# MatrixRTC SDK architecture

`@element-hq/matrixrtc-sdk` is the model under Element Call, on its own: one
`MatrixRTCClient` per room that joins and leaves the MatrixRTC session, discovers
and connects to the media transports, publishes the local media, distributes the
encryption keys, and hands out every member with its media as observables. It has
no UI, reads no configuration and no settings, and never names LiveKit in its
interface.

Element Call becomes one consumer of it: `CallViewModel` keeps what makes a session
a _call_ (ringing, auto-leave, layout, sounds, reactions, settings, the host
bridge) and takes everything else from the client. The dependency direction is
therefore `Element Call → sdk`, never the reverse. The migration from today's
`CallViewModel` to that shape is in [`SdkMigration.md`](./SdkMigration.md).

## The architecture

```
 host application (Element Call, a whiteboard, watch-together, …)
   │
   │  createMatrixRTCClient(scope, matrixClient, room, options)
   ▼
┌──────────────────────────────────────────────────────────────────────────────┐
│ MatrixRTCClient                                                              │
│                                                                              │
│   public API, all Behaviors:                                                 │
│     join() leave() state$                                                    │
│     localMember$ remoteMembers$ memberCount$ sendData() data$                │
│     keyRotationSuppressed$ connectedTransports$                              │
│                                                                              │
│   per member    RTCMember   { id userId deviceId membership$ displayName$    │
│                               avatarUrl$ transport$ tracks$ encryptionError$ }│
│   per track     MediaTrack  { source kind muted$ encrypted stats$ attach() } │
│                                                                              │
│  ┌───────────────────────────┐  MediaBackend   ┌───────────────────────────┐ │
│  │ session: MatrixRTC        │ media-backend/  │ media backend             │ │
│  │                           │   api.ts        │ media-backend/livekit/    │ │
│  │ js-sdk MatrixRTCSession,  │                 │ today, or options.backend │ │
│  │   later the rust-rtc      │ prepareLocal    │                           │ │
│  │   crate behind a bridge   │   Transport ───▶│ token service:            │ │
│  │ memberships from room     │ delegateDelayed │   OpenID → JWT,           │ │
│  │   state and sticky events │   Leave ───────▶│   delegation probe        │ │
│  │ own membership, delayed   │                 │ ConnectionManager:        │ │
│  │   leave, retries          │ mediaKeys$ ────▶│   one LiveKit room per    │ │
│  │ discovery of the local    │ mediaFor$ ─────▶│   transport url, local    │ │
│  │   transport (homeserver)  │ ◀─ tracks$,     │   published, remote       │ │
│  │ membership state machine  │ encryptionError$│   subscribed              │ │
│  │   (LocalMembership)       │ local.publish,  │ Publisher: local tracks,  │ │
│  │ transport registry        │   setPublishing▶│   upstream paused until   │ │
│  │   (TransportMetadata)     │ ◀─ connections$ │   joined                  │ │
│  │ data packet → member      │ ◀─ data$        │ LivekitMemberMedia:       │ │
│  │                           │                 │   participant → tracks    │ │
│  └───────────────────────────┘                 └───────────────────────────┘ │
│                                                                              │
│   RTCMember = membership + mediaFor$(membership): the backend matches the    │
│   member's rtcBackendIdentity to the identity it sees on the transport       │
└──────────────────────────────────────────────────────────────────────────────┘
   │                                                            │
   ▼                                                            ▼
 MatrixClient (sync, room state, to-device, OpenID)         LiveKit SFU
```

The left box is MatrixRTC: who is in the session, with which transport, the
keys, the delayed leave, and the membership state machine. The right box is a
media backend: whatever carries the media, behind the `MediaBackend` interface in
`sdk/src/media-backend/api.ts`. The client is the join of the two: a member exists
once its membership does, and gets its media once the backend has something
for that membership on its transport; `state$` is the membership state combined
with the local media connection. LiveKit is the one backend today; a
cascading SFU or full mesh is another folder under `sdk/src/media-backend/`, handed
in through `options.backend`, and nothing in the left box changes.

### How Element Call uses it

```
 Element Call, as app, widget or component
┌────────────────────────────────────────────────────────────────┐
│ views: InCallView, tiles, header, footer, settings             │
│    ▲ Behaviors                                                 │
│ CallViewModel                                                  │
│   layout, ringing and notifications, auto-leave, sounds,       │
│   reactions, hand raise, settings, audio routing, host bridge  │
│    ▲ state$, localMember$, remoteMembers$, tracks$             │
│ MatrixRTCClient                   @element-hq/matrixrtc-sdk    │
└────────────────────────────────────────────────────────────────┘
     │ MatrixClient + Room                        │ livekit-client
     ▼                                            ▼
 homeserver                                      SFU
```

All three modes already hold a `MatrixClient` and a `Room` by the time a call
starts: standalone after `useLoadGroupCall`, the component from the host's `client`
prop, the widget from `createRoomWidgetClient`. The view model passes them on and
builds its media view models from `RTCMember` and its `tracks$` instead of from
LiveKit participants.

