# Federation System (Server-to-Server)

> **Companion spec:** This document covers **S2S (server-to-server)** federation — the relay protocol, HMAC auth, identity resolution, and background workers. For the **client-side** multi-instance architecture (how the web/desktop app connects to multiple instances, federated account creation, origin-aware routing), see [`client-federation.md`](client-federation.md). Both systems work together.

Source files:
- `packages/server/src/routes/federation.ts` -- API endpoints (peer handshake, relay, sync) + all inbound event processors + identity resolution functions
- `packages/server/src/utils/federationAuth.ts` -- HMAC signing, verification, header parsing, `getOurOrigin()`
- `packages/server/src/utils/federationOutbox.ts` -- Event queuing, coalescing, relay payload construction, mutation log, participant/target resolution
- `packages/server/src/utils/federationWorker.ts` -- Background workers: outbox delivery, file download, health check, janitor, initial sync
- `packages/server/src/utils/storageJanitor.ts` -- Federation GC: outbox expiry, mutation log retention, file queue cleanup, DM channel purge
- `packages/server/src/routes/social.ts` -- Friend request/accept/cancel/remove endpoints that queue federation events
- `packages/server/src/routes/dm.ts` -- DM REST endpoints that queue federation events (message relay, group lifecycle)
- `packages/server/src/ws/events.ts` -- WebSocket event handlers that queue DM message/reaction relay events
- `packages/web/src/utils/profileSync.ts` -- Client-side profile sync via LWW timestamps (not S2S relay)
- `packages/web/src/utils/identity.ts` -- Client-side federated identity resolution helpers

DB tables: `federation_peers`, `federation_outbox`, `federation_file_queue`, `federation_mutation_log`, plus `users` (identity), `dm_channels`/`dm_members`/`dm_messages` (DM federation), `friends`/`friend_requests` (friend federation), `attachments` (file replication).
See `docs/systems/database.md` for full schemas.

---

## Architecture Overview

Backspace federation is peer-to-peer with no central authority. Each instance maintains its own copy of all data. Peers exchange real-time events for DMs and friendships via a signed relay protocol.

**Canonical identity:** The `(homeUserId, homeInstance)` pair is globally unique. Local users have `homeInstance = NULL` and `homeUserId = NULL`. Federated users are represented as **replicated user stubs** -- minimal user records with `passwordHash = '!federation-replicated'` (bcrypt never produces this value, so login is impossible).

**Trust model:** Symmetric shared-secret HMAC. Both peers share the same 256-bit secret. Events are attributed to users by `homeUserId + homeInstance` in the payload, with authority checks verifying the source instance matches the claimed origin of the acting user.

---

## 1. Peer Handshake & Discovery

### 2-Phase Flow

**Phase 1 -- Initiate** (`POST /api/federation/peer/initiate`)
- Auth: JWT + admin role required
- Validates `remoteOrigin` is a well-formed HTTP(S) URL via `validateOrigin()`
- Prevents self-peering (`localOrigin === remoteOrigin`)
- Handles existing peers: active -> return 200, pending -> return 409, revoked -> delete and re-initiate
- Generates HMAC secret: `generateHmacSecret()` -> `randomBytes(32).toString('hex')` (256-bit)
- Generates challenge: `randomBytes(16).toString('hex')` (128-bit, currently unused by acceptor)
- Creates local peer record with `status='pending'`
- POSTs to `{remoteOrigin}/api/federation/peer/accept` with `{ sourceOrigin, challenge, hmacSecret }`
- Timeout: 10 seconds (`AbortSignal.timeout`)
- On remote acceptance: updates local peer to `status='active'`, sets `lastSeenAt`
- On failure: deletes pending peer, returns 502 (network error) or 504 (timeout)

**Phase 2 -- Accept** (`POST /api/federation/peer/accept`)
- Auth: **none** (first contact -- no JWT, no HMAC)
- Rate-limited: 10 requests per minute per IP (in-memory sliding window, buckets cleaned every 60s)
- Validates `sourceOrigin`, `challenge`, and `hmacSecret` from body
- Handles existing peers: active -> return 200 (idempotent), revoked -> return 403, pending -> update with new secret and activate
- New peer: creates record with provided `hmacSecret`, sets `status='active'`
- Returns `{ accepted: true }` on success

### Secret Storage & Rotation

Both instances store the **same** HMAC secret. The initiating instance generates it and sends it in the accept request.

**Rotation protocol:** Either peer (or automatically on a configurable interval, default 90 days) can trigger rotation:

1. Initiator generates a new secret, stores it as `pendingHmacSecret`, and POSTs to `{peer}/api/federation/peer/rotate` signed with the current secret
2. Acceptor verifies the HMAC, stores `pendingHmacSecret`, and returns `{ accepted: true }`
3. Both sides enter a 15-minute grace period where either secret is accepted for verification, while outbound requests are signed with the new secret
4. After the grace period, the health check worker promotes `pendingHmacSecret` → `hmacSecret` and clears the pending fields

**Conflict guard:** If `pendingHmacSecret` is already set, the rotate endpoint returns 409.

**Schema columns:** `pending_hmac_secret` (TEXT NULL), `secret_rotation_at` (INTEGER NULL), `secret_rotated_at` (INTEGER NULL), `auto_rotate_interval_days` (INTEGER NOT NULL DEFAULT 90).

### Peer Status Lifecycle

```
                     initiate
  (none) ──────────► pending ──────────► active
      ▲                                    │
      │              10+ consecutive        │ delivery failures
      │              failures              ▼
      │                               unreachable
      │              health check OK        │
      │                                    ▼
      │                                 active
      │              admin revoke           │
      │                                    ▼
      │                                 revoked ──► (delete) ──► re-initiate
      │
      │  auto-peer rejected (403 PEERING_REQUIRES_APPROVAL)
      └──────────────────────────── rejected
```

| Status | Outbox delivery | Health check | Relay accepts | Re-initiation |
|--------|----------------|--------------|---------------|---------------|
| `active` | Yes | No | Yes | No (returns existing) |
| `pending` | No | No | No | No (returns 409) |
| `unreachable` | No (entries wait) | Yes (1h interval) | Yes (resets to active) | No |
| `revoked` | No (entries purged) | No | No (returns 403) | Yes (old record deleted) |
| `rejected` | No | No | No | Yes (admin deletes record, then re-initiates) |

### PEER_UNREACHABLE_THRESHOLD

Defined in `federationWorker.ts:45` as `10`. After 10 consecutive delivery failures for a peer, the worker sets `status = 'unreachable'`. The health check worker (1h interval) pings `GET /api/instance/info` on unreachable peers and reverts to `active` on success.

### Auto-Peering

When the server needs to relay events to an instance it has not yet peered with, `ensurePeered()` in `federationPeering.ts` automatically initiates the handshake. This removes the requirement for an admin to manually initiate every peering relationship.

**Integration points:**
- **Outbox worker** (`federationWorker.ts`) — after delivering active peers, calls `ensurePeered()` for any pending-placeholder entries whose peer has not yet been activated.
- **Connection flow** (`POST /api/federation/peer/ensure`) — called by the client when establishing a cross-instance connection, ensuring the two instances are peered before any relay traffic is sent.

**`rejected` status** — Added to the peer lifecycle. Set when a remote instance explicitly rejects auto-peering with `403 PEERING_REQUIRES_APPROVAL`. This status is sticky: no automatic retry occurs. An admin can clear it by deleting the peer record (then re-initiating), or by manually initiating via `peer/initiate`. An incoming `peer/accept` from a remote admin can also override `rejected` → `active` (treated the same as a `pending` record).

**`autoAcceptPeering` instance setting** — Controls whether `POST /api/federation/peer/accept` accepts unsolicited peering requests. Default: `true`. When `false`, the endpoint returns `403 PEERING_REQUIRES_APPROVAL` for requests where no local `pending` record exists (i.e., a request the local admin did not initiate). The determination is made by checking the local peer table — not a client-provided flag.

