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A pure-Go, production-grade implementation of Yjs. Runs real collaborative apps: a horizontally-scaling, Hocuspocus-compatible websocket server with versioned persistence and compaction. Hardened by differential fuzzing and a byte-level conformance suite. Also embeddable on iOS/Android via gomobile.

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# ygo

[![CI](https://github.com/reearth/ygo/actions/workflows/ci.yml/badge.svg)](https://github.com/reearth/ygo/actions/workflows/ci.yml)
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**ygo** is a pure-Go implementation of [Yjs](https://github.com/yjs/yjs), the CRDT (Conflict-free Replicated Data Type) framework for real-time collaborative editing. Use it to build collaborative applications in Go — shared text, rich text, maps, arrays, and XML trees that merge concurrent edits without conflicts, without a central authority, and without operational transformation.

ygo is **binary-compatible** with the JavaScript Yjs reference implementation: it speaks both the V1 and V2 update wire formats byte-for-byte, along with the [y-protocols](https://github.com/yjs/y-protocols) sync handshake and awareness layer. Updates produced by ygo apply cleanly in Yjs (JavaScript) and yrs (Rust), and vice versa. That claim is enforced, not asserted — a byte-level cross-language conformance suite generated from `yjs@13.6.30` and a differential convergence fuzzer both run in CI.

Where ygo goes further than a port is on the server. It ships a [Hocuspocus](https://tiptap.dev/docs/hocuspocus)-compatible WebSocket server that scales horizontally across instances through a Redis-backed cluster relay, with versioned persistence (including a CGo-free SQLite store), snapshots, and a turnkey `ygo-server` binary. The same library also embeds natively on iOS and Android through `gomobile` — no JavaScript runtime, no CGO, anywhere.

**[Quick start](#quick-start) · [How ygo compares](#how-ygo-compares) · [WebSocket server](#websocket-server) · [Benchmarks](#performance) · [Documentation](#documentation)**

## Contents

- [Capabilities](#capabilities) · [Features](#features)
- [Installation](#installation) · [Quick Start](#quick-start) · [Examples](#examples)
- [How ygo compares](#how-ygo-compares) · [Goals and non-goals](#goals-and-non-goals)
- [WebSocket Server](#websocket-server) · [Server-side document injection](#server-side-document-injection)
- [Attribution](#attribution) · [Persistence](#persistence) · [Subdocuments](#subdocuments) · [Mobile (iOS / Android)](#mobile-ios--android)
- [Running in production](#running-in-production) · [Performance](#performance) · [Architecture](#architecture)
- [Compatibility](#compatibility) · [Versioning and API stability](#versioning-and-api-stability) · [Gotchas](#gotchas)
- [Documentation](#documentation) · [Contributing](#contributing) · [Security](#security) · [License](#license)

## Capabilities

ygo is a pure-Go CRDT library that interoperates with Yjs (JavaScript) and yrs (Rust):

- All Y-types: `YText`, `YArray`, `YMap`, `YXmlFragment`, `YXmlElement`, `YXmlText`
- Both update wire formats (V1 and V2, with V1↔V2 conversion)
- The y-protocols sync handshake and awareness layer
- WebSocket and HTTP transport bindings (the core is transport-agnostic)
- Native iOS/Android embedding via `gomobile` (the `mobile/` subpackage) — no JS runtime, no CGO
- Snapshots, garbage collection, undo manager, persistence adapters

See [the latest release](https://github.com/reearth/ygo/releases/latest) for the current version, [CHANGELOG.md](CHANGELOG.md) for per-release detail, and [docs/HISTORY.md](docs/HISTORY.md) for the longer arc.

## Features

- **Pure Go** — no CGO, no V8, no embedded JavaScript engine.
- **Binary-compatible** with Yjs JS and yrs. Updates round-trip across all three implementations.
- **Full Y-type coverage** — `YText`, `YArray`, `YMap`, `YXmlFragment`, `YXmlElement`, `YXmlText`.
- **Both update formats** — V1 and V2, with V1↔V2 conversion.
- **Sync protocol** — implements [y-protocols](https://github.com/yjs/y-protocols) `SyncStep1`, `SyncStep2`, and incremental updates.
- **Awareness** — presence, cursor sharing, ephemeral state.
- **Snapshots** — point-in-time document history and restore.
- **Transport-agnostic** — core logic has no transport dependency; WebSocket and HTTP handlers are addons.
- **Mobile bindings** — embed natively in iOS/Android via `gomobile bind` (the [`mobile/`](mobile/) subpackage). Pure Go, no CGO; full on-device editing, sync, presence, and change-notification observers.

Post-v1.0 hardening:

- **Panic-safe transactions** (v1.1.1). If `fn` inside `Transact` panics, the document lock is still released, observers fire with the partial state that was committed before the panic, and the original panic propagates to the caller. No rollback — that's by design, matching Yjs JS and yrs. For atomic batching, recover above `Transact` and reconcile via sync.
- **Cooperative cancellation** (v1.1.2). `Doc.TransactContext` accepts a `context.Context` and exposes it inside `fn` via `txn.Ctx()`. `fn` can poll `txn.Ctx().Err()` to bail out early when the request context is cancelled.
- **Error-returning variants** (v1.3.0, v1.6.0, v1.7.0). Sibling methods that surface errors instead of panicking or silently succeeding: `TransactE`, `TransactContextE`, `Awareness.SetLocalStateContext`, `Awareness.ApplyUpdateContext`, `UndoManager.UndoContext`, `UndoManager.RedoContext`, `Encoder.WriteVarIntE`. All additive; original methods unchanged.
- **Out-of-order delta convergence** (v1.2.0). When an update references an item whose dependency hasn't arrived yet, the item is parked in a per-doc pending queue and integrated automatically when the missing predecessor arrives. Mirrors `pendingStructs` in Yjs JS and `Store.pending` in yrs.
- **WebSocket hardening** (v1.4.0). Structured logging via `*slog.Logger`, per-message size cap (`MaxMessageBytes`), bounded per-peer broadcast queue with disconnect-on-overflow (`PeerWriteQueueSize`), and the bounded broadcast pattern itself — replacing the previous goroutine-per-broadcast fan-out.
- **Operational observability** (v1.5.0). `Doc.PendingStats()` returns counts of items parked waiting for dependencies — useful when monitoring convergence in production.
- **Semaphore-backed hard caps** (v1.5.0). `MaxConnections` and `MaxPeersPerRoom` are now hard guarantees, not optimistic atomic counters with race windows.
- **`crypto/rand` ClientID** (v1.5.0). Predictable IDs in multi-tenant deployments are a footgun; the default `ClientID` is now cryptographically random. `crdt.NewClientID()` is exposed for callers who want to generate IDs externally.
- **Context-aware persistence** (v1.7.0). Adapters can opt into `PersistenceAdapterContext` to receive a context cancelled when `Server.Shutdown` begins, letting them abort in-flight DB calls instead of blocking shutdown.
- **Security hardening** (v1.8.0–v1.8.1). Pending-items queue cap (`Server.MaxPendingItems`), WebSocket handshake read deadline (`Server.HandshakeTimeout`), CSWSH documentation for `AllowedOrigins`, and a per-room awareness state cap (`Server.MaxAwarenessBytesPerRoom` plus `Awareness.SetMaxBytes`).
- **lib0 wire-format parity** (v1.8.0, v1.10.0). Float byte-order fixed to big-endian (contributed by @zombiek731), lib0 `Any` tag 122 (BigInt) support, integer dispatch by magnitude matching lib0, lossless float64→float32 narrowing, strict UTF-8 in `ReadVarString` (`ErrInvalidUTF8`), and acceptance of Go's full numeric tower in `WriteAny`.
- **Cross-reference audit** (v1.9.0–v1.14.0). A systematic comparison of ygo against Yjs JS and yrs reference implementations surfaced ten correctness gaps, tracked under the [`gaps` label](https://github.com/reearth/ygo/issues?label=gaps). Notable fixes: YATA `OriginRight` boundary (#65, #68), awareness self-state protection (#73), `Item.delete` cascade into nested types + `DeleteSet` partial-overlap split (#72), YText format-marker correctness (#71: bleed, accumulation, gap cleanup, current-attribute inheritance), `YText.InsertEmbed` (#76), and `YArray/YMap.ToJSON` recursive unwrap of nested types (#75). See [`gaps` label](https://github.com/reearth/ygo/issues?label=gaps) for the full list.
- **Sync read-loop resilience** (v1.9.0). `sync.WithErrorHandler` option lets `ApplySyncMessage` route a single malformed update to a caller-supplied handler rather than tearing down the connection.
- **Awareness heartbeat** (v1.11.0). `Awareness.Heartbeat()` re-emits local state at an incremented clock so peers learn we're still alive without the local state needing to change. Pairs with `StartAutoExpiry` on the peer side.
- **Hocuspocus compatibility** (v1.18.0–v1.19.0). The websocket server handles Hocuspocus message types (sync-reply, stateless, broadcast-stateless, close, ping/pong), exposes room lifecycle hooks (`OnLoadDocument`, `OnUnloadDocument`, `OnFirstPeer`, `OnLastPeer`) and an `OnStateless` hook, and ships a [`provider/webhook`](provider/webhook/) subpackage with HMAC-signed, debounced, retrying delivery.
- **Horizontal scale** (v1.20.0–v1.21.0). A [`cluster`](cluster/) relay (Redis-backed adapter in [`cluster/redis`](cluster/redis/)) shares one logical document per room across multiple server instances behind a load balancer, alongside a versioned persistence layer.
- **SQLite persistence & turnkey server** (v1.23.0). A pure-Go (CGo-free, `modernc.org/sqlite`) `VersionedPersistence` store with WAL mode, full versioned history, and crash-safe two-phase prune ([`persistence/sqlite`](persistence/sqlite/)), plus a ready-to-run [`cmd/ygo-server`](cmd/ygo-server/) binary.
- **CRDT convergence & Yjs-interop fixes** (v1.23.1). Map-key last-writer-wins via scan-right, out-of-order `rightOrigin` parking, self-encode reload of nested types, and `RelativePosition` / `Snapshot` wire formats aligned to the Yjs JS reference.
- **Awareness DoS hardening** (v1.25.0). A per-room cap on distinct presence entries (`Awareness.SetMaxClients`, `Server.MaxAwarenessClientsPerRoom`) plus server-side auto-expiry (`Server.AwarenessExpiry`) that reclaims ghost presence from peers that died silently.
- **Server secure-by-default & decode-ceiling alignment** (v1.26.0). `cmd/ygo-server` binds loopback by default and warns loudly on a public bind (it has no built-in auth); a single wire field is now bounded by the message size rather than a fixed 16 MiB ceiling (large documents no longer fail to sync below the configured cap); malformed inbound frames are logged; and a `RelativePosition` anchored to a root type resolves to the end of the type, matching Yjs.
- **CRDT correctness batch** (v1.27.0). Three Yjs-parity fixes, all verified against `yjs@13.6.30`: `YText.Format` ports the Yjs `formatText` algorithm so re-applying/toggling a format over a sub-range no longer strips the surrounding run (`ToDelta` also coalesces adjacent equal-attribute inserts); `UndoManager` undo of a deletion re-inserts content as a new item so it propagates to peers instead of being silently lost on the next sync; and `MergeUpdatesV1`/`DiffUpdateV1` merge at the struct level so non-integrable structs are no longer dropped. Adds struct-level `MergeUpdatesV2`/`DiffUpdateV2`/`EncodeStateVectorFromUpdate` and exports `crdt.SharedType`.
- **Snapshot reconstruction & conflict-scan perf** (v1.28.0). Adds `crdt.CreateDocFromSnapshot` (Yjs-parity name for rebuilding a historic doc from a `Snapshot`), with a `WithGC(false)` safety guard (`ErrSnapshotSourceGCed`) so reconstruction can't silently return incomplete history; `RestoreDocument` now shares that guard. `Item.integrate` also reuses its YATA conflict-tracking map via `clear()` instead of reallocating it, cutting convergence allocations ~92% under high same-position contention with no change to the common path.
- **Provider security hardening** (v1.29.0). The HTTP provider gains `AuthFunc` (401), room-name validation (400, the same rule the WebSocket provider uses), and a configurable `MaxUpdateBytes` (413) — parity with the WebSocket provider. The WebSocket provider gains optional per-peer message rate limiting (`MessageRateLimit`/`MessageRateBurst`): a peer that floods past the limit is disconnected rather than diverged. `AllowedOrigins` also gains `*` wildcard matching (e.g. `https://*.netlify.app`), anchored so a wildcard can't spoof a host. The new config fields are additive (zero values preserve current behaviour); the one behaviour change is that the HTTP provider now rejects invalid room names with 400.
- **Read-only WebSocket connections** (v1.30.0). New `Server.Authorize func(*http.Request) (ConnectionConfig, bool)` both accepts/rejects a connection and reports per-connection config — currently `ReadOnly`. A read-only peer receives document and awareness broadcasts but its inbound writes (sync step-2/update and awareness) are dropped server-side, while still being able to request state (SyncStep1) and query awareness. The existing bool `AuthFunc` is unchanged; `Authorize` takes precedence when both are set. Matches Hocuspocus's `readOnly` connection flag (#59).
- **Attribution API** (v1.30.0). `IDSet`/`IDMap`/`ContentMap` + wire codec tracking yjs v14-rc `14.0.0-16` — stamp CRDT content with per-item authorship (`userid`, `ts`, …) and exchange it with JS. Non-goals documented (no storage integration; `diffDocsToDelta` deferred). See [Attribution](#attribution) below.
- **Subdocument lifecycle** (#63). A `Doc` can embed another `Doc` as a subdocument via `YMap.Set(txn, key, subdoc)`; `GetSubdocs`/`GetSubdocGUIDs`/`OnSubdocs`/`Load` plus `WithAutoLoad`/`WithShouldLoad`/`WithCollectionID` track and drive the add/remove/load lifecycle — the local half of Yjs's subdocuments feature. Also: `New()` now defaults a Doc's `guid` to a random uuidv4 instead of `""` (Yjs parity). Live cross-peer subdoc sync is a separate, tracked follow-up (#142). See [Subdocuments](#subdocuments) below.
- **Coalesced persistence** (v1.36.0). The websocket server's persistence worker now debounces backing-store writes by default — a 2s window, capped by a 10s max wait — merging bursts into a single `StoreUpdate` instead of one write per update (Hocuspocus parity). This is a behaviour change for servers with a `PersistenceAdapter` configured; set `Server.PersistCoalesceWindow = -1` to restore strict per-update writes (#175).
- **Persistence durability + compaction** (v1.37.0). The room-teardown paths now flush a pending coalesced batch durably before evicting the room, closing a gap where a fast reconnect could reload stale state and miss the just-made edit. Adapters can also bound stored-version growth by implementing `CompactableAdapter`, driven by `Server.CompactEvery` (`persistence.LegacyAdapter` supports this via its new `KeepVersions` field) (#175).
- **Positional-access performance** (v1.39.0). `YText`/`YArray` positional operations (`Get`, `Slice`, positional insert/delete, `Format`, `ApplyDelta`) now route through a Yjs-style bidirectional, move-aware search-marker cache instead of the old forward-only position cache — internal only, no public API change. Measured on a 100k-node document: random-position insert ~101× faster, reverse insert ~916× faster (the old cache's O(n²) worst case), random `Get` ~114× faster. Also fixes a `YText.ApplyDelta` bug where an attribute-less `insert` could inherit a preceding retain's `attributes` (Yjs/Quill-aligned fix; behaviour change for callers relying on the old bleed-through) (#181).
- **Room load off the global lock + idle-room residency** (v1.39.0). Concurrent connects to distinct rooms now load in parallel instead of serializing behind the server's single rooms lock (#182). New `Server.RoomIdleTimeout`/`Server.MaxResidentRooms` let a room stay warm in memory for a bounded time (and bounded count) after its last peer leaves so a quick reconnect reuses the live doc instead of reloading; both default to zero, preserving the previous eager-evict behaviour (#183).
- **Per-room relay isolation + honest delivery guarantee** (v1.42.0). Inbound (`cluster/redis`) and outbound (`provider/websocket`) relay delivery are each isolated per room — one bounded lane and worker per room, in both directions — so one room's slow `Sink.Inject` call (inbound) or slow publish (outbound) can no longer block delivery for every other room on the node (#187). This is `cluster/redis`-specific, not a property of `cluster.Relay` in general: the interface now permits any relay to deliver rooms concurrently, but the in-process reference `MemRelay` still delivers every room from one goroutine per node and does not (yet) get this isolation. A saturated lane coalesces its queued `KindSync` backlog via `crdt.MergeUpdatesV1` instead of dropping it (a saturated awareness slot instead replaces the queued blob with the newest one — `AwarenessSuperseded`, not a drop); this trades a small, bounded, amortized merge cost for boundedness, so a wedged room's merge attempts can still delay *other* rooms briefly on the shared inbound subscriber goroutine (rare; the same condition that produces `HardDrops`). Also fixes a pre-existing bug, shipped since v1.20.0, where `Server.Apply`'s origin sentinel could alias the relay's echo-guard sentinel (Go's zero-size-allocation guarantee) — silently disabling relay publish for every `Apply` write, and separately letting a concurrent relay-injected update bleed into the delta `Apply` returns to its caller. Contract change: `cluster.Sink.Inject` (for a custom `Sink`) and `cluster.Relay.Publish` (for a custom `Relay`) may now each be called concurrently for distinct rooms; `Publish` additionally permits two concurrent calls for the SAME room during a room's eviction/reload handoff, with no per-room ordering guaranteed (`Inject` still serialises same-room calls). `*websocket.Server`, `MemRelay`, and `cluster/redis` are already safe for both; third-party `Sink`/`Relay` implementations must confirm they are too. Reality check, unchanged by this release: Redis pub/sub itself remains at-most-once by Redis's own definition — a subscriber that can't keep up loses the message for good, and persistence only heals that on the room's next reload, which a hot room never gets. Watch `cluster/redis.Relay.Stats()` / `websocket.Server.RelayStats()`: `Coalesced`/`AwarenessSuperseded` are routine on a busy room (alert on rate, not presence), inbound-only `RouterDrops` is routine under ordinary room churn (alert on rate), and `HardDrops`/`Dropped` should always be zero (alert on presence — they mean data was lost).

