# Client-Side Federation System

> **Companion spec:** This document covers the **client-side** multi-instance architecture. For server-to-server relay (HMAC auth, outbox pipeline, relay events, identity resolution), see [`federation.md`](federation.md). Both systems work together — S2S relay distributes data between servers, while this client system enables users to interact with multiple instances from a single app session.

Source files:
- `packages/web/src/stores/instanceStore.ts` — Core multi-instance connection management, token caching, topology sync
- `packages/web/src/hooks/useWebSocket.ts` — WebSocket multiplexing (one connection per instance), origin-aware event routing
- `packages/web/src/stores/spaceStore.ts` — Origin-aware space/channel store, `channelOriginMap`, `getChannelOrigin()`, `getApiForOrigin()`, DM deduplication
- `packages/web/src/utils/identity.ts` — Cross-instance user identity resolution (`isSelf`, `canonicalUserMatch`, self-ID registry)
- `packages/web/src/hooks/useInstanceConnect.ts` — Connection flow hook for the Connections UI
- `packages/web/src/components/modals/ConnectedInstances.tsx` — Connections settings panel

---

## Architecture Overview

Backspace supports **client-side federation**: a single app session (web or desktop — both are feature-identical) can connect to multiple Backspace instances simultaneously. The user has a **home instance** (their primary identity) and zero or more **connected remote instances**.

```
┌─────────────────────────────────────────────────┐
│               Electron / Web App                │
│                                                 │
│  ┌──────────────┐    ┌──────────────────────┐   │
│  │ Home Instance │    │ Remote Instance(s)   │   │
│  │ nova.ddns.net│    │ orbit.ddns.net│   │
│  │              │    │                      │   │
│  │ WS ──────────┤    │ WS ──────────────────┤   │
│  │ API ─────────┤    │ API ─────────────────┤   │
│  │ JWT ─────────┤    │ JWT ─────────────────┤   │
│  └──────────────┘    └──────────────────────┘   │
│                                                 │
│  instanceStore manages all connections          │
│  spaceStore merges data from all origins        │
│  channelOriginMap routes operations to origin   │
└─────────────────────────────────────────────────┘
```

**What this enables:**
- Join Spaces on any connected instance
- See friends across instances (friend discovery)
- DMs between users on different instances (via S2S relay — see [federation.md](federation.md))

**What each instance provides:**
- Its own JWT token and authenticated API client
- Its own WebSocket connection (heartbeat, events)
- Its own user identity (different Snowflake ID per instance)

---

## 1. Federated Account Creation

When a user connects to a remote instance, the client creates (or logs into) an account on that instance. This is a **real account with a real bcrypt password** — not a replicated stub.

### Username Format

| Account type | Username | passwordHash | homeInstance | Can log in? |
|---|---|---|---|---|
| Local (native) | `youruser` | bcrypt hash | `NULL` | Yes |
| Federated (client-created) | `youruser@nova.ddns.net` | bcrypt hash | `nova.ddns.net` | Yes |
| Replicated stub (S2S-created) | `youruser@nova.ddns.net` | `!federation-replicated` | `nova.ddns.net` | No |

Key distinction: **Federated accounts** and **replicated stubs** can have the same username format (`user@instance`), but federated accounts have real passwords and can log in. Replicated stubs are server-created placeholders for identity resolution and cannot log in.

The merge migration in `migrate.ts` detects when both exist for the same remote user and merges them (real account always wins).

### Connection Flow (`connectToRemote`)

When a user adds a remote instance via the Connections settings:

1. **Verify home password** — client confirms the user's password against the home instance
2. **Compute federated username** — `{bareUsername}@{homeHost}` (e.g., `youruser@nova.ddns.net`)
3. **Try registration** on remote instance with:
   - Username: `youruser@nova.ddns.net`
   - Password: same as home instance password
   - `homeInstance`: `nova.ddns.net` (bare domain)
   - `homeUserId`: user's Snowflake ID on home instance
4. **If registration fails** (account already exists) — fall back to login
5. **On success** — store JWT token, create API client, open WebSocket, sync profile

The same password is used across all instances. Password changes on the home instance are synced to remote instances automatically.