### Admin Endpoints

| Endpoint | Method | Auth | Purpose |
|----------|--------|------|---------|
| `/api/federation/peer/initiate` | POST | JWT + admin | Start peering handshake |
| `/api/federation/peer/accept` | POST | None (rate-limited) | Accept incoming handshake |
| `/api/federation/peer/ensure` | POST | JWT (any user), rate-limited 3/15min/user | Trigger auto-peering to a remote instance |
| `/api/federation/peers` | GET | JWT + admin | List all peers (secret excluded) |
| `/api/federation/peers/:id` | PATCH | JWT + admin | Update peer settings (auto-rotation interval) |
| `/api/federation/peers/:id` | DELETE | JWT + admin | Revoke peer, purge outbox |
| `/api/federation/peers/:id/permanent` | DELETE | JWT + admin | Hard-delete revoked peer record |
| `/api/federation/peers/:id/rotate` | POST | JWT + admin | Trigger immediate secret rotation |

**`POST /api/federation/peer/ensure`** — Wraps `ensurePeered()`. Accepts `{ remoteOrigin: string }` in body. Returns `{ peeringStatus, peerId?, error? }` where `peeringStatus` is one of `active`, `pending`, `rejected`, `unreachable`, or `revoked`.

### S2S Endpoints

| Endpoint | Method | Auth | Purpose |
|----------|--------|------|---------|
| `/api/federation/peer/rotate` | POST | HMAC | Accept secret rotation from peer |
| `/api/federation/identity` | DELETE | HMAC | Delete federated user identity (soft/full mode) |

### S2S Identity Deletion (`DELETE /api/federation/identity`)

Allows a home instance to remove a user's replicated identity from a remote instance.

**Request body:**
```json
{ "homeUserId": "<string>", "homeInstance": "<string>", "mode": "soft" | "full" }
```

**Behavior:**

- **Attribution guard:** Rejects with `403` if the user's `homeInstance` doesn't match the `X-Federation-Origin` of the signing peer. Prevents one instance from deleting another instance's users.
- **Idempotent:** Returns `{ success: true }` for already-deleted or nonexistent users (no error).
- **Owned spaces check:** Returns `409` with `{ ownedSpaces: string[] }` if the user owns any spaces on the remote. The user must transfer or delete those spaces before identity removal proceeds.
- **Mode `"soft"`:** Calls `tombstoneUser(uid, { purgeContent: false })` — anonymizes the user row and removes memberships, but skips reaction deletion and orphaned DM purge.
- **Mode `"full"`:** Calls `tombstoneUser(uid, { purgeContent: true })` — full tombstone including reactions and orphaned DM cleanup.
- **Post-deletion:** Broadcasts `member_left` WS events for all spaces the user belonged to before removal.

### WebSocket Events (Peering)

These S→C events are pushed to the acting user's connected clients by the federation subsystem.

| Event | Pushed when | Payload |
|-------|-------------|---------|
| `federation_peer_rejected` | Outbox worker receives `403 PEERING_REQUIRES_APPROVAL` from a remote instance during auto-peering | `{ peerId: string, origin: string }` |
| `federation_peer_active` | A previously `rejected` peer transitions to `active` (e.g., via manual `peer/initiate` or incoming `peer/accept`) | `{ peerId: string, origin: string }` |

---

## 2. HMAC Request Authentication

### Signing Format

```
HMAC-SHA256(secret, "${timestamp}.${requestBody}")
```

Where `timestamp` is `Date.now()` (Unix milliseconds) and `requestBody` is the JSON string.

### HTTP Headers

| Header | Format | Example |
|--------|--------|---------|
| `X-Federation-Signature` | `sha256=<hex>` | `sha256=a1b2c3...` |
| `X-Federation-Origin` | Full URL | `https://nova.ddns.net` |
| `X-Federation-Timestamp` | Unix ms string | `1711619400000` |
| `Content-Type` | `application/json` | -- |

### Verification (`federationAuth.ts:verifySignature`)

1. Validate inputs: reject empty/missing body, signature, or secret
2. **Timestamp window:** `Math.abs(Date.now() - timestamp) <= maxAgeMs` (default 15 minutes)
3. Recompute: `HMAC-SHA256(secret, "${timestamp}.${body}")`
4. **Constant-time comparison:** `crypto.timingSafeEqual` on hex-decoded buffers
5. Length check: mismatched buffer lengths are rejected before `timingSafeEqual`

### Replay Attack Prevention

Two layers of replay protection:

1. **Timestamp window:** Requests older than 15 minutes are rejected (`DEFAULT_MAX_AGE_MS`).
2. **Nonce:** Each request includes a `X-Federation-Nonce` header (UUID v4). The nonce is included in the HMAC payload (`${timestamp}.${nonce}.${body}`) so it cannot be stripped. The receiver stores seen nonces in memory (keyed by peer origin, evicted after 15 min) and rejects duplicates with `409 Conflict`.

**Auto-ratchet:** The `nonceSupported` column on `federation_peers` tracks whether a peer has ever sent a nonce. Once set to `1`, nonce-less requests from that peer are permanently rejected (`401`). This allows graceful rollout — new peers get nonce enforcement automatically, legacy peers are warned in logs until they upgrade.

### Inbound Verification Flow (`POST /api/federation/relay`)

1. `parseFederationHeaders()` extracts origin, timestamp, signature, and nonce from headers
2. Look up peer by `origin` in `federation_peers` -- must exist and be `status = 'active'`
3. Re-serialize request body to JSON: `JSON.stringify(request.body)`
4. `verifySignature(bodyString, signature, peer.hmacSecret, timestamp, nonce)` -- reject if false
5. Nonce enforcement: duplicate nonce → 409, missing nonce from ratcheted peer → 401, legacy peer → warn

**Important:** The body is re-serialized server-side. This means Fastify's JSON parsing and re-stringification must produce identical output to the sender's `JSON.stringify`. In practice this works because both sides use standard `JSON.stringify` with no custom replacers.

---

## 3. Identity Resolution

### Functions

**`extractDomain(homeInstance)`** -- `federation.ts`
- Extracts bare domain from a homeInstance value (full URL or bare domain)
- `"https://nova.ddns.net"` → `"nova.ddns.net"`, `"nova.ddns.net"` → `"nova.ddns.net"`
- **Use when:** Normalizing homeInstance for comparison or storage

**`findFederatedUser(homeUserId, homeInstance, db, hints?)`** -- `federation.ts`
- Three-tier lookup: homeUserId match → domain + username hint match → not found
- Tier 1: delegates to `resolveLocalUser` (fast path)
- Tier 2: uses `extractDomain(homeInstance)` + `hints.username` to match stubs created by the auth registration path (which may have a different homeUserId)
- Side-effect-free — does not modify any records
- When multiple candidates match in tier 2, prefers real accounts over stubs, then most profile data
- **Use when:** Read-only lookup that needs to find users created by either auth or relay path

**`resolveLocalUser(homeUserId, db)`** -- `federation.ts`
- Read-only lookup. Returns `undefined` if not found.
- Matches: `(users.homeUserId = homeUserId)` OR `(users.id = homeUserId AND homeInstance IS NULL)`
- Excludes deleted users (`isDeleted = 0`)
- When multiple candidates exist: prefers the one with `homeUserId` set (replicated stub) over a local ID match
- **Use when:** Optional lookups where null is acceptable (member_remove, reaction processing, friend_remove)

**`resolveOrCreateReplicatedUser(homeUserId, homeInstance, db, hints?)`** -- `federation.ts`
- Calls `findFederatedUser` first. If found, backfills `homeUserId` for future fast-path lookups and returns.
- Accepts optional `hints: { username?: string | null }` for tier-2 matching
- If not found, creates a stub with `homeInstance` normalized to bare domain via `extractDomain`
- Collision-safe: appends `_1`, `_2`, ..., `_10` suffix if username exists; after 10 attempts, uses `_<random hex>`
- **Use when:** You MUST have a valid user ID. Always pass `{ username: profile?.username }` when profile data is available.

