map.mdx

keywords	map crdt, last write win, key value, conflict
description	how to use loro map crdt and show all APIs of loro map crdt.

Map

Loro's Map uses LWW (Last-Write-Wins) semantics. When concurrent edits conflict, it compares Lamport logic timestamps to determine the winner.

Map slot conflicts are still resolved by LWW. If the value at a key is a child container, regular child containers have operation-derived identities, so two peers that create a child at the same key can create two different children. If the child should be shared by logical key, use a mergeable child container:

const body = doc.getMap("notes").ensureMergeableText(noteId);
body.insert(0, "Hello");

Here is how to use it:

import { LoroDoc, LoroText } from "loro-crdt";
// ---cut---
const docA = new LoroDoc();
docA.setPeerId("0");
const docB = new LoroDoc();
docB.setPeerId("1");

const mapA = docA.getMap("map");
const mapB = docB.getMap("map");

mapA.set("a", 1);
const textB = mapB.setContainer("a", new LoroText());
textB.insert(0, "Hi");

console.log(docA.toJSON()); // OUTPUT: { map: { a: 1 } }
console.log(docB.toJSON()); // OUTPUT: { map: { a: "Hi" } }

docA.import(docB.export({ mode: "snapshot" }));
docB.import(docA.export({ mode: "snapshot" }));

// docB wins because it has the larger peerId, thus the larger logical timestamp
console.log(docA.toJSON()); // OUTPUT: { map: { a: "Hi" } }
console.log(docB.toJSON()); // OUTPUT: { map: { a: "Hi" } }

Note: When calling map.set(key, value) on a LoroMap, if map.get(key) already returns value, the operation will be a no-op (no operation recorded).