See [CHANGELOG.md](CHANGELOG.md) for the full per-release picture.

## Requirements

- Go 1.23 or later

## Installation

```bash
go get github.com/reearth/ygo
```

## Quick Start

```go
package main

import (
    "fmt"
    "github.com/reearth/ygo/crdt"
)

func main() {
    // Create two peers
    alice := crdt.New()
    bob := crdt.New()

// Obtain the shared type before entering a transaction —
    // GetText and Transact both acquire the document mutex.
    text := alice.GetText("content")

// Alice makes edits
    alice.Transact(func(txn *crdt.Transaction) {
        text.Insert(txn, 0, "Hello, world!", nil)
    })

// Encode Alice's state and send to Bob
    update := alice.EncodeStateAsUpdate()

// Bob applies the update — both docs now converge
    if err := crdt.ApplyUpdateV1(bob, update, nil); err != nil {
        panic(err)
    }

fmt.Println(bob.GetText("content").ToString()) // "Hello, world!"
}
```

## Examples

The [`examples/`](examples/) directory contains four runnable programs with detailed inline comments:

| Example | What it shows |
|---------|---------------|
| [`examples/peer-sync`](examples/peer-sync/) | In-process two-peer sync via the y-protocols handshake — no network needed |
| [`examples/http-sync`](examples/http-sync/) | Pull/push sync over HTTP with incremental state-vector diffs |
| [`examples/collab-editor`](examples/collab-editor/) | Real-time multi-tab collaborative editor with a browser client |
| [`examples/snapshot-history`](examples/snapshot-history/) | Document versioning — capture, store, and restore past states |

Run any example from the repository root:

```bash
go run ./examples/peer-sync
go run ./examples/http-sync
go run ./examples/snapshot-history
go run ./examples/collab-editor/server   # then open http://localhost:8080
```

**New users**: start with `peer-sync` for the smallest end-to-end demonstration of two docs converging in-process. Jump to `collab-editor` when you want to wire the WebSocket server to a real browser client.