**`hydrateReplicatedUserProfile(user, profile, db)`** -- `federation.ts:2041`
- Updates replicated stubs only (`homeInstance` must be set)
- Only updates null/empty fields (preserves manually-set local values)
- Exception: avatar/banner are overwritten if the current value is a bare filename (not an absolute URL)
- Resolves bare filenames to `{homeInstance}/api/uploads/{filename}` absolute URLs
- Sets `displayName` from `profile.displayName || profile.username` -- ensures federated users show a human-readable name instead of `user@instance`

### Critical Rule

Any code path that sets `ownerId`, creates a `dm_members` row, or inserts a message MUST use `resolveOrCreateReplicatedUser`. Using `resolveLocalUser` with a `?? null` fallback can cause data corruption (e.g., `ownerId` set to null for group DMs).

### Origin Normalization

**Two formats exist in the database:**

| Location | Format | Example |
|----------|--------|---------|
| `users.home_instance` | Bare domain (normalized) | `nova.ddns.net` |
| `federation_peers.origin` | Full URL | `https://nova.ddns.net` |
| `getOurOrigin()` return | Full URL | `https://orbit.ddns.net` |
| `resolveOrCreateReplicatedUser` stores | Bare domain (normalized via `extractDomain`) | `nova.ddns.net` |
| Auth registration stores | Bare domain | `nova.ddns.net` |

Both user creation paths now store bare domain. A self-healing migration in `migrate.ts` normalizes any existing full-URL `homeInstance` values to bare domain on startup.

**Normalization pattern used in code:**
```typescript
const normalized = homeInstance.startsWith('http') ? homeInstance : `https://${homeInstance}`;
```

Locations where normalization is applied:
- `getGroupDmTargetOrigins()` (`federationOutbox.ts:294`) -- normalizes before comparing to `ourOrigin`
- `dm.ts:655` -- `isLocalMember` broadcast filter checks both formats
- `dm.ts:743` -- normalizes target homeInstance before peer origin comparison

**Attribution verification (`verifyAttribution`):**
- `verifyAttribution(actingUserHomeInstance, sourceInstance)` normalizes both via `extractDomain` and compares
- Two valid cases:
  1. **Direct**: `authorDomain === sourceDomain` — standard S2S, peer sends events for its own users
  2. **Homeward relay**: `authorDomain === extractDomain(getOurOrigin())` — a client-federation user sent a message on a remote server, and the S2S relay forwards it back to the author's home instance
- Applied as the FIRST check in every relay event processor (13+ handlers) — before user resolution or DB writes
- Prevents malicious peers from forging events attributed to users on *unrelated* instances (neither source nor receiver)

All origin comparisons use `extractDomain()` or `getOurOrigin()` with normalization, handling both bare domains and full URLs consistently.
- `federation.ts:2447` -- same pattern in `processFriendRemoveEvent`

---

## 4. DM Message Relay

### 1-on-1 DMs

**Outbound (origin instance):**
1. Message created via REST (`POST /api/dm/:id/messages`) or WS (`dm_message_create`)
2. `queueDmRelay(message, channelId, 'create')` called from `dm.ts` / `events.ts`
3. `buildRelayPayload()` constructs the message portion with `homeUserId`, `homeInstance`, `content`, `replyToId`, `editedAt`, `createdAt`
4. `getDmParticipants(channelId)` resolves all members to `(homeUserId, homeInstance)` pairs with profile snapshots
5. `getGroupDmTargetOrigins(channelId)` returns `undefined` (no owner -> broadcast to all)
6. `queueOutboxEvent(messageId, channelId, 'create', payload, undefined)` -> queued to ALL active peers

**Channel creation (1-on-1 with federated user):**
- When `POST /api/dm` creates a channel where either participant has `homeInstance` set, the deterministic `federatedId = SHA256(sorted([homeUserIdA, homeUserIdB])).slice(0, 32)` is computed and stored immediately
- This ensures `findOrCreateDmChannel` on the receiving instance finds the existing channel when the S2S reply arrives, preventing duplicate channels

**Inbound (receiving instance -- `processCreateEvent`):**
1. Validate: `event.message` and `event.participants` (>= 2) required
2. Dedup: check `(sourceInstance, sourceMessageId)` -- reject if exists
3. Resolve ALL participants via `resolveOrCreateReplicatedUser`, hydrate profiles
4. No `event.federatedId` -> 1-on-1 path
5. Compute deterministic `federatedId = SHA256(sorted([homeUserIdA, homeUserIdB])).slice(0, 32)`
6. `findOrCreateDmChannel(federatedId, [localUserA.id, localUserB.id], db)`:
   - Find by `federatedId` in `dm_channels`
   - If exists: ensure both users are members (idempotent insert)
   - If not: create channel with `federatedId`, add both members
7. Insert `dm_messages` with `sourceInstance` and `sourceMessageId`
8. Process attachments (see File Replication)
9. Broadcast `dm_message_created` to local members, **skipping** members whose `homeInstance === sourceInstance` (they already have the original)

### Group DMs

**Outbound (origin instance):**
Same as 1-on-1 except:
- `getGroupDmTargetOrigins(channelId)` returns a list of peer origins that have at least one participant
- Normalizes `homeInstance` to full URL before comparison
- `queueOutboxEvent` receives `targetPeerOrigins` and only queues to those peers
- Payload includes `federatedId` (random UUID assigned at channel creation)

**Inbound (receiving instance -- `processCreateEvent`):**
1. `event.federatedId` present -> group DM path
2. Find channel by `federatedId` -- must already exist (bootstrapped by prior `member_add`)
3. If not found -> reject with `channel_not_found`
4. Insert message, broadcast to local members

### Federated ID Generation (`federationOutbox.ts:computeFederatedId`)

```typescript
// 1-on-1: deterministic 32-char hex hash
const sorted = [homeUserIdA, homeUserIdB].sort();
return sha256(sorted.join(':')).slice(0, 32);

// Group: random 36-char UUID with dashes
return crypto.randomUUID();
```

The format difference (32-char hash vs 36-char UUID) is used by the self-healing migration to detect channel type independently of `owner_id`.

### Message Deduplication

Every relayed message is stored with:
- `source_instance`: the relay request's `sourceInstance` header value
- `source_message_id`: the `event.messageId` (original message ID on source instance)

The `(source_instance, source_message_id)` pair is checked before insertion. Duplicates are rejected with reason `'duplicate'`. A unique partial index enforces this at the DB level: `idx_dm_messages_source_unique ON dm_messages(source_instance, source_message_id) WHERE source_instance IS NOT NULL`.

### Typing Indicator Relay

**Event types:** `dm_typing_start`, `dm_typing_stop`

**Model:** Fire-and-forget, same as call signaling. No outbox, no retry, no mutation log. Typing is ephemeral — lost packets are acceptable.

**Channel identification:** Uses `federatedId` (not instance-local `dmChannelId`) for cross-instance channel lookup, plus `participants` for resolution context.

