name: yjs
description: Yjs CRDT patterns, shared types, conflict resolution, and meta data structures. Use when building collaborative apps with Yjs, handling Y.Map/Y.Array/Y.Text, implementing drag-and-drop reordering, or optimizing document storage.
metadata:
author: epicenter
version: '1.0'

Yjs CRDT Patterns

Core Concepts

Shared Types

Yjs provides six shared types. You'll mostly use three:

• Y.Map - Key-value pairs (like JavaScript Map)
• Y.Array - Ordered lists (like JavaScript Array)
• Y.Text - Rich text with formatting

The other three (Y.XmlElement, Y.XmlFragment, Y.XmlText) are for rich text editor integrations.

Client ID

Every Y.Doc gets a random clientID on creation. This ID is used for conflict resolution—when two clients write to the same key simultaneously, the higher clientID wins, not the later timestamp.

const doc = new Y.Doc();
console.log(doc.clientID); // Random number like 1090160253

From dmonad (Yjs creator):

"The 'winner' is decided by ydoc.clientID of the document (which is a generated number). The higher clientID wins."

— GitHub issue #520

The actual comparison in source (updates.js#L357):

return dec2.curr.id.client - dec1.curr.id.client; // Higher clientID wins

This is deterministic (all clients converge to same state) but not intuitive (later edits can lose).

Shared Types Cannot Move

Once you add a shared type to a document, it can never be moved. "Moving" an item in an array is actually delete + insert. Yjs doesn't know these operations are related.

Critical Patterns

1. Single-Writer Keys (Counters, Votes, Presence)

Problem: Multiple writers updating the same key causes lost writes.

// BAD: Both clients read 5, both write 6, one click lost
function increment(ymap) {
    const count = ymap.get('count') || 0;
    ymap.set('count', count + 1);
}

Solution: Partition by clientID. Each writer owns their key.

// GOOD: Each client writes to their own key
function increment(ymap) {
    const key = ymap.doc.clientID;
    const count = ymap.get(key) || 0;
    ymap.set(key, count + 1);
}

function getCount(ymap) {
    let sum = 0;
    for (const value of ymap.values()) {
        sum += value;
    }
    return sum;
}

2. Fractional Indexing (Reordering)

Problem: Drag-and-drop reordering with delete+insert causes duplicates and lost updates.

// BAD: "Move" = delete + insert = broken
function move(yarray, from, to) {
    const [item] = yarray.delete(from, 1);
    yarray.insert(to, [item]);
}

Solution: Add an index property. Sort by index. Reordering = updating a property.

// GOOD: Reorder by changing index property
function move(yarray, from, to) {
    const sorted = [...yarray].sort((a, b) => a.get('index') - b.get('index'));
    const item = sorted[from];

    const earlier = from > to;
    const before = sorted[earlier ? to - 1 : to];
    const after = sorted[earlier ? to : to + 1];

    const start = before?.get('index') ?? 0;
    const end = after?.get('index') ?? 1;

    // Add randomness to prevent collisions
    const index = (end - start) * (Math.random() + Number.MIN_VALUE) + start;
    item.set('index', index);
}

3. Nested Structures for Conflict Avoidance

Problem: Storing entire objects under one key means any property change conflicts with any other.

// BAD: Alice changes nullable, Bob changes default, one loses
schema.set('title', {
    type: 'text',
    nullable: true,
    default: 'Untitled',
});

Solution: Use nested Y.Maps so each property is a separate key.

// GOOD: Each property is independent
const titleSchema = schema.get('title'); // Y.Map
titleSchema.set('type', 'text');
titleSchema.set('nullable', true);
titleSchema.set('default', 'Untitled');
// Alice and Bob edit different keys = no conflict

Storage Optimization

Y.Map vs Y.Array for Key-Value Data

Y.Map tombstones retain the key forever. Every ymap.set(key, value) creates a new internal item and tombstones the previous one.

For high-churn key-value data (frequently updated rows), consider YKeyValue from yjs/y-utility:

// YKeyValue stores {key, val} pairs in Y.Array
// Deletions are structural, not per-key tombstones
import { YKeyValue } from 'y-utility/y-keyvalue';

const kv = new YKeyValue(yarray);
kv.set('myKey', { data: 'value' });

When to use Y.Map: Bounded keys, rarely changing values (settings, config).
When to use YKeyValue: Many keys, frequent updates, storage-sensitive.

Epoch-Based Compaction

If your architecture uses versioned snapshots, you get free compaction:

// Compact a Y.Doc by re-encoding current state
const snapshot = Y.encodeStateAsUpdate(doc);
const freshDoc = new Y.Doc({ guid: doc.guid });
Y.applyUpdate(freshDoc, snapshot);
// freshDoc has same content, no history overhead

Common Mistakes

1. Assuming "Last Write Wins" Means Timestamps

It doesn't. Higher clientID wins, not later timestamp. Design around this or add explicit timestamps with y-lwwmap.

2. Using Y.Array Position for User-Controlled Order

Array position is for append-only data (logs, chat). User-reorderable lists need fractional indexing.

3. Forgetting Document Integration

Y types must be added to a document before use:

// BAD: Orphan Y.Map
const orphan = new Y.Map();
orphan.set('key', 'value'); // Works but doesn't sync

// GOOD: Attached to document
const attached = doc.getMap('myMap');
attached.set('key', 'value'); // Syncs to peers

4. Storing Non-Serializable Values

Y types store JSON-serializable data. No functions, no class instances, no circular references.

5. Expecting Moves to Preserve Identity

// This creates a NEW item, not a moved item
yarray.delete(0);
yarray.push([sameItem]); // Different Y.Map instance internally

Any concurrent edits to the "moved" item are lost because you deleted the original.

Debugging Tips

Inspect Document State

console.log(doc.toJSON()); // Full document as plain JSON

Check Client IDs

// See who would win a conflict
console.log('My ID:', doc.clientID);

Watch for Tombstone Bloat

If documents grow unexpectedly, check for:

• Frequent Y.Map key overwrites
• "Move" operations on arrays
• Missing epoch compaction

References

• Learn Yjs - Interactive tutorials
• Yjs Documentation - API reference
• Yjs INTERNALS.md - How Yjs works internally
• GitHub issue #520 - Conflict resolution discussion with dmonad
• yjs/y-utility - YKeyValue and helpers
• y-lwwmap - Timestamp-based LWW
• fractional-indexing - Production library
• YATA paper - Academic foundation