## How ygo compares

The Yjs ecosystem is mostly JavaScript, with a Rust core. If you are searching for a Go equivalent of one of these, here is where ygo sits:

| Project | Runtime | What it provides | Relationship to ygo |
|---|---|---|---|
| [Yjs](https://github.com/yjs/yjs) | JavaScript | The reference CRDT implementation and wire format | ygo is a pure-Go implementation of it, byte-compatible on the wire in both directions |
| [y-websocket](https://github.com/yjs/y-websocket) | Node.js | Minimal WebSocket sync server for Yjs clients | ygo's [WebSocket server](#websocket-server) is the Go-native equivalent, plus auth hooks, rate limiting, resource caps, and persistence |
| [Hocuspocus](https://tiptap.dev/docs/hocuspocus) | Node.js | Production Yjs backend — hooks, webhooks, persistence, read-only peers | ygo speaks the Hocuspocus message types and mirrors its hook and webhook model, so a Go backend can replace it without touching the client |
| [yrs](https://github.com/y-crdt/y-crdt) | Rust (+ FFI) | The Rust CRDT core used for non-JS bindings | The alternative when your host language is Go: no CGO, no FFI boundary, no cross-language build. See [docs/comparison/ygo-vs-yrs.md](docs/comparison/ygo-vs-yrs.md) |
| [y-leveldb](https://github.com/yjs/y-leveldb) / [y-redis](https://github.com/yjs/y-redis) | Node.js | Document persistence and multi-node fan-out for Yjs servers | ygo's [persistence adapters](#persistence) and [cluster relay](cluster/) cover the same ground in-process, including a CGo-free SQLite store |

Choose ygo when the collaborative backend is Go and you would rather not run a Node.js sidecar, an FFI boundary, or an embedded JavaScript runtime to get there. Choose Yjs or Hocuspocus directly when your backend is already Node.js — ygo interoperates with both, so the choice is per-service rather than all-or-nothing.

## Goals and non-goals

**Goals**

- **Wire compatibility with Yjs**, verified byte-for-byte against fixtures generated from the JavaScript reference implementation, in both directions and for both V1 and V2.
- **Convergence correctness**, verified by a differential fuzzer that compares ygo's merge results against Yjs across randomized concurrent-edit histories.
- **An idiomatic Go API** — explicit transactions, `error` returns, `context` support, `*slog.Logger`, no reflection-driven magic.
- **Production-ready server operation** — horizontal scale, versioned persistence, bounded resources, structured logs, graceful shutdown.
- **Pure Go**, no CGO, on every supported target including iOS and Android.

**Non-goals**

- **Being a browser client.** ygo runs in Go processes and on mobile. Browser clients stay on Yjs itself, which ygo syncs with.
- **A new or "improved" wire format.** Compatibility with Yjs is the point; divergence would defeat it.
- **Authentication and authorization.** ygo provides the hooks (`Authorize`, `AuthFunc`, `OnInject`) and expects your application to own the policy. See [Security](#security).

## WebSocket Server

```go
package main

import (
    "net/http"
    "github.com/reearth/ygo/provider/websocket"
)

func main() {
    server := websocket.NewServer()
    http.Handle("/yjs/{room}", server)
    http.ListenAndServe(":8080", nil)
}
```

### Run a server

For a ready-to-run binary — with flags for origins, connection/room limits, an optional Redis cluster relay (`-redis`), and SQLite persistence (`-store`) — use [`cmd/ygo-server`](cmd/ygo-server/):

```bash
# Binds 127.0.0.1:1234 by default (loopback only). The server has no built-in
# auth, so it logs a loud warning if you bind a public address (e.g. -addr :1234)
# — put an authenticating reverse proxy in front in that case.
go run github.com/reearth/ygo/cmd/ygo-server -store data.db
```

## Server-side document injection

Backend services — AI agents, HTTP handlers, content pipelines — can push
changes into a live room without simulating a WebSocket peer. Three APIs
are available on `*websocket.Server`.

### `BroadcastUpdate(ctx, room, update)`

Fans a pre-encoded V1 update out to all peers currently connected to a
room. Does **not** apply the update to the server's doc — callers who
want the server's state to reflect the broadcast must call
`crdt.ApplyUpdateV1` first (or use `Apply` below).

```go
doc := server.GetDoc("my-room")
if err := crdt.ApplyUpdateV1(doc, update, nil); err != nil {
    return err
}
if err := server.BroadcastUpdate(ctx, "my-room", update); err != nil {
    return err
}
```

**Skipping `ApplyUpdateV1` creates divergence.** Live peers see the
update, but peers joining afterwards receive the server's stale state
via sync step 2.

### `Apply(ctx, room, fn)`

Applies a callback to the doc and broadcasts the resulting delta atomically.
Auto-creates the room if needed. Persistence runs via the existing
`OnUpdate` hook — callers do not need to persist separately.

```go
err := server.Apply(ctx, "my-room",
    func(doc *crdt.Doc, transact func(func(*crdt.Transaction))) {
        frag := doc.GetXmlFragment("content") // OUTSIDE transact — see note
        transact(func(txn *crdt.Transaction) {
            elem := crdt.NewYXmlElement("p")
            frag.InsertElement(txn, 0, elem)
        })
    },
)
```

**Important:** calls to `doc.GetXmlFragment`, `doc.GetText`, `doc.GetMap`,
and the other root-type accessors must happen **outside** the `transact`
callback. These methods acquire the doc's write lock, which `transact`
already holds — calling them inside deadlocks.

`fn` should be fast. It runs inside the doc's write lock and blocks all
peer reads and writes to the room for the duration.

**On `ErrUpdateTooLarge`, the mutation sticks.** The size check runs
after `fn`'s transaction commits and after persistence has enqueued the
update, so the server's doc reflects `fn`'s changes and the update IS
persisted — but peers do NOT see it. Size-bound `fn`'s effects
explicitly or reconcile peers via a sync step 1/2 exchange.

### `CloseRoom(name, force)`

Explicit teardown for rooms created by `Apply` that never accumulated
peer connections. Without `CloseRoom`, such rooms linger until process
exit.

```go
if err := server.CloseRoom("my-room", false); err != nil { /* ... */ }
// force=true closes connected peers first.
```

### Access control: `Server.OnInject`

An optional hook gates all server-side writes:

```go
server.OnInject = func(ctx context.Context, info websocket.InjectInfo) error {
    tenant, _ := ctx.Value(tenantKey{}).(string)
    if !allowed(tenant, info.Room) {
        return fmt.Errorf("tenant %q may not write to %q", tenant, info.Room)
    }
    if info.Op == websocket.OpBroadcastUpdate && info.UpdateSize > 1<<20 {
        return errors.New("update too large for this tenant")
    }
    return nil
}
```

`info.Op` is `OpBroadcastUpdate` or `OpApply`. `info.UpdateSize` is the
length of the update bytes for `BroadcastUpdate`; zero for `Apply` (the
delta has not yet been produced — size capping for `Apply` is handled
by `MaxUpdateBytes`, post-hoc).