**Outbound (`events.ts` / `dm.ts`):**
- `handleDmTypingStart()` → after local broadcast, calls `sendTypingRelay(dmChannelId, 'dm_typing_start', userId)`
- `broadcastDmMessage()` → after local `dm_typing_stop` broadcast, calls `sendTypingRelay(dmChannelId, 'dm_typing_stop', message.userId)`

**`sendTypingRelay()` (`federationOutbox.ts`):**
- Fetches channel's `federatedId` and `getDmParticipants()` for target resolution
- Builds `FederationRelayEvent` with `typing: { homeUserId, homeInstance, username }`
- Reuses `sendCallRelay()` for the actual POST to each remote peer origin

**Inbound (`federation.ts`):**
- `processDmTypingStartEvent` → look up channel by `federatedId`, resolve user via `resolveLocalUser()` (no stub creation for ephemeral events), broadcast `dm_typing` to local members
- `processDmTypingStopEvent` → same, broadcast `dm_typing_stop` to local members
- **Implicit clear:** `processCreateEvent()` also emits `dm_typing_stop` for the message author after processing an inbound relay — primary typing clear mechanism for relayed messages

---

## 5. Outbox & Relay Pipeline

### Event Queuing (`federationOutbox.ts:queueOutboxEvent`)

```
Trigger (API/WS handler)
  -> isFederationRelayEnabled()? No -> return silently
  -> Fetch active peers from federation_peers
  -> Filter to targetPeerOrigins (if specified) -- EXACT string match against peer.origin
  -> If zero peers match -> logs warning and returns
  -> For each peer, in a transaction:
      -> Check for existing outbox entry by (peerId, entityId)
      -> COALESCE:
          - delete + existing create -> delete both (net: never relayed)
          - update + existing create -> update payload, keep 'create' eventType
          - update + existing update -> update payload and eventType
          - no existing -> insert new entry
      -> TTL: now + (relayTtlDays * 86400000)
```

### Coalescing Rules (per-peer, per-entity)

| Incoming | Existing | Result |
|----------|----------|--------|
| `delete` | `create` | Entry removed (message was never relayed) |
| `update` | `create` | Payload updated, keeps `create` type (peer gets full message) |
| `update` | `update` | Payload updated, type becomes latest |
| `delete` | `update` | Payload updated, type becomes `delete` |
| any | none | New entry inserted |

### Outbox Delivery Worker (`federationWorker.ts:processOutboxTick`)

**Interval:** 10 seconds (`OUTBOX_INTERVAL_MS`)
**Batch size:** 50 (`OUTBOX_BATCH_LIMIT`)
**Timeout:** 30 seconds per request (`OUTBOX_FETCH_TIMEOUT_MS`)

1. Query entries where `nextRetryAt <= now` joined with active peers, ordered by `createdAt ASC`, limit 50
2. Group by peer
3. For each peer, reconstruct `FederationRelayEvent[]` from stored payloads:
   - Parse JSON payload
   - Copy fields: `federatedId`, `participants`, `message`, `reactions`, `reaction`, `membership`, `ownership`, `group`, `friendship`, file_rejected fields
   - Set `eventType`, `contextType`, `messageId`, `dmChannelId`, `encryptionVersion`, `timestamp`
4. Build `FederationRelayRequest` with `version: 1`, `sourceInstance: ourOrigin`
5. Sign with `buildFederationHeaders(body, peerHmacSecret, ourOrigin)`
6. POST to `{peerOrigin}/api/federation/relay`
7. On success (200):
   - Delete accepted entries from outbox (matched by `entityId` -> `outboxId`)
   - Log rejected entries (remain in outbox for retry)
   - Store `result.maxUploadSize` on peer record
   - Update peer: `lastSeenAt = now`, `consecutiveFailures = 0`
8. On failure (non-200 or network error):
   - `handleOutboxDeliveryFailure()`:
     - Increment `attempts` per entry, compute `nextRetryAt = now + backoff`
     - Increment peer `consecutiveFailures`, set `lastFailureAt`
     - If `consecutiveFailures >= PEER_UNREACHABLE_THRESHOLD (10)` -> mark peer `unreachable`

### Retry Backoff Schedule

| Attempt | Delay |
|---------|-------|
| 1 | 30 seconds |
| 2 | 1 minute |
| 3 | 5 minutes |
| 4 | 15 minutes |
| 5 | 1 hour |
| 6 | 6 hours |
| 7+ | 24 hours (cap) |

### Relay Request/Response Format

**Request:**
```typescript
interface FederationRelayRequest {
  version: 1;
  sourceInstance: string;          // Full URL, e.g., "https://nova.ddns.net"
  events: FederationRelayEvent[];  // Max 50 per batch
}
```

**Response:**
```typescript
interface FederationRelayResponse {
  accepted: string[];              // messageIds successfully processed
  rejected: Array<{
    messageId: string;
    reason: string;                // e.g., 'duplicate', 'unknown_message', 'missing_participants'
  }>;
  maxUploadSize: number;           // This instance's max upload size in bytes
}
```

### Inbound Relay Dispatch (`POST /api/federation/relay`)

Body limit: 10 MB. Max 50 events per batch. Rate-limited to 90 requests/min per peer (sliding window, keyed by `peer.origin`). Returns 429 when exceeded. Raised from 30 after FED-009 reduced the outbox worker interval from 10s to 1s — a busy sender can now hit 60 req/min during sustained traffic.

| eventType | Processor | contextType |
|-----------|-----------|-------------|
| `create` | `processCreateEvent` | dm |
| `update` | `processUpdateEvent` | dm |
| `delete` | `processDeleteEvent` | dm |
| `reaction_add` | `processReactionAddEvent` | dm |
| `reaction_remove` | `processReactionRemoveEvent` | dm |
| `member_add` | `processMemberAddEvent` | dm |
| `member_remove` | `processMemberRemoveEvent` | dm |
| `ownership_transfer` | `processOwnershipTransferEvent` | dm |
| `read_state_update` | `processReadStateUpdateEvent` | dm |
| `friend_request_create` | `processFriendRequestCreateEvent` | friend |
| `friend_request_update` | `processFriendRequestUpdateEvent` | friend |
| `friend_request_cancel` | `processFriendRequestCancelEvent` | friend |
| `friend_add` | `processFriendAddEvent` | friend |
| `friend_remove` | `processFriendRemoveEvent` | friend |
| `file_rejected` | `processFileRejectedEvent` | dm |
| `dm_typing_start` | `processDmTypingStartEvent` | dm (fire-and-forget, no outbox) |
| `dm_typing_stop` | `processDmTypingStopEvent` | dm (fire-and-forget, no outbox) |
| `dm_close` | `processDmCloseEvent` | dm |
| `dm_reopen` | `processDmReopenEvent` | dm |

After processing all events, the relay endpoint updates the peer's `lastSeenAt` and resets `consecutiveFailures`, then returns accepted/rejected arrays plus `maxUploadSize`.

---

## 6. Group DM Lifecycle over Federation

### member_add (`processMemberAddEvent` -- `federation.ts:1618`)

**Required fields:** `event.federatedId`, `event.membership.user`

**Two paths:**

**Bootstrap path** (channel does not exist locally by `federatedId`):
1. Requires `event.group` metadata (owner + full member roster)
2. Creates `dm_channels` row with `federatedId`, `ownerId` (resolved via `resolveOrCreateReplicatedUser`), `ownerHomeUserId`, `ownerHomeInstance`
3. Adds ALL roster members from `event.group.members` (each resolved via `resolveOrCreateReplicatedUser`)
4. Sends `dm_channel_created` to **local-only members** (home instance matches `getOurOrigin()`, with normalization for bare domain)
5. Sets `bootstrapped = true` to skip redundant system messages and member_add broadcasts below

**Incremental path** (channel already exists):
1. Validates authority: any HMAC-verified peer is accepted (relaxed — the `sourceInstance === channel.ownerHomeInstance` check was removed to support cross-instance access). The per-user attribution check (`verifyAttribution`) still applies.
2. Cancels soft-delete if channel was pending GC
3. Resolves added user via `resolveOrCreateReplicatedUser`
4. Enforces max 10 members
5. Inserts `dm_members` row (idempotent -- skip if exists)
6. Inserts system message, broadcasts `dm_member_added` to local WebSocket clients