### Joining, in order

```
 host          MatrixRTCClient / backend        homeserver / JWT service       LiveKit SFU
  │  create ───▶│                                       │                         │
  │             │ GET rtc/transports (session) ────────▶│                         │
  │             │◀── first transport of the backend's type                        │
  │             │ prepareLocalTransport (backend):      │                         │
  │             │   OpenID token, then /sfu/get ───────▶│                         │
  │             │◀── sfu url, jwt, alias, identity ─────│   = resolved$ of the     │
  │             │   probe delegate_delayed_leave ──────▶│     local transport      │
  │             │ connect(url, jwt) ───────────────────────────────────────────▶│
  │  join() ───▶│                                       │                         │
  │             │ membership state event + delayed leave ──▶│                     │
  │             │ delegateDelayedLeave(delayId) (backend) ─▶│                     │
  │             │ create tracks, resume upstream ──────────────────────────────▶│
  │             │ media key to every other member (to-device) ──▶│               │
  │             │◀── other memberships (sync) ──────────│◀── participants ────────│
  │◀─ state$ connected, localMember$, remoteMembers$ with tracks$ ───────────────│
```

The session discovers the transport from the homeserver and the backend prepares
it, both before `join()`, so that the membership can name it and the first frames
go out as soon as the state event is sent. Preparing is the backend's own
business: for LiveKit the token exchange, and the probe that decides whether the
SFU can take over the delayed leave, which picks the delayed leave timings the
join uses. Until `join()` the tracks exist but their upstream is paused: a host
can show a preview without anyone hearing it. The upstream flows exactly while the
membership is `joined`: a sync outage pauses it, and `leave()` pauses it for good
and sends the leave.

## Vocabulary

The words the SDK is written in. They name the shape after the slot and
participation split in
[`client-into-slot-session-split-plan.md`](./client-into-slot-session-split-plan.md);
the sections below still describe `MatrixRTCClient` until that lands.

| Word                        | Meaning here                                                                                                                                                                                                                                           | Elsewhere                                                                                                   |
| --------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ | ----------------------------------------------------------------------------------------------------------- |
| slot                        | an `RTCSlot`: the namespace in a room that memberships join into, `application` plus `id`. It exists while the room does, with nobody in it too. Holds `members$`, its open or closed `status$`, and `join()`                                          | the js-sdk `MatrixRTCSession` is one per slot; the MSC4143 slot event                                       |
| participation               | an `RTCParticipation`: the span between our `join()` of a slot and our `leave()`. Holds the connection and its `state$`, `localMember$`, `remoteMembers$`, the connected transports and the data channel. Made by `slot.join()`, never reused          | "session" is not used here, because in the js-sdk it means the slot                                         |
| member                      | an `RTCMember`: one membership in a slot with what the SDK derives from it, `rtcBackendIdentity`, `userId`, `deviceId`, `memberId`, `displayName$`, `avatarUrl$`, `transport$` and `applicationData$`. No media. What the slot hands out in `members$` | the js-sdk `CallMembership` behind it, which never leaves the SDK; Element Call's old "matrixLivekitMember" |
| local member, remote member | `LocalRTCMember` and `RemoteRTCMember`: an `RTCMember` plus its `MemberMedia`, which only a participation can carry. The local one's tracks carry the controls. What the participation hands out in `localMember$` and `remoteMembers$`                | LiveKit `LocalParticipant` and `RemoteParticipant`, which never leave the SDK                               |
| media                       | a `MemberMedia`: what a member sends once it has arrived on its transport, `tracks$`, null while waiting, and `encryptionError$` beside it. The media backend supplies exactly these two fields per member                                             | Element Call's `MediaViewModel` is a view of it                                                             |
| track                       | a `MediaTrack`: one publication of a member, audio or video, with `source`, `kind`, `id` and the behaviors on it. `LocalAudioMediaTrack` and `LocalVideoMediaTrack` add the controls over our own                                                      | LiveKit `TrackPublication` and `Track`, which never leave the SDK                                           |
| publish request             | a `PublishRequest`: a source, where to capture it from and how to encode it. Given at the join in `publish`, or later to `publish` on the participation                                                                                                |                                                                                                             |
| attach, detach              | handing a `<video>` or `<audio>` element to a track and taking it back. The SDK owns the stream and the observers on the element                                                                                                                       | LiveKit `Track.attach`                                                                                      |
| transport                   | an `RTCTransport`: where media is exchanged, a LiveKit service url today. Carries the raw js-sdk `Transport` from the membership and, in `resolved$`, what the backend fetched to connect to it                                                        | MSC4143 "focus"                                                                                             |
| media backend               | a `MediaBackend`: what carries the media, behind `sdk/src/media-backend/api.ts`. One per participation, chosen with `backend` in the join options; LiveKit under `sdk/src/media-backend/livekit/` is the one that exists                               | "SFU", a LiveKit room, a peer connection                                                                    |