Refusals are returned wrapped with `ErrInjectRefused`, so callers can
match either the sentinel or the hook's own error via `errors.Is`.

### Resource caps

- `Server.MaxUpdateBytes` — per-update size cap, default 64 MiB (matches
  the peer frame limit).
- `Server.MaxRooms` — total-room cap applied uniformly to peer upgrades
  (HTTP 503) and `Apply` (`ErrTooManyRooms`). Default unlimited.

### Trust model

`Server.Apply` and `Server.BroadcastUpdate` grant total write authority
on the document. Treat the `*Server` handle with the same care as a
database connection — do not expose it directly to untrusted code.
`OnInject` is defense-in-depth, not a substitute for caller-side
authorization. A caller who can reach either API can craft updates that
spoof any client ID, which is equivalent to the authority already
granted by `GetDoc` + `ApplyUpdateV1`.

## Attribution

The `crdt` package exposes yjs-v14's attribution primitives for stamping CRDT
content with per-item authorship metadata — who inserted or deleted which
item, and any other attribute you want to attach (`userid`, `ts`, a request
ID, …) — without integrating the update into a doc. This is the pattern
[y-redis](https://github.com/yjs/y-redis) (also known as y/hub, the Yjs
cluster backend) uses to store per-character authorship in Postgres.

- **`IDSet`** — the set of item IDs touched by an update or a doc (yjs-v14
  `IdSet`): per-client sorted, merged `(clock, len)` runs.
- **`IDMap`** — `IDSet` plus attribution data per range (yjs-v14 `IdMap`):
  overlapping ranges are split and their attributes joined on read.
- **`ContentAttribute`** — one `{Name, Value}` fact, built via
  `NewContentAttribute`/`MustContentAttribute`, which validate `Value` against
  the lib0 `any` domain so a bad value fails at the call site instead of
  panicking deep inside the encoder.
- **`ContentIDs`** / **`ContentMap`** — the insert/delete pair of `IDSet`s or
  `IDMap`s for one update or doc (yjs-v14 `ContentIds` / `ContentMap`).
- Builders: `ContentIDsFromUpdateV1`/`V2` (extract touched IDs from an update
  without applying it), `InsertSetFromDoc`/`DeleteSetFromDoc` (extract from a
  live doc's store), `CreateContentMapFromContentIDs` (stamp IDs with
  attributes).
- Set algebra mirroring yjs-v14: `MergeIDSets`/`MergeIDMaps`,
  `ExcludeIDSet`/`ExcludeIDMap`, `IntersectIDSets`/`IntersectIDMaps`,
  `FilterIDMap`, plus the `ContentMap`-level wrappers
  `MergeContentMaps`/`ExcludeContentMap`/`IntersectContentMaps`/`FilterContentMap`.

Typical server-side flow: a collaborator's update arrives, the server stamps
it with attribution and stores both side by side, without ever integrating
the update into a live doc.

```go
// A collaborator produced an update…
src := crdt.New(crdt.WithClientID(7))
txt := src.GetText("t") // resolve OUTSIDE Transact
src.Transact(func(txn *crdt.Transaction) { txt.Insert(txn, 0, "hi", nil) })
update := crdt.EncodeStateAsUpdateV1(src, nil)

// …the server stamps it without integrating it.
ids, _ := crdt.ContentIDsFromUpdateV1(update)
userid := crdt.MustContentAttribute("userid", "alice")
cm := crdt.CreateContentMapFromContentIDs(ids, []*crdt.ContentAttribute{userid}, nil)

// Store EncodeContentMap(cm) next to the update; later, read it back.
decoded, _ := crdt.DecodeContentMap(crdt.EncodeContentMap(cm))
for _, r := range decoded.Inserts.Slice(7, 0, 2) {
    fmt.Printf("clocks [%d,%d): %s=%v\n", r.Clock, r.Clock+r.Len, r.Attrs[0].Name, r.Attrs[0].Value)
}
// clocks [0,2): userid=alice
```

See `Example_attribution` in [`crdt/example_attribution_test.go`](crdt/example_attribution_test.go)
for the runnable version (`go test ./crdt/ -run Example_attribution`).

### Interop scope — read this before treating attribution as wire-stable with a specific yjs release

- **`IDSet`/`IDMap` are byte-compatible with published yjs v14.** Verified
  byte-for-byte against a pinned yjs14 build (`npm:yjs@14.0.0-16`) — see
  `crdt/attribution_js_compat_test.go` and `testutil/gen_fixtures_attribution.js`.
- **`ContentMap`/`ContentIDs` follow yjs-main's `writeContentMap`/`writeContentIds`
  composition** (two `IdMap`s / `IdSet`s, inserts then deletes) — each half is
  byte-verified as above, but **no published yjs v14 rc exposes a top-level
  `ContentMap`/`encodeContentMap` function to pin the wrapper against**; that
  API exists only on yjs's unreleased `main` branch as of this writing. Treat
  `ContentMap`'s wire format as "matches yjs-main's composition of
  byte-verified parts," not "byte-compatible with a shipped `yjs.encodeContentMap`."
  Tracked as a follow-up to re-verify once yjs v14.0.0 final publishes that API.
- **`diffDocsToDelta`** (yjs-main's Prosemirror-style delta-with-attribution
  renderer) is **not implemented**. It depends on yjs v14's delta/renderer
  subsystem, which is still changing on `main`; porting it is a tracked
  follow-up once that subsystem stabilizes in a published release.
- **No storage integration.** ygo does not persist `ContentMap`s for you —
  callers store `EncodeContentMap(cm)` next to the update in their own
  database, exactly as shown above.
- **Garbage collection erases attributed history.** `IDSet`/`IDMap` reference
  item IDs by `(client, clock)`; once GC (the default) frees deleted items'
  content, ranges that pointed at deleted content no longer correspond to
  retrievable data. To render attributed history later (e.g. "who wrote this
  paragraph, including deleted-and-restored text"), either retain the raw
  updates you stamped (so `ContentIDsFromUpdateV1` can re-derive IDs from the
  update itself) or create the doc with `crdt.WithGC(false)` — see
  `CreateDocFromSnapshot` and the snapshot-history pattern above.

## Persistence

The WebSocket server takes an optional `PersistenceAdapter` so room state survives restarts:

```go
type PersistenceAdapter interface {
    LoadDoc(room string) ([]byte, error)
    StoreUpdate(room string, update []byte) error
}
```

`LoadDoc` is called once when the first peer connects to a room; the result seeds the in-memory doc. `StoreUpdate` is called on every committed transaction. Writes run on a per-room worker goroutine — slow storage doesn't block peers. Wire an adapter in via `NewServerWithPersistence(adapter)`.