### member_remove (`processMemberRemoveEvent` -- `federation.ts:1825`)

1. Find channel by `federatedId` -- if not found, accept idempotently
2. Validate authority: owner's instance for kicks (`reason !== 'leave'`), any instance for self-leave
3. Resolve user via `resolveLocalUser` -- if not found, accept idempotently
4. Insert system message (before deletion, so broadcast includes the leaving user)
5. Delete `dm_members` row, clean up `read_states`
6. Broadcast `dm_member_removed` to remaining local members
7. If zero members remain -> soft-delete channel (`deletedAt = now`)

### ownership_transfer (`processOwnershipTransferEvent` -- `federation.ts:1938`)

1. Find channel by `federatedId` -- if not found, accept idempotently
2. Validate authority: `sourceInstance === channel.ownerHomeInstance`
3. Resolve new owner via `resolveOrCreateReplicatedUser` (**never** `resolveLocalUser` -- must guarantee valid ID)
4. Update `dm_channels`: `ownerId`, `ownerHomeUserId`, `ownerHomeInstance`
5. Broadcast `dm_owner_updated` WebSocket event
6. Insert system message with previous owner as actor

### Local-Only Broadcast Principle

Users connected to multiple instances must see each DM channel exactly once (from their home instance). All structural broadcasts (`dm_channel_created`, system messages) filter to **local members only**:

```typescript
const isLocalMember = (u: { homeInstance?: string | null }) =>
  !u.homeInstance || !domainOrigin ||
  u.homeInstance === domainOrigin ||
  `https://${u.homeInstance}` === domainOrigin;
```

**Does NOT apply to:** Regular DM messages (`dm_message_created` for user messages). These broadcast to all local `dm_members` regardless of home instance.

### System Messages

System messages (`type = 'system'` in `dm_messages`) are **instance-local** -- they are NOT relayed via federation. Each instance creates its own when processing events.

| Event | Content JSON | Actor (`userId`) |
|-------|-------------|-----------------|
| `member_added` | `{event, targetUserId, targetDisplayName}` | User who added them |
| `member_removed` | `{event, targetUserId, targetDisplayName, reason}` | User who left/was removed |
| `owner_changed` | `{event, newOwnerId, newOwnerDisplayName}` | Previous owner |

### Outbound Queuing (Origin Instance -- `dm.ts`)

When a group DM is created or modified locally, the origin instance queues federation events:

**Group DM creation** (`POST /api/dm/group`):
- Iterates each remote target user (those with `homeInstance !== domainOrigin`)
- Builds a `member_add` event per remote user, carrying the full roster in `event.group`
- Computes `finalTargets` by starting from `getGroupDmTargetOrigins()` and adding the new member's normalized homeInstance
- Calls `appendMutationLog` + `queueOutboxEvent` per event

**Add member to existing group** (`POST /api/dm/:id/members`):
- Same structure as creation -- builds `member_add` with full group metadata
- Normalizes new member's homeInstance to full URL before including in targets

**Leave group** (`DELETE /api/dm/:id/members`):
- Computes `fedTargetOrigins` **before** deleting the member (so the leaving user's peer is still included)
- Queues `member_remove` event with `reason: 'leave'`

**Ownership transfer** (`PATCH /api/dm/:id`):
- Queues `ownership_transfer` event with `previousOwner` and `newOwner`

---

## 7. File Replication

### Outbound (origin instance)

When `queueDmRelay` constructs the relay payload, each attachment gets a `sourceUrl`:
```
sourceUrl: `${getOurOrigin()}/api/uploads/${attachment.filename}`
```

### Inbound (receiving instance -- `processCreateEvent`)

1. For each attachment in `event.message.attachments`:
   - SSRF check: `isUrlFromPeer(sourceUrl, peerOrigin)` -- hostname of sourceUrl must match peer origin hostname
   - Create `attachments` row with `filename = sourceUrl` (remote URL as interim filename)
   - Queue `federation_file_queue` entry with `status = 'pending'`, `expiresAt = now + 30 days`
2. Initial WebSocket broadcast uses sourceUrl directly (frontend's `AttachmentRenderer` detects `http` prefix)

### File Download Worker (`federationWorker.ts:processFileQueueEntry`)

**Interval:** 30 seconds. **Batch:** 5 files. **Timeout:** 60 seconds per download.

1. SSRF protection: validate sourceUrl hostname matches peerOrigin hostname
2. Pre-download size check against `maxUploadSizeBytes` from instance settings
3. Download via `fetch` with streaming pipeline to disk (`Readable.fromWeb` -> `fs.createWriteStream`)
4. Post-download size verification (defense in depth)
5. Generate thumbnail via `sharp` (same as local upload flow)
6. Update `attachments` row: `filename = localFilename`, `size`, `thumbnailFilename`
7. Fallback: if no existing attachment row was found (legacy queue entry), insert a new one
8. Mark file queue entry as `completed` with `targetFilename`
9. Broadcast `dm_message_updated` to refresh client-side attachment display

### Size Rejection Flow (`handleSizeRejection`)

When a file exceeds the local instance's size limit:

1. Mark file queue entry as `rejected` with `reason = 'size_limit_exceeded'`
2. Update local attachment: `federationStatus = 'remote'`, `federationMeta` = source info JSON
3. Determine affected local users (native to this instance -- `!user.homeInstance || user.homeInstance === ourOrigin`)
4. Queue `file_rejected` reverse relay event to the sender's instance (`sourceInstance`)
5. Broadcast `dm_message_updated` locally so clients see the 'remote' badge

### Inbound file_rejected (`processFileRejectedEvent` -- `federation.ts:2458`)

When the origin instance receives a `file_rejected` event:

1. Find local message by `event.messageId` (the original local message ID)
2. Match attachment by `sourceFilename` or fallback to single attachment
3. Resolve `affectedUserIds` (homeUserIds) to local replicated user stubs
4. Merge rejection info into `federationMeta` (accumulates from multiple peers)
5. Set `federationStatus = 'remote_partial'`
6. Broadcast `dm_message_updated` + targeted `federation_file_rejected` toast to message author

### Federation Status on Attachments

| Status | Meaning |
|--------|---------|
| `null` | Local upload, no federation involvement |
| `'local'` | Successfully downloaded from peer |
| `'remote'` | Rejected (size limit), `federationMeta` has source instance info |
| `'remote_partial'` | Rejected by some peers, `federationMeta` has per-user rejection array |

### File Download Retry

Uses the same backoff schedule as outbox delivery. Max attempts: 10 (`MAX_FILE_ATTEMPTS`). After exceeding max attempts: `status = 'failed'`, `rejectionReason = 'max_attempts_exceeded'`.

---

## 8. Read State Relay

### `read_state_update` Event

When a user marks a DM channel as read (`channel_ack`) or marks it unread (`mark_unread`), the read state is relayed to all peer instances so cross-instance sessions stay in sync.

**Outbound (`events.ts:handleChannelAck` / `handleMarkUnread`):**
- Fires after writing `read_states` locally
- Only triggers for DM channels with a `federatedId` (cross-instance DMs)
- Calls `queueReadStateRelay(channelId, messageId, userId)` in `federationOutbox.ts`
- Queued via the standard outbox pipeline — durable, retried by the background worker
- Entity key `read_state:{federatedId}:{userId}` enables coalescing (rapid acks collapse to latest)
- `mark_unread` with the `'0'` sentinel (delete read state entirely) is NOT relayed — it cannot be mapped to a message

**Event payload:**
```typescript
{
  eventType: 'read_state_update',
  dmChannelId: string,
  messageId: string,            // unique event ID: 'read_state:{userId}:{timestamp}'
  federatedId: string,          // DM channel's federatedId (cross-instance channel lookup)
  encryptionVersion: 0,
  timestamp: number,            // LWW tiebreaker
  readState: {
    user: { homeUserId: string; homeInstance: string },
    messageRef: { sourceInstance: string; sourceMessageId: string }
  }
}
```

`messageRef` identifies the acked message in federation coordinates. If the message originated on this instance, `sourceInstance` is our own origin and `sourceMessageId` is the local message ID. If the message was relayed here, `sourceInstance` and `sourceMessageId` come from the `dm_messages` row's `source_instance`/`source_message_id` columns.

