A data structure for distributed arrays using the LSeq allocation strategy
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README.md

LSEQTree

Keywords: distributed systems, collaborative editing, CRDT, LSEQ allocation strategy, unique identifiers, tree-based array

This project aims to provide an implementation of a CRDT-based array [1] with an underlying exponential tree and the allocation strategy LSeq [2].

Installation

$ npm install lseqtree

Usage

To include LSeqTree within your web browser, put the following line in your html:

<script src="./path/to/bundle/lseqtree.bundle.js"></script>

In your JavaScript file:

const LSeqTree = require('lseqtree');

Example

// #1 We create a first distributed data structure for sequences'
const lseq1 = new LSeqTree(1);

// #2 We insert an element in the structure
const idInsert = lseq1.insert('A', 0);
console.log('\tSize of the 1st structure: ' + lseq1.length);
// > Size of the 1st structure: 1

console.log('#A We initialize a second structure');
const lseq2 = new LSeqTree(2);

// #B We insert the element of the first structure
lseq2.applyInsert(idInsert);
console.log('\tSize of the 1st structure: ' + lseq1.length);
console.log('\tSize of the 2nd structure: ' + lseq2.length);
// > Size of the 1st structure: 1
// > Size of the 1st structure: 1

console.log('\tCharacter at index 0: ' + lseq1.get(0));
// > Character at index 0: A

// #C We remove the element at position 0, ie, character "A"
const idDelete = lseq2.remove(0);
console.log('\tSize of the 1st structure: ' + lseq1.length);
console.log('\tSize of the 2nd structure: ' + lseq2.length);
// > Size of the 1st structure: 1
// > Size of the 2nd structure: 0

// #3 We apply the removal to the first structure
lseq1.applyRemove(idDelete);
console.log('\tSize of the 1st structure: ' + lseq1.length);
console.log('\tSize of the 2nd structure: ' + lseq2.length);
// > Size of the 2nd structure: 0
// > Size of the 2nd structure: 0

Misc

  • Clojure LSEQTree from Travis McNeill

  • Prior project follows the specification of LSEQTree. Nevertheless, the former is a linearization of the tree into an array. As such, the memory usage is high. On the other hand, LSEQTree uses a tree, and therefore, it has a better space complexity. LSEQTree uses the core of the prior project to generate its identifiers. Despite being less efficient (obviously, the code must be improved), it provides interoperability and interchangeability between the two projects.

References

[1] M. Shapiro, N. Preguiça, C. Baquero, and M. Zawirski. A comprehensive study of Convergent and Commutative Replicated Data Types. Research Report. 2011.

[2] B. Nédelec, P. Molli, A. Mostéfaoui, and E. Desmontils. LSEQ: an Adaptive Structure for Sequences in Distributed Collaborative Editing. DocEng '13 Proceedings of the 2013 ACM symposium on Document engineering. Pages 37-46. Sept. 2013.