By default (v1.36.0) these writes are coalesced: the worker debounces bursts of updates into a single merged `StoreUpdate` call using a 2s window (reset on each new update) capped by a 10s max wait, matching Hocuspocus's default debounce behaviour. Tune it via `Server.PersistCoalesceWindow` / `Server.PersistCoalesceMaxWait`, or set `PersistCoalesceWindow` to a negative value (e.g. `-1`) to disable coalescing and restore strict one-write-per-update persistence.

A room's pending coalesced batch is flushed durably before the room unloads (v1.37.0), so a peer that reconnects during a quick refresh reuses the live document instead of racing a stale reload from the backing store. Adapters that also want to bound stored-version growth can implement `CompactableAdapter` (`Compact(ctx, room) error`); the server invokes it on room unload and, when `Server.CompactEvery > 0`, after every N persistence flushes. `persistence.LegacyAdapter` implements this via its `KeepVersions` field.

`Server.RoomIdleTimeout` (v1.39.0) goes a step further: instead of unloading a room the instant its last peer disconnects, it keeps the room resident — the durable flush above still runs immediately, but the worker and in-memory doc stay warm — so a peer that reconnects within the timeout reuses the live doc with no `LoadDoc` at all. `Server.MaxResidentRooms` puts an LRU bound on how many idle rooms stay warm at once. Both default to zero, which is the original eager-evict-on-last-peer behaviour; see [Resource limits](#resource-limits) below for the field details.

For a ready-made durable backend, [`persistence/sqlite`](persistence/sqlite/) provides a pure-Go (CGO-free, `modernc.org/sqlite`) `VersionedPersistence` store with WAL mode, full versioned history, and a crash-safe two-phase prune. Open it with `sqlite.Open("data.db")`.

For backend examples (Postgres, Redis, file-system) and the v1.7.0 context-aware extension that lets adapters abort in-flight writes during `Server.Shutdown`, see [docs/PERSISTENCE.md](docs/PERSISTENCE.md).

## Subdocuments

A `Doc` can embed another `Doc` as a **subdocument** — a separate CRDT
document (its own clock space, its own GUID) nested inside a parent doc's
`YMap`. This is the same pattern Yjs uses to split a large workspace into
independently-loadable documents (e.g. one subdoc per page in a multi-page
app) while still tracking them from a single root doc.

Embed one by setting a `*crdt.Doc` as a `YMap` value — there is no separate
"embed" method, it's just `Set`:

```go
parent := crdt.New()
child := crdt.New(crdt.WithGUID("page-1"), crdt.WithAutoLoad(true))

root := parent.GetMap("pages") // resolve OUTSIDE Transact
parent.Transact(func(txn *crdt.Transaction) {
    root.Set(txn, "page-1", child)
})
```

`YMap.Get` returns the embedded `*crdt.Doc` back out (type-assert on read,
same as any other `any`-typed map value). A `Doc` can only be embedded once;
embedding the same `*Doc` a second time panics with `ErrSubdocAlreadyIntegrated`
— create a second `Doc` with the same GUID instead.

- **`Doc.GetSubdocs()` / `Doc.GetSubdocGUIDs()`** — the subdocuments currently
  resident on this doc (sorted by GUID). Reflects adds/removes even if nothing
  is observing via `OnSubdocs`.
- **`Doc.OnSubdocs(func(crdt.SubdocsEvent))`** — fires once per transaction
  that changes subdocument state. `SubdocsEvent{Added, Removed, Loaded []*Doc}`
  reports docs newly embedded, docs detached (their `YMap` entry deleted), and
  docs that should now be synced (see `Load` below). Embedding and then
  deleting the same doc within one transaction cancels out — no event fires.
- **`Doc.Load()`** — signals that a subdocument's data should be synced now.
  A locally-created, locally-embedded subdoc loads immediately on integrate
  (`ShouldLoad()` defaults to `true`); a subdoc decoded off the wire starts
  with `ShouldLoad() == false` until the receiving application decides to
  page it in and calls `Load()`, which flips `ShouldLoad()` to `true` and
  emits a `Loaded` event on the **parent**. `Load` opens a transaction on the
  parent, so — like `GetText` — it must not be called from inside a `Transact`
  closure.
- **`crdt.WithAutoLoad(bool)`** — marks a subdocument so remote peers/providers
  auto-load it instead of waiting for an explicit `Load()` call. Default `false`.
- **`crdt.WithShouldLoad(bool)`** — sets the initial `ShouldLoad()` value.
  Default `true` for a doc you create yourself; a doc materialized from a
  decoded update starts `false` (derived from `autoLoad`) until `Load()`.
- **`crdt.WithCollectionID(string)`** — an optional grouping label
  (`Doc.CollectionID()`), e.g. to batch-load every subdoc in a collection.

```go
child := crdt.New(
    crdt.WithGUID("page-1"),
    crdt.WithAutoLoad(true),
    crdt.WithCollectionID("workspace-42"),
)
```

See `Example_subdocs` in
[`crdt/example_subdocs_test.go`](crdt/example_subdocs_test.go) for the
runnable version (`go test ./crdt/ -run Example_subdocs`).

**Scope**: this is the local lifecycle surface — creating, embedding,
enumerating, and observing subdocuments on a single doc, matching Yjs's
`Y.Doc` subdocs API. Actually syncing a subdocument's *contents* across peers
(a provider recognizing `Added`/`Loaded` events and opening a connection per
subdocument) is a separate, not-yet-implemented layer, tracked in
[#142](https://github.com/reearth/ygo/issues/142).

## Mobile (iOS / Android)

The [`mobile/`](mobile/) subpackage is a [`gomobile bind`](https://pkg.go.dev/golang.org/x/mobile/cmd/gomobile)-able façade over `crdt` and `awareness`, so you can embed ygo natively in iOS and Android apps — **no JavaScript runtime and no CGO**. It is a **full on-device editor**: a Swift/Kotlin app can edit locally, sync, render, exchange presence, and subscribe to push change-notifications — not just receive and display.

```
gomobile bind -target=ios                ./mobile   # → Mobile.xcframework
gomobile bind -target=android -androidapi 21 ./mobile  # → mobile.aar
```

`Doc` exposes sync (`ApplyUpdate`, `EncodeStateAsUpdate`, `EncodeStateVector`, `EncodeDiff`), read accessors (`GetText`, `GetTextJSON`, `GetMapJSON`, `GetArrayJSON`), on-device mutators (`InsertText`, `FormatText`, `DeleteText`, `InsertArray`, `DeleteArray`, `SetMap`, `DeleteMapKey`, …), and change observers (`Observe`); `Awareness` exposes presence (`SetLocalState`, `StatesJSON`, `EncodeAll`, `ApplyUpdate`) plus its own `Observe`. Every exported signature uses only gomobile-safe types (`string` / `int64` / `bool` / `[]byte` / `error`, plus the bound `*Doc` / `*Awareness` / `*Subscription` and observer interfaces), and `Close()` releases the native state. `gomobile` is a build-time tool, not a dependency — `go.mod` is unchanged. See [`mobile/README.md`](mobile/README.md) for the build matrix, threading and lifecycle guidance, binary size / ABI notes, and Kotlin / Swift snippets.