**Inbound (`processReadStateUpdateEvent`):**
1. Resolve channel by `federatedId` — reject if not found
2. Resolve user via `resolveLocalUser` — reject if not found
3. Translate `messageRef` to local message ID:
   - If `sourceInstance` matches our origin: `sourceMessageId` IS our local ID
   - Otherwise: look up `dm_messages` by `source_instance + source_message_id`
4. If no local message found (relay hasn't arrived yet): silently accept (no-op)
5. Upsert `read_states` using timestamp-only LWW (`event.timestamp > existing.updatedAt`)
6. Echo `channel_ack` to the user's local WebSocket connections (multi-tab sync)

---

## 8b. DM Close/Reopen Relay

### Overview

When a user closes or reopens a DM on their home instance, the action is relayed to all peer instances that hold a copy of the channel. This keeps the visibility state of a DM consistent across all instances that participate in it.

Only DMs with a `federatedId` are eligible. Legacy local-only DMs (created before federation was added, with no `federatedId`) are silently skipped.

### Event Types

| Event | Trigger |
|-------|---------|
| `dm_close` | User calls `DELETE /api/dm/:id` (soft-close) |
| `dm_reopen` | User calls `POST /api/dm` and reopens a closed 1-on-1 DM |

### Payload

```typescript
{
  eventType: 'dm_close' | 'dm_reopen',
  dmChannelId: string,          // local channel ID (context only)
  federatedId: string,          // cross-instance channel lookup key
  messageId: string,            // unique event ID: 'dm_close:{federatedId}:{userId}:{ts}'
  encryptionVersion: 0,
  timestamp: number,
  dmCloseReopen: {
    homeUserId: string,         // acting user's home user ID
    homeInstance: string,       // acting user's home instance (full URL)
  }
}
```

### Outbound (`federationOutbox.ts:queueDmCloseRelay`)

Called from `dm.ts` after the local close or reopen is committed.

1. Fetch the channel's `federatedId` — if null (local-only DM), return silently
2. Fetch the acting user's `(homeUserId, homeInstance)` federation identity
3. Build `FederationRelayEvent` with `eventType` and `dmCloseReopen` payload
4. `getGroupDmTargetOrigins(dmChannelId)` resolves the delivery targets:
   - 1-on-1 DMs (`ownerId = NULL`): returns `undefined` → broadcast to ALL active peers
   - Group DMs: returns the set of peer origins that have at least one participant
5. Enqueue via `appendMutationLog` + `queueOutboxEvent`

### Inbound

**`processDmCloseEvent` (`federation.ts`):**
1. Look up channel by `federatedId` — if not found, accept silently (idempotent)
2. Resolve acting user via `resolveLocalUser` (lookup-only; no stub creation for close/reopen) — if not found, accept silently
3. If the user has no `dm_members` row in this channel, accept silently
4. Set `dm_members.closed = 1` for the resolved local user
5. Broadcast `dm_channel_closed` to the user's local WebSocket connections

**`processDmReopenEvent` (`federation.ts`):**
1. Look up channel by `federatedId` — if not found, accept silently
2. Resolve acting user via `resolveLocalUser` — if not found, accept silently
3. If the user has no `dm_members` row, accept silently
4. Set `dm_members.closed = 0`
5. Build full `DmChannel` payload and broadcast `dm_channel_created` to the user's local WebSocket connections (mirrors the automatic reopen path in `broadcastDmMessage`)

### Closed-State Reopen on Message Relay (Bug Fix)

`processCreateEvent` (inbound message relay) now mirrors the local `broadcastDmMessage` logic: before broadcasting `dm_message_created`, it checks each recipient's `dm_members.closed` flag. For any recipient with `closed = 1`:

1. Set `closed = 0`
2. Broadcast `dm_channel_created` (with the new message as `lastMessage`) to resurface the DM in the recipient's sidebar
3. Then broadcast `dm_message_created`

This fixes a gap where relayed messages bypassed the closed-state reopen logic, leaving the DM hidden for recipients whose `closed` flag was set on the receiving instance.

---

## 9. Friend Relay

### Event Flow (social.ts)

| User Action | Federation Event | Authority Check |
|-------------|-----------------|-----------------|
| Send friend request | `friend_request_create` | `from.homeInstance === sourceInstance` |
| Accept/decline request | `friend_request_update` | `to.homeInstance === sourceInstance` |
| Cancel outgoing request | `friend_request_cancel` | `from.homeInstance === sourceInstance` |
| Accept creates friendship | `friend_add` | `to.homeInstance === sourceInstance` |
| Remove friend | `friend_remove` | Either side's instance |

### Target Resolution (`getFriendEventTargets`)

Computes which peer origins need the event. Compares `fromHomeInstance` and `toHomeInstance` against `getOurOrigin()` with normalization applied, correctly handling both bare domain and full URL formats.

### Context ID

Friend events use a deterministic context ID: `friend:${sorted[homeUserIdA, homeUserIdB].join(':')}`.

### Outbound Payload Construction

Each friend endpoint builds a `FederationRelayEvent` with:
- `contextType: 'friend'`
- `friendship` payload containing `from` and `to` as `FederationRelayParticipant` objects
- `fromProfile` and/or `toProfile` snapshots (`FederationRelayProfileSnapshot`)
- `entityId` formatted as `friend_req:${sorted_ids}:${timestamp}` (for requests) or `friend_remove:${sorted_ids}:${timestamp}`

The full event payload is stored in both `appendMutationLog` (for sync) and `queueOutboxEvent` (for delivery).

### Inbound Processing

**`processFriendRequestCreateEvent` (`federation.ts:2082`):**
- Authority check: `from.homeInstance !== sourceInstance` -> reject
- Resolve sender via `resolveOrCreateReplicatedUser` + hydrate profile
- Resolve recipient via `resolveLocalUser` (must be native to this instance)
- Idempotency: if already friends or pending request exists, accept as no-op
- Create `friend_requests` row, broadcast `friend_request_received` to recipient

**`processFriendRequestUpdateEvent` (`federation.ts:2178`):**
- Authority check: `to.homeInstance !== sourceInstance` -> reject
- Resolve sender (original requester) via `resolveLocalUser` (must exist locally)
- Resolve recipient (acceptor/decliner) via `resolveOrCreateReplicatedUser`
- Find pending request, update status
- Broadcast `friend_request_accepted` or `friend_request_declined` to the original sender

**`processFriendRequestCancelEvent` (`federation.ts:2254`):**
- Authority check: `from.homeInstance !== sourceInstance` -> reject
- Both users must exist locally. If not, accept idempotently.
- Delete the pending friend request. Broadcast `friend_request_cancelled` to recipient.

**`processFriendAddEvent` (`federation.ts:2318`):**
- Authority check: `to.homeInstance !== sourceInstance` -> reject
- Resolve both users via `resolveOrCreateReplicatedUser` + hydrate profiles
- Insert `friends` row (idempotent)
- Auto-resolve any pending `friend_requests` to `'accepted'` (handles out-of-order delivery)
- Determine which user is local (`from.homeInstance === ourOrigin`) and broadcast `friend_request_accepted`

**`processFriendRemoveEvent` (`federation.ts:2404`):**
- Authority check: either `from.homeInstance` or `to.homeInstance` must be `sourceInstance`
- Both users resolved via `resolveLocalUser`. If not found, accept idempotently.
- Delete `friends` row in both directions
- Determine local user (whose `homeInstance` is NOT the source) and broadcast `friend_removed`

---

## 10. Profile Sync

Profile sync uses **two mechanisms** that operate independently:

### S2S Profile Hydration (Server-side)

When relay events carry `FederationRelayProfileSnapshot` data:
- `processCreateEvent`: hydrates participant profiles on message relay
- `processFriendRequestCreateEvent` / `processFriendAddEvent`: hydrates friend profiles

`hydrateReplicatedUserProfile` only fills null/empty fields. Avatar/banner are overwritten only if the current value is not an absolute URL (catches stale bare filenames).

