observed-remove-set

An "Observed-Remove Set" or "OR-Set", is a set that can be modified concurrently and will eventually reach the same state everywhere (it is "eventually consistent").

A casuality-tracking method is also included, allowing operations to be delivered in any order and more-than-once.

Also see observed-remove-map, which is built on top of this module.

example

var set1 = new OrSet('bob')
var set2 = new OrSet('alice')
 
// let's provide a "delay" function to simulate concurrency on a network
function delay (cb) {
  setTimeout(cb, 3000) // 3 seconds of delay!
}
 
// let's "connect" our two sets
set1.on('op', op => delay(() => set2.receive(op)))
set2.on('op', op => delay(() => set1.receive(op)))
 
// both sets start with a single a
set1.add('a') 
set2.add('a')
 
// now below, we introduce a conflict
set1.add('a')
set2.delete('a')

Look at the last two lines. There is a conflict there! Did set1 delete the 'a' and re-add it? Or did it add a redundant 'a' only to delete it soon after? With normal sets, we cannot know and we may end up with set1 = [] and set2 = ['a'].

However, these are OR-Sets and have eventual consistency. Therefore we can be certain that set1 = set2. We can't predict the end state, but we are guaranteed that all connected sets will reach the same state at the end of our operations.

install

npm install observed-remove-set

<script src="dist/or-set.js"></script>

api

orSet = new OrSet(uuid, [opts])

Create a new OR-Set.

Required uuid is some universally unique identifer for this set.

Optional opts defaults to the following object:

{
  state: null, // an initial state from getState()
  serialize: JSON.serialize, // function used to internally serialize elements
  parse: JSON.parse // function used to internally parse elements
}

orSet.add(element)

Add an element to the set.

element is any Javascript object. Changes to this object will NOT be replicated.

orSet.delete(element)

Remove an element from the set.

orSet.has(element)

Returns true if element is contained within the set, false otherwise.

orSet.values()

Returns an array with all elements within the set.

orSet.size()

Returns an integer that is the size of set.

orSet.receive(op)

Receive an operation from a remote set. Must be called exactly once per remote operation.

orSet.on('op', function (op) {})

Fires when an operation needs to be sent to connected sets. Operations can be delivered in any order but should be delivered at least once.

op is the operation object that needs to be passed into otherOrSet.receive(op) for all other replicas.

orSet.on('add', function (element) {})

Fires when an element is added to the set by a remote operation. (will not fire when orSet.add() is called locally.)

orSet.on('delete', function (element) {})

Fires when an element is removed from the set by a remote operation. (will not fire when orSet.delete() is called locally.)

orSet.getState()

Get the underlying state of the CRDT. Can be used to transfer state to peers that are just joining and have missed operations. Keep in mind that operations can be sent while transfering state and a 2-step sync is required.

orSet.setState(state)

Set the underlying state of the CRDT. Equivalent to passing state into the constructor.