## Running in production

The library ships several operational hooks. See package godoc for the full reference; here's the short version of what to wire up.

### Logging

`Server.Logger *slog.Logger` defaults to `slog.Default()`. Surfaces slow-peer write failures, sync-dispatch errors, and awareness apply errors at `Warn` level with `room` and `peer` context.

```go
server := websocket.NewServer()
server.Logger = slog.New(slog.NewJSONHandler(os.Stdout, nil))
```

### Observability

`Doc.PendingStats()` returns a snapshot of the per-doc pending queue: how many items are parked, how many delete-set ranges are queued, which clients we're blocked on. Cheap (one read-lock). Useful when monitoring out-of-order delivery in production.

```go
stats := doc.PendingStats()
metrics.PendingItems.Set(float64(stats.Items))
metrics.PendingDeleteRanges.Set(float64(stats.DeleteRanges))
```

### Resource limits

All hard-capped (semaphore-backed for connection counts):

- `Server.MaxConnections` — server-wide cap on simultaneous WebSocket peers.
- `Server.MaxPeersPerRoom` — per-room cap.
- `Server.MaxRooms` — total-room cap (applies to peer upgrades and `Server.Apply`).
- `Server.MaxUpdateBytes` — per-update size cap (default 64 MiB).
- `Server.MaxMessageBytes` — per-message size on the WebSocket read path (default 64 MiB).
- `Server.PeerWriteQueueSize` — per-peer broadcast queue depth (default 256). When the queue fills, the peer is disconnected.
- `Server.MaxPendingItems` — per-document cap on items parked in the out-of-order pending queue (default 100,000). When the cap is reached, updates that would park additional items return `ErrInvalidUpdate`. Defends against a crafted update full of far-future-clock items that would otherwise grow the queue unboundedly. Same cap is available at the doc level via `crdt.WithMaxPendingItems(n)`.
- `Server.HandshakeTimeout` — first-read deadline applied after WebSocket upgrade (default 30s). Closes connections that complete the handshake but never send a message (slow-loris defense). Cleared after the first successful read.
- `Server.MaxAwarenessBytesPerRoom` — cap on the cumulative byte size of awareness state held in one room across all remote clients (default unlimited; suggested production value: 100 MiB). Without this cap, a single peer can claim up to 10,000 clientIDs each holding the 1 MiB per-state maximum. Forwarded to each room's `Awareness` via `awareness.Awareness.SetMaxBytes`.
- `Server.RoomIdleTimeout` (v1.39.0) — keeps a room resident and warm (durably flushed, in-memory doc intact) for this long after its last peer disconnects, so a quick reconnect skips a full `LoadDoc` reload. Default 0: evict immediately, matching prior releases.
- `Server.MaxResidentRooms` (v1.39.0) — LRU bound on how many idle-resident rooms stay warm at once when `RoomIdleTimeout > 0`; a background sweeper evicts the least-recently-idle room first once the count is exceeded. Default 0: unbounded.

Each defaults to a sensible value or unlimited where noted.

### Auth

`Server.AuthFunc func(*http.Request) bool` runs before the WebSocket upgrade. Return false to reject:

```go
server.AuthFunc = func(r *http.Request) bool {
    return validateBearer(r.Header.Get("Authorization"))
}
```

For **read-only connections** (v1.30.0, #59), use `Server.Authorize` instead — it both accepts/rejects and reports per-connection config. A read-only peer receives document and awareness broadcasts but its inbound writes (sync step-2/update and awareness) are dropped server-side; it can still request state (SyncStep1) and query awareness. When set, `Authorize` takes precedence over `AuthFunc`:

```go
server.Authorize = func(r *http.Request) (ygws.ConnectionConfig, bool) {
    if !validateBearer(r.Header.Get("Authorization")) {
        return ygws.ConnectionConfig{}, false // reject → 401
    }
    return ygws.ConnectionConfig{ReadOnly: !isEditor(r)}, true
}
```

### Graceful shutdown

`Server.Shutdown(ctx)` drains pending writes and closes peers. Adapters that implement `PersistenceAdapterContext` (v1.7.0) receive a context derived from the shutdown signal so they can abort in-flight DB calls instead of waiting for the driver's timeout.

## Performance

### Running the benchmarks

```bash
# Run all benchmarks with memory allocation stats
go test ./... -run='^$' -bench='^Benchmark' -benchmem

# Run a specific package only
go test ./crdt/ -run='^$' -bench='^Benchmark' -benchmem

# Run with more iterations for tighter confidence intervals
go test ./... -run='^$' -bench='^Benchmark' -benchmem -benchtime=5s -count=3
```

To compare two branches (e.g. before and after an optimization), install [`benchstat`](https://pkg.go.dev/golang.org/x/perf/cmd/benchstat):

```bash
go install golang.org/x/perf/cmd/benchstat@latest

# Capture baseline
git checkout main
go test ./... -run='^$' -bench='^Benchmark' -benchmem -count=5 | tee old.txt

# Capture candidate
git checkout my-branch
go test ./... -run='^$' -bench='^Benchmark' -benchmem -count=5 | tee new.txt

# Compare
benchstat old.txt new.txt
```

The CI benchmark workflow (`.github/workflows/benchmark.yml`) runs this comparison automatically on every pull request.

### Reference numbers

Measured on Apple M4 Max (arm64, Go 1.23). Your numbers will vary by hardware.

**Encoding (`encoding/`)** — the codec runs on every item; these are sub-10 ns, zero-alloc:

| Benchmark | ns/op | Allocs |
|-----------|-------|--------|
| ReadVarUint (1 byte) | 1.0 | 0 |
| WriteVarUint (1 byte) | 1.7 | 0 |
| WriteVarString (1000 chars) | 15 | 0 |
| ReadVarString (1000 chars) | 89 | 1 (string copy) |
| Encoder reuse (`Reset`) vs new | 7.7 vs 12.4 | 0 vs 1 |

**CRDT core (`crdt/`)** — realistic document operations:

| Benchmark | ns/op | Notes |
|-----------|-------|-------|
| `YText_InsertBulk` (1000 chars) | 2 006 | Single transaction — fast path |
| `YText_Insert` (1000 × 1 char) | 344 048 | ~344 ns per keystroke |
| `YText_Delete` (1000 × 1 char) | 891 456 | ~891 ns per delete |
| `EncodeStateAsUpdateV1` (1000 items) | 21 360 | ~21 µs to serialise a document |
| `ApplyUpdateV1` (1000 items) | 109 806 | ~110 µs to integrate a full state |
| `EncodeStateAsUpdateV2` | 33 029 | V2 is ~1.5× larger to encode… |
| `ApplyUpdateV2` | 679 207 | …and ~6× slower to decode |
| `TwoPeerConvergence` | 16 284 | Encode + apply incremental sync |
| `YMap_Set` (100 keys) | 19 557 | |
| `YArray_Push` (100 elements) | 59 209 | |

**Sync protocol (`sync/`)** — message framing overhead is negligible:

| Benchmark | ns/op |
|-----------|-------|
| `EncodeSyncStep1` | 179 |
| `ApplySyncMessage_Step1` | 631 |
| `ApplySyncMessage_Update` (1000-item doc) | 1 404 |
| `FullHandshake` | 1 303 |

**Awareness (`awareness/`)** — per-peer ephemeral state:

| Benchmark | ns/op |
|-----------|-------|
| `SetLocalState` | 65 |
| `EncodeUpdate` (1 client) | 226 |
| `EncodeUpdate` (50 clients) | 12 901 |
| `ApplyUpdate` (50 clients) | 19 801 |

## Architecture

See [docs/ARCHITECTURE.md](docs/ARCHITECTURE.md) for a detailed explanation of the CRDT algorithm, data model, and package design.

