In distributed computing, a conflict-free replicated data type (CRDT) is a type of specially-designed data structure used to achieve strong eventual consistency (SEC) and monotonicity (absence of rollbacks).

One way of implementing CRDT is by using LWW-element-set.

LWW-element-set is set with timestamped adds and removes

LWW-element-set elements are stored with timestamp. Add and remove will save the operation timestamp along with data in two different sets. For each set new operation of add/remove will update the timestamp for that element.

Queries like Get/List and Len over LWW-set will check both add and remove timestamps to decide if latest state of each element is "exists" or "removed".

crdt package uses two libraries, lww and qset to create a conflict-free replicated storage.

lww implements the logic of an LWW-element-set. It is design in a modular way to use different types of underlying sets. Each set can have a different characteristic like using Go internal maps to be fast or using Redis to share state will other processes and staying persistence.

Is one implementation of what lww can use as underlying set. It mixes Go internal maps and Redis storage to provide lww package with a both fast and persistent underlying set.

$ go get github.com/kavehmz/crdt

package main

import (
	"fmt"
	"github.com/kavehmz/crdt"
)

func main() {
	c := CRDT{RedisURL: "redis://localhost:6379/0"}
	c.Connect()
	c.Add("Item", time.Now())
	c.Add("Item2", time.Now())
	c.Remove("Item", time.Now())
	c.Remove("Item2", time.Now().Add(-1*time.Second))

	l := c.Get()
	fmt.Println(l)

	c.Add("Item", time.Now())
	c.Remove("Item2", time.Now())
	l = c.Get()
	fmt.Println(l)

	// Output:
	// [Item2]
	// [Item]
}