#### Profile Sync (S2S)

Profile data is synced server-to-server. The home instance is authoritative — when a user updates their profile, the home server queues a `profile_update` relay event to all active peers via the outbox.

**Event:** `profile_update` (contextType: `profile`)

**Payload:** `FederationProfileUpdatePayload` — full snapshot of 6 durable fields:
- `homeUserId`, `homeInstance`, `profileUpdatedAt` (monotonic version)
- `displayName`, `avatar` (absolute URL or null), `banner` (absolute URL or null)
- `accentColor`, `avatarColor`, `bio`

**Targeting:** Broadcast to all active peers. Peers silently accept if they have no replica.

**Coalescing:** `entityId = homeUserId`. Rapid successive edits coalesce to one delivery per peer.

**Processing:** Remote overwrites all 6 fields unconditionally. Rejects if incoming `profileUpdatedAt ≤ stored`. Broadcasts `user_updated` to local WS clients.

#### Profile Image File Replication

When a `profile_update` relay carries avatar or banner absolute URLs, the receiving instance downloads the image files locally rather than storing remote URLs. This eliminates cross-origin dependencies — avatars render from the local `/api/uploads/` endpoint.

**Flow:** `processProfileUpdateEvent` calls `downloadProfileAsset(url, sourceInstance)` for each of avatar/banner:
1. SSRF check (URL hostname must match authenticated source instance)
2. `fetch` with 10s timeout
3. Content-type validation (`image/*` only)
4. Stream to temp file (`temp_{snowflake}{ext}`), atomic rename to `{snowflake}{ext}`
5. Store local bare filename in user row

**Fallback:** On any download failure (timeout, HTTP error, non-image content, SSRF mismatch), the absolute URL is stored instead. This degrades to the pre-replication behavior — the avatar loads cross-origin from the home instance.

**File cleanup:** When avatar/banner changes, the old local file is deleted via `deleteUploadFile()`. The check `!oldValue.startsWith('http')` ensures only locally-downloaded files are deleted, not absolute URL strings.

**No migration:** Existing absolute URLs self-heal — the next profile change on the home instance triggers a relay and download.

**Write protection:** Remote instances reject PATCH /users/@me profile field updates for replicated users (homeInstance set). Profile data is read-only on remote — only updated via S2S relay.

**Bootstrap:** When a new origin appears in a user's `replicatedInstances`, the home server queues a targeted `profile_update` to that peer.

**File handling:** Profile images are downloaded locally on relay receipt (see "Profile Image File Replication" above).

**Replaces:** Client-driven `profileSync.ts` (deleted). `hydrateReplicatedUserProfile` still bootstraps null fields during DM/friend relay — it is not replaced.

---

## 11. Reaction Relay

### Outbound

Reactions are queued by WS event handlers in `events.ts`:
- `dm_reaction_add` -> `queueOutboxEvent(reactionId, channelId, 'reaction_add', payload, targetOrigins)`
- `dm_reaction_remove` -> `queueOutboxEvent(messageId, channelId, 'reaction_remove', payload, targetOrigins)`

Payload includes `userId`, `homeUserId`, `emoji`, `createdAt`, plus `messageId` and `messageHomeInstance` for cross-instance message resolution.

The mutation log entry for reactions stores a simpler payload (no `messageId`/`messageHomeInstance`), while the outbox entry carries the full reaction payload including those fields.

### Inbound

**`processReactionAddEvent` (`federation.ts:1480`):**
1. Resolve message via `resolveLocalDmMessage(canonicalMessageId, messageHomeInstance, sourceInstance, db)`:
   - If `messageHomeInstance === getOurOrigin()` -> find by local ID (the message originated here)
   - Otherwise -> find by `(messageHomeInstance || sourceInstance, canonicalMessageId)` tracking -- uses `messageHomeInstance` when available (correct origin in 3-instance relay), falls back to `sourceInstance`
2. Resolve reacting user via `resolveLocalUser` (must already exist)
3. Dedup: check existing reaction by `(dmMessageId, userId, emoji)`
4. Insert `dm_reactions`, broadcast `reaction_added` to local clients

**`processReactionRemoveEvent` (`federation.ts:1561`):**
- Same resolution logic
- Delete matching reaction, broadcast `reaction_removed` if changes > 0

---

## 12. Initial Sync

### `runInitialSyncForNewPeers()` (`federationWorker.ts:739`)

Triggered once at server startup (async, non-blocking). Finds peers with `status = 'active'` and `lastSyncedAt = 0`.

**For each unsynced peer:**
1. **DM sync pass:** Paginate through `POST {peerOrigin}/api/federation/sync` with `sinceTimestamp = 0`, `limit = 100`
2. **Direct processing:** Call `processRelayEvents()` to process received events in-process (no HTTP round-trip)
3. **Friend sync pass:** Same pagination with `contextType: 'friend'`, also processed via `processRelayEvents()`
4. Update `lastSyncedAt = Date.now()` after completion
5. On failure: don't update `lastSyncedAt` -- retried on next startup

### Sync Endpoint (`POST /api/federation/sync`)

HMAC-authenticated. Returns events from the `federation_mutation_log`.

**Request:**
```typescript
{ sinceTimestamp: number, dmChannelId?: string, federatedId?: string, contextType?: 'dm'|'friend', limit?: 1-500 }
```

**Response:**
```typescript
{ events: FederationRelayEvent[], hasMore: boolean, checkpoint: number }
```

**DM sync:**
- Queries all `dm_channels` with non-null `federated_id` (not soft-deleted)
- Joins `federation_mutation_log` with `dm_messages` to reconstruct events
- Only returns locally-created messages (`source_instance IS NULL` via the LEFT JOIN)
- Handles delete mutations separately (message rows don't exist for deletes)
- For create/update: fetches current message state from DB, builds full relay event with attachments and participants
- Membership/friend mutations store the full event payload in the mutation log, so they are returned directly

**Friend sync:**
- Queries `federation_mutation_log WHERE context_type = 'friend'`
- Returns stored payloads directly (friend events carry their complete data)

### Relay Event Processing

The event processing logic is extracted into `processRelayEvents()` (exported from `federation.ts`), shared by both the HTTP relay endpoint and the initial sync worker. This avoids a DNS hairpin issue where the server would HTTP-request itself through public DNS, which fails on networks without hairpin NAT.

---

## 13. DM Calls over Federation

DM calls work across federated instances. The caller's instance hosts the LiveKit room. Remote clients connect directly to the caller's LiveKit server using a token passed through S2S relay — no media is routed through the federation layer.

```
User A's client <--WS--> Instance 1 (hosts LiveKit) <--S2S HTTP--> Instance 2 <--WS--> User B's client
                              |                                                            |
                              +------------- LiveKit (direct client connection) -----------+
```

### S2S Event Types

Four relay event types are processed in `processRelayEvents()`:

| Event Type | Direction | Key Payload Fields |
|---|---|---|
| `dm_call_start` | Host → Peers | `federatedId`, `livekitUrl`, `tokens: Record<string, string>` (keyed by `homeUserId`), `caller: { homeUserId, homeInstance, displayName }`, `participants` |
| `dm_call_accept` | Participant → Host, then Host → All Peers | `federatedId`, `acceptor: { homeUserId, homeInstance }` |
| `dm_call_reject` | Participant → Host, then Host → All Peers | `federatedId`, `rejector: { homeUserId, homeInstance }` |
| `dm_call_end` | Any → Host (if not host), then Host → All Peers | `federatedId`, `endedBy: { homeUserId, homeInstance }` |

All events carry standard relay fields: `eventType`, `messageId`, `encryptionVersion: 0`, `timestamp`. All events pass through `verifyAttribution()` before any DB or state mutations.

