GSet

A set that only grows. Elements can be added but never removed. Merging two GSets is just set union.

Converges to: the union of all elements ever added across all replicas.

Merge rule

piece(a, b) is a.elements ∪ b.elements. Set union is idempotent (re-adding something doesn't change the result), commutative, and associative — the three laws that guarantee convergence.

Code examples

Add and merge:

    @Test
    fun addProducesADeltaThatAddsAnElement() {
        val empty = GSet.empty<String>()
        val delta = empty.add("x")
        val next = empty.piece(delta)
        assertTrue(next.contains("x"))
        assertEquals(setOf("x"), next.elements)
    }

    @Test
    fun mergeIsUnion() {
        val left = GSet.of("x", "y")
        val right = GSet.of("y", "z")
        assertEquals(setOf("x", "y", "z"), left.piece(right).elements)
    }

When to use

GSet is the right choice when elements are only ever added — completed tasks, acknowledged events, registered participants. For a set where elements can be removed, see ORSet (observe-remove, add-wins on conflict) or TwoPhaseSet (remove-wins, permanent tombstone).

Last modified: 22 June 2026