convergent

Lightweight, composable CRDTs (Conflict-Free Replicated Data Types) for decentralized systems.

CRDTs are data structures that can be replicated across nodes, modified independently, and merged with a mathematically guaranteed consistent result — no central coordination required.

Types

Type	Description	Use case
GCounter	Grow-only counter	View counts, event tracking
PNCounter	Positive-Negative counter	Likes/dislikes, stock levels
LWWRegister	Last-Writer-Wins register	User profile fields, settings
ORSet	Observed-Remove set	Shopping carts, tag lists

Usage

[dependencies]
convergent = "0.1"

use convergent::{GCounter, Merge};

let mut node_a = GCounter::new("a");
let mut node_b = GCounter::new("b");

node_a.increment(3);
node_b.increment(5);

node_a.merge(&node_b);
assert_eq!(node_a.value(), 8);

Design

Every type implements the Merge trait, which guarantees:

• Commutative — a.merge(b) == b.merge(a)
• Associative — a.merge(b).merge(c) == a.merge(b.merge(c))
• Idempotent — a.merge(a) == a

The LWWRegister uses hlc_id for causally-ordered timestamps, connecting this crate to the broader problem of time in distributed systems.

Related

• hlc_id — Hybrid Logical Clock IDs (used by LWWRegister)
• A comprehensive study of CRDTs — Shapiro et al., 2011

License

MIT