s2)

Joins two states (or deltas) and returns the result.

const GCounter = CRDT.type('gcounter')

const state = GCounter.join(delta1, delta)

const value = GCounter.value(state)

Example of using static methods:

const GCounter = CRDT('gcounter')

deltas = []
deltas.push(replica1.inc())
deltas.push(replica1.inc())
deltas.push(replica1.inc())

const bigDelta = deltas.reduce(GCounter.join, GCounter.initial())

replica2.apply(bigDelta)

License

MIT