## Compatibility

ygo targets compatibility with:

- [Yjs](https://github.com/yjs/yjs) v13.x (JavaScript reference implementation)
- [y-protocols](https://github.com/yjs/y-protocols) sync and awareness protocol
- [lib0](https://github.com/dmonad/lib0) binary encoding format

Compatibility is verified by golden-file tests that compare binary output byte-for-byte with Yjs-generated fixtures, in both directions, plus a differential convergence fuzzer that checks ygo's merge results against Yjs over randomized concurrent-edit histories. Both run in CI.

For a Go-vs-Rust port comparison, see [docs/comparison/ygo-vs-yrs.md](docs/comparison/ygo-vs-yrs.md).

## Versioning and API stability

ygo follows [semantic versioning](https://semver.org/). The v1.x public API is stable: new functionality lands as minor releases; bug fixes as patch releases; breaking changes are deferred to v2.

Note that the bump size follows the change's **API surface**, not its intent — a bug-fix release that adds a new exported symbol is still a minor.

## Gotchas

### No read methods or observer registration inside `Transact`

`Transact` acquires the document **write lock** for the duration of its callback.
Calling any of the read methods (`Get`, `ToSlice`, `Keys`, `Entries`, `ToString`,
`ToDelta`) or registering/unregistering observers (`Observe`, `ObserveDeep`) from
**inside** a `Transact` callback will **deadlock** because those operations try to
acquire the same lock.

```go
// ✗ WRONG — deadlocks
doc.Transact(func(txn *crdt.Transaction) {
    arr.Get(0)         // tries to RLock — deadlock
    arr.Observe(fn)    // tries to Lock  — deadlock
})

// ✓ CORRECT — acquire references and register observers before Transact
arr.Observe(func(e crdt.YArrayEvent) { /* ... */ })
doc.Transact(func(txn *crdt.Transaction) {
    arr.Push(txn, []any{"value"})
})
fmt.Println(arr.ToSlice()) // read after Transact returns
```

This constraint applies to `YArray`, `YText`, `YMap`, `YXmlFragment`, and
`YXmlElement`. UndoManager callbacks (`OnStackItemAdded`) also run outside
the lock and are safe to use normally.

### `Doc.ClientID` is read-only after creation

Use `crdt.WithClientID(id)` at construction time. Changing the ID after the
document has started accepting operations will corrupt the item store.

## What's changed since v1.0

Forty-plus minor and patch releases, summarised under [Post-v1.0 hardening](#features) above. Broadly: the early arc (v1.1.x–v1.7.x) focused on production hardening — panic safety, out-of-order convergence, WebSocket hooks, observability, error-returning variants, context-aware persistence. The middle arc (v1.8.x–v1.14.x) delivered a systematic cross-reference audit against Yjs JS and yrs, closing correctness gaps in YATA boundary handling, the awareness protocol, delete-path cascade, lib0 wire-format parity, YText format markers, and JSON serialisation of nested shared types — tracked under the [`gaps` label](https://github.com/reearth/ygo/issues?label=gaps). Since then the work has been mostly server-side: Hocuspocus compatibility, horizontal scale, versioned and coalesced persistence, mobile bindings, and positional-access performance.

See [CHANGELOG.md](CHANGELOG.md) for per-release detail and [docs/HISTORY.md](docs/HISTORY.md) for the design narrative.

## Documentation

| Document | What it covers |
|---|---|
| [docs/ARCHITECTURE.md](docs/ARCHITECTURE.md) | The CRDT algorithm (YATA), data model, and package layout |
| [docs/INTERNALS.md](docs/INTERNALS.md) | Implementation detail below the public API — item store, integration, encoding |
| [docs/CLUSTERING.md](docs/CLUSTERING.md) | Running multiple server instances behind a load balancer via the cluster relay |
| [docs/PERSISTENCE.md](docs/PERSISTENCE.md) | Persistence adapters, versioning, compaction, and durability guarantees |
| [docs/comparison/ygo-vs-yrs.md](docs/comparison/ygo-vs-yrs.md) | Go-vs-Rust port comparison against yrs |
| [docs/HISTORY.md](docs/HISTORY.md) | The design narrative across releases |
| [CHANGELOG.md](CHANGELOG.md) | Per-release changes |
| [SECURITY.md](SECURITY.md) | Threat model and vulnerability reporting |
| [CONTRIBUTING.md](CONTRIBUTING.md) | Development workflow, benchmarking discipline, PR checklist |

API reference: [pkg.go.dev/github.com/reearth/ygo](https://pkg.go.dev/github.com/reearth/ygo).

## Contributing

Contributions are welcome! Please read [CONTRIBUTING.md](CONTRIBUTING.md) before submitting a pull request.

For significant changes, open an issue first to discuss what you'd like to change.

## Security

ygo's security model is **defense-in-depth**, not authentication:

- **`ClientID` is collision-avoidance, not authentication.** The protocol does not validate that incoming updates match a peer's declared `ClientID`. Use `Server.AuthFunc` and/or transport-level auth.
- **Transport security is the embedder's responsibility.** ygo does not enforce TLS, signed updates, or peer authentication. Wrap the WebSocket server behind your usual reverse proxy.
- **`Server.Apply` and `Server.BroadcastUpdate` grant total write authority.** Treat the `*Server` handle like a database connection. `OnInject` is defense-in-depth, not a substitute for caller-side authorization.
- **`ClientID` generation uses `crypto/rand`** (v1.5.0). 32-bit space matches Yjs JS for wire compatibility; collision probability at multi-tenant scale is documented in [SECURITY.md](SECURITY.md).

Please report vulnerabilities by following the process in [SECURITY.md](SECURITY.md). Do not open public issues for security problems.

## License

MIT License — see [LICENSE](LICENSE).

This project is not affiliated with the Yjs authors. Yjs is developed by [Kevin Jahns](https://github.com/dmonad) and contributors.

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