### Direct Delivery (No Outbox)

Call signaling is time-critical and bypasses the outbox entirely. `sendCallRelay()` sends a synchronous HTTP POST to the peer's `/api/federation/relay` endpoint using existing HMAC signing (`buildFederationHeaders`). If delivery fails, the call operation fails — there is no retry.

### Call Flows

**Start:** Host validates membership, generates LiveKit tokens for all DM members (local + remote), broadcasts `dm_call_incoming` to local WS clients, then sends `dm_call_start` S2S to each remote instance with per-user tokens.

**Accept:** Remote instance sends `dm_call_accept` S2S to host. Host transitions `ringing → active`, broadcasts `dm_call_accepted` locally, fans out `dm_call_accept` to all other remote instances.

**Reject:** Remote sends `dm_call_reject` to host. Host destroys room, sends `dm_call_end` to all peers. (For 1-on-1 DMs, reject = end.)

**End:** Initiating instance (host or not) routes through the host. Host destroys room, fans out `dm_call_end` to all remote instances.

**Timeout:** Both host and remote instances auto-clean stale ringing calls after 60 seconds.

### LiveKit Room Naming

Room name = `federatedId` (the cross-instance stable UUID), never the local `dmChannelId` (which differs per instance).

- 1-on-1 DMs: `federatedId` is a deterministic SHA-256 hash of the sorted `homeUserId` pair
- Group DMs: `federatedId` is a UUID assigned at creation

### LiveKit Token Generation

`generateFederatedCallToken(roomName, homeUserId, displayName)` generates tokens with:
- **TTL:** 5 minutes (short join window; local calls use 1 hour)
- **Room:** scoped to exact `federatedId`
- **Identity:** `${homeUserId}:${displayName}`
- **Permissions:** full DM grants (mic, camera, screen share, subscribe, data channel)

### Public LiveKit URL

The URL sent in S2S payloads is always `https://${DOMAIN}/livekit` (the Caddy-proxied public address). The internal `LIVEKIT_URL` env var (`http://livekit:7880`) is never sent to peers.

Instances without LiveKit configured can still receive federated calls — they pass the host's URL and token to the client, which does all the heavy lifting.

### In-Memory Call Registry

When a remote instance receives `dm_call_start`, it creates a `FederatedCallEntry` in memory:

```typescript
interface FederatedCallEntry {
  dmChannelId: string;          // local dmChannelId for this DM
  federatedId: string;          // cross-instance room identifier
  callerId: string;             // local userId of caller's stub
  callerHomeUserId: string;
  federatedCallHost: string;    // peer origin of the host instance
  livekitUrl: string;
  tokens: Map<string, string>;  // homeUserId → LiveKit token
  state: 'ringing' | 'active';
  startedAt: number;
}
```

This registry ensures tokens and `livekitUrl` survive browser refreshes via the `activeCalls` array in the `ready` WS payload. The server filters to the per-user token at payload assembly time.

---

## 14. Background Workers

All workers are started by `startFederationWorkers()` on server boot and stopped by `stopFederationWorkers()` on shutdown. Each worker uses `setTimeout` chains (not `setInterval`) with abort controllers for graceful shutdown.

| Worker | Interval | Batch | Timeout | Source |
|--------|----------|-------|---------|--------|
| Outbox delivery | 10s | 50 | 30s | `processOutboxTick` |
| File download | 30s | 5 | 60s | `processFileQueueTick` |
| Health check | 1h | all unreachable | 10s | `processHealthCheckTick` |
| Janitor | 1h | -- | -- | `runFederationJanitor` (sync) |
| Initial sync | Once at startup | -- | 30s per page | `runInitialSyncForNewPeers` |

### Janitor Cleanup (`storageJanitor.ts:runFederationJanitor`)

| Target | Condition | Retention |
|--------|-----------|-----------|
| `federation_outbox` | `expiresAt < now` | Configurable via `federationRelayTtlDays` (default 30) |
| `federation_mutation_log` | `mutatedAt < (now - 90 days)` | 90 days |
| `federation_file_queue` (completed) | `createdAt < (now - 7 days)` | 7 days |
| `federation_file_queue` (any) | `expiresAt < now` | 30 days (set at queue time) |
| `dm_channels` (soft-deleted) | `deletedAt < (now - 24h)` | 24-hour grace period |

DM channel hard-delete cascades: reactions, embeds, attachments (DB rows + disk files), messages, members, outbox entries, mutation log entries, file queue entries.

---

## 15. Settings Cache

`federationOutbox.ts` caches `federationRelayEnabled` and `federationRelayTtlDays` from `instance_settings` for 30 seconds (`CACHE_TTL_MS`). This prevents repeated DB reads on every message send. The cache is invalidated by TTL only -- there is no explicit cache bust on settings change.

Relevant settings in `instance_settings`:

| Column | Default | Purpose |
|--------|---------|---------|
| `federation_relay_enabled` | 1 | Master toggle for all federation relay |
| `federation_relay_ttl_days` | 30 | Outbox entry TTL |
| `max_upload_size_bytes` | `null` (uses `config.maxUploadSize`) | File download size limit |

---

## 16. Client-Side Identity Helpers (`identity.ts`)

The frontend needs to resolve federated identities for display purposes:

**`parseFederatedUsername(username)`** -- splits `"youruser@nova.ddns.net"` into `{baseName: "youruser", domain: "nova.ddns.net"}`.

**`isSelf(user, homeUser)`** -- determines if a user object is the logged-in user or their replicated stub. Uses cascading checks: same ID, known self-ID set, homeInstance + baseName match.

**`canonicalUserMatch(a, b)`** -- federation-safe check for whether two user objects represent the same person. Cascades through: same local ID, `homeUserId` cross-match, username + homeInstance fallback.

**`resolveDisplayIdentity(user, homeUser)`** -- returns `homeUser` for display if `user` is a replicated stub of `homeUser`, enabling consistent avatars and display names across instances.

**Cross-instance self-ID registry:** `registerSelfId(id)` / `clearSelfIds()` track all Snowflake IDs belonging to the current user across connected instances, populated from WS `ready` events.

---

## 17. Self-Healing Migrations (`migrate.ts`)

The migration system includes several data integrity checks that run on every server startup:

**Group DM ownerId repair:**
Detects group DMs with UUID-format `federated_id` (length 36, matches `________-____-____-____-____________`) but `NULL owner_id`. Restores the owner from the first remaining member or from `owner_home_user_id`/`owner_home_instance`. Root cause: a bug in `processOwnershipTransferEvent` (fixed in cd7aff0) could set `ownerId = NULL` via `resolveLocalUser` fallback.

**Federated ID backfill:**
Finds 1-on-1 DM channels without `federated_id`, computes deterministic SHA-256 hash from home user IDs, and sets it. Also detects relay-created duplicate channels with the same `federated_id` and merges messages into the oldest channel.

**Duplicate channel merge:**
Finds `federated_id` values appearing on multiple channels and merges them into the oldest, moving messages, members, and cleaning up the duplicates.

**Mutation log backfill:**
If the `federation_mutation_log` table exists but is empty, populates it with `create` entries for all existing DM messages where `source_instance IS NULL` (locally-created messages).

---

## Known Issues

No critical known issues. See `docs/federation-production-roadmap.md` for open items (FED-001 through FED-013).