---

## 2. Instance Store (`instanceStore.ts`)

The central store for multi-instance state.

### State

```typescript
interface ConnectedInstance {
  origin: string;           // 'https://orbit.ddns.net'
  label: string;            // Instance display name
  token: string;            // JWT for this instance
  user: User;               // User record on this instance
  username: string;         // e.g., 'youruser@nova.ddns.net'
  status: 'connected' | 'connecting' | 'disconnected' | 'error';
  error?: string;
  api: BackspaceApiClient;  // Authenticated API client
}

interface InstanceState {
  instances: ConnectedInstance[];
  _autoConnectDone: boolean;    // Has startup reconnection finished?
  pendingSyncOrigins: string[]; // Instances needing password sync
}
```

### Token Caching

Tokens are persisted to `localStorage` keyed by `backspace_instances_${userId}`. This allows automatic reconnection on app restart without re-entering passwords.

### Auto-Connect on Startup (`autoConnectAll`)

Called once per session after login:

1. Read `currentUser.replicatedInstances` from the home server (list of known remote origins)
2. Load cached tokens from `localStorage`
3. For instances **with cached tokens**: attempt reconnection in parallel — verify token, open WebSocket, sync profile
4. For instances **without cached tokens**: create error placeholders (visible in Connections UI with "re-authenticate" prompt)
5. Set `_autoConnectDone = true` to unblock topology sync

### Topology Sync (`syncInstanceList`)

After connections change, the client notifies all instances of the current topology. Each instance receives a perspective-correct list:
- **Home instance** gets: list of all remote origins
- **Remote instances** get: home origin + all other remote origins (excluding self)

This allows S2S federation to know which peers to relay to.

---

## 3. Origin-Aware Routing

### Origin String Convention

- `''` (empty string) = home instance
- `'https://domain.com'` = remote instance (full URL with protocol)

### channelOriginMap

Every channel (space channels and DM channels) is tagged with its origin instance:

```typescript
// In spaceStore:
channelOriginMap: Map<string, string>  // channelId → origin

// Usage:
getChannelOrigin(channelId): string    // Returns '' for home, origin URL for remote
```

Built during `populateFromReady()` when WS ready events arrive from each instance.

> **DM channels** are always mapped to `''` (home origin). `channelOriginMap` is only relevant for Space channels. DM operations always route to the home instance; S2S relay handles cross-instance distribution.

### API Client Resolution

```typescript
getApiForOrigin(origin: string): BackspaceApiClient
```

Returns the correct API client for the given origin. Uses a resolver pattern to break circular dependencies between stores:

- `instanceStore` registers the resolver at module init
- `spaceStore` exposes `getApiForOrigin()` which calls the registered resolver
- Consumers call `getApiForOrigin(getChannelOrigin(channelId))` to get the right client

### User Origin Resolution

```typescript
resolveUserOrigin(user: { homeInstance?: string | null }): string
```

Determines which connected instance a user belongs to, based on their `homeInstance` field. Returns the origin string or `''` for local users.

---

## 4. WebSocket Multiplexing (`useWebSocket.ts`)

The client maintains **one WebSocket connection per instance** (home + each remote). Each connection has:
- Independent heartbeat (15-second ping via Web Worker)
- Exponential backoff reconnection
- Origin-aware event dispatching

### Sending

```typescript
wsSend(event, origin)    // Send to specific instance
wsSendAll(event)         // Broadcast to all instances
```

### Receiving

All incoming WS events pass through `handleEvent(origin, event)`. The `origin` parameter identifies which instance sent the event, enabling origin-aware state updates.

