LWWMap

A map where each key is an LWWRegister. Concurrent writes to the same key are resolved by timestamp — the later write wins. Keys are independent, so a conflict on one key never affects another.

Converges to: a map where each key holds the value written at the highest (timestamp, replicaId) for that key.

Merge rule

LWWMap is a map from key to LWWRegister<V>. piece merges each key's register independently using the LWW rule. Keys with no conflict simply union.

Code examples

Set and read:

    @Test
    fun setReturnsTheValue() {
        val m = LWWMap.empty<String, String>().set(a, 10L, "lang", "en")
        assertEquals("en", m["lang"])
        assertEquals(mapOf("lang" to "en"), m.entries)
    }

Per-key LWW: later timestamp wins:

    @Test
    fun perKeyLwwSemantics_laterWins() {
        val m1 = LWWMap.empty<String, String>().set(a, 10L, "lang", "en")
        val m2 = LWWMap.empty<String, String>().set(b, 20L, "lang", "fr")
        assertEquals("fr", m1.piece(m2)["lang"])
        assertEquals("fr", m2.piece(m1)["lang"]) // commutative
    }

Different keys compose independently:

    @Test
    fun differentKeysComposeIndependently() {
        val m1 = LWWMap.empty<String, String>().set(a, 10L, "lang", "en")
        val m2 = LWWMap.empty<String, String>().set(b, 5L, "tz", "UTC")
        val merged = m1.piece(m2)
        assertEquals("en", merged["lang"])
        assertEquals("UTC", merged["tz"])
    }

When to use

LWWMap is a good fit for converging metadata — display names, preferences, labels — where per-key last-write-wins semantics are acceptable. For a map whose keys are ORSet-managed (add-wins on key presence) and whose values merge via their own CRDT, use ORMap.

Last modified: 22 June 2026