### Ready Event Processing

When a WS connection opens and authenticates, the server sends a `ready` event containing spaces, DM channels, voice states, etc. The client processes this via `populateFromReady()`:

1. Tag all spaces with `_instanceOrigin`
2. Merge into the unified space list (replacing stale data from same origin)
3. Build/update `channelOriginMap`, `channelToSpaceMap`
4. Normalize remote asset URLs to absolute paths
5. Skip DM channels from remote origins — DMs are managed exclusively by the home instance
6. Last-write-wins layout merge for sidebar order

---

## 5. Cross-Instance Identity (`identity.ts`)

Users have **different Snowflake IDs on each instance**. The identity system resolves these:

### Self-ID Registry

```typescript
registerSelfId(id)   // Called on each WS ready event
isSelf(user)         // Checks all registered IDs
```

Tracks all IDs belonging to the current user across instances.

### Display Identity Resolution

```typescript
resolveDisplayIdentity(user, homeUser): User
```

If a user is `isSelf()`, returns the home user for consistent avatar/display name rendering. Prevents the same person appearing with different profiles across instances.

### Canonical User Match

```typescript
canonicalUserMatch(a, b): boolean
```

Determines if two user records represent the same person across instances. Cascade: same local ID → same homeUserId → username+homeInstance match.

---

## 6. Connections Settings UI

The **Connections** panel (in user settings) allows managing remote instance connections:

- **Home Instance** — always shown, cannot be removed. Desktop app has a "Change" button.
- **Remote Instances** — each shows status (connected/disconnected/error), hostname, username. Actions: Reconnect, Re-authenticate, Sync Password, Disconnect.
- **Add Instance** — multi-step form: enter hostname → verify password → register/login → connected.

---

## 7. Federation Registry

The federation registry is a persistent server-side record of all instances a user has federated with. Unlike the `replicatedInstances` field (which serves S2S topology relay), the registry tracks the full lifecycle of each connection.

### Storage

- **Server:** `user_federation_registry` table — composite PK `(userId, origin)`
- **Client:** `registry` Map in `instanceStore` (Zustand)
- **LWW timestamp:** `federationRegistryUpdatedAt` on `users` table

### Lifecycle States

| State | Meaning |
|-------|---------|
| `connected` | Active WebSocket, valid token |
| `disconnected` | User intentionally disconnected; account exists on remote |
| `unreachable` | Remote is down/unresponsive |
| `auth_expired` | Token invalid; needs re-authentication |

### Sync Pattern

Client-driven LWW whole-registry push (same pattern as `profileSync.ts`):
1. User mutates registry → `registryUpdatedAt = Date.now()`
2. Client calls `PUT /api/users/@me/federation-registry` on all connected instances
3. Server rejects if `updatedAt <= stored` (409 Conflict)
4. On startup, client fetches registry from home via `GET`, merges with localStorage tokens

### API

- `GET /api/users/@me/federation-registry` — fetch registry + updatedAt
- `PUT /api/users/@me/federation-registry` — LWW whole-registry push

### Relationship to replicatedInstances

`replicatedInstances` continues to serve S2S topology relay — it tells the federation layer which peers to relay events to. The registry is a superset that also includes disconnected, unreachable, and auth-expired entries. The two are maintained independently.

---

## 8. Relationship to S2S Federation

Client-side and S2S federation serve different purposes:

| Aspect | Client-Side Federation | S2S Federation |
|---|---|---|
| **Purpose** | User interacts with multiple instances | Instances exchange data automatically |
| **Scope** | Spaces, friend discovery | DM relay, friend relay, file replication |
| **Authentication** | Per-user JWT on each instance | Per-peer HMAC shared secret |
| **Initiated by** | User (Connections settings) | Admin (peer handshake) |
| **Connection** | Client → each server directly | Server → server via outbox |

**How they work together:**
1. User adds a remote instance via Connections (client-side)
2. The client triggers S2S peering between the two servers (automatic)
3. User joins Spaces on the remote instance (client-side — API calls go directly to remote)
4. User sends DMs — all DM writes go to the home instance. S2S relay distributes messages, reactions, and membership changes to peer instances.
5. Friend requests and discovery work across instances (client loads friends from all connected instances, S2S relays friend events)