Names use `RTC` in capitals, as the js-sdk does: `RTCSlot`, `RTCParticipation`,
`RTCMember`, `matrixRTCMode`.

## Reactive primitives

The whole public API is made of `Behavior`s: an rxjs observable that always holds a
current value (`.value`) and replays it on subscription. A consumer subscribes to
what it renders and reads `.value` where it needs the present state. Everything is
bound to an `ObservableScope` the host creates; ending the scope ends the client,
its connections and its tracks. Both come from the SDK (`Behavior`, `constant`,
`ObservableScope`); the host needs rxjs and nothing else.

## What a host must provide

Found the hard way while building the harness; each one fails in a way that looks
like an SDK bug.

- **The room the sync loop maintains.** `client.getRoom(roomId)` once the join has
  synced. `joinRoom` returns a detached copy for a room joined just now, and a
  client built on it never sees a member.
- **Power to send the membership event.** Every member needs to write
  `org.matrix.msc3401.call.member` (and the sticky event in Matrix 2.0 mode). A
  plain room only lets its creator send state; Element Call's rooms set the level
  to 0.
- **An encrypted room for per-participant keys.** Media keys travel in encrypted
  to-device messages, which the crypto only delivers to devices it tracks, which
  are the members of encrypted rooms. In an unencrypted room the key never arrives
  and every remote track decodes nothing.
- **A logged-in, syncing client with crypto set up**, and the transport advertised
  by the homeserver (`rtc/transports`). The SDK has no fallback url of its own.

## The client: `MatrixRTCClient`

Rule of thumb for what belongs here: if it would be the same for a different
application on top of the same MatrixRTC session, it is in the client. If it is
specific to calls, or depends on the window, a setting, a platform, or a React
context, it is in the view model.

### Creating one

```ts
import { type MatrixClient, type Room } from "matrix-js-sdk";
import { type RTCNotificationType } from "matrix-js-sdk/lib/matrixrtc";

export interface MatrixRTCClientOptions {
  encryptionSystem: EncryptionSystem;
  /** Resolved by the host; the SDK reads neither config.json nor settings. */
  matrixRTCMode: MatrixRTCMode;
  /** Published at the join; `publish` on the local member adds to it from then on. */
  publish: PublishRequest[];
  /**
   * MSC4075 notification sent with the join. A parameter of the MatrixRTC
   * join itself, so it is here even though it is named after calls; reacting
   * to a notification (ringing, timeouts, declines) is the application's job.
   */
  sendNotificationType?: RTCNotificationType;
  /** The application the session belongs to, as named in the membership. Default `m.call`. */
  application?: string;
  /** The application's slot in the room. Default `ROOM`. */
  slot?: string;
  /**
   * Whatever the application wants to say about itself in the membership.
   * Opaque to the SDK. Today the js-sdk carries one key, `m.call.intent`
   * (`"audio"` or `"video"`); anything else is dropped until it can.
   */
  applicationData?: Record<string, unknown>;
  /** Session timings the host has configured; the defaults otherwise. */
  timings?: Partial<SessionTimings>;
  /** Limits on what is published; LiveKit's defaults otherwise. */
  mediaQuality?: MediaQuality;
  /** Use this transport instead of asking the homeserver. */
  transportUrl?: string;
  /** Use this transport when the homeserver advertises none. */
  fallbackTransportUrl?: string;
}

/**
 * One thing to publish: a source, where to capture it from and how to encode
 * it. Where the device and the settings are left out, the browser's and
 * LiveKit's defaults apply.
 */
export type PublishRequest =
  | { source: "microphone"; deviceId?: string; capture?: AudioCaptureSettings }
  | {
      source: "camera";
      deviceId?: string;
      /** Background blur and the like. */
      processor?: VideoProcessor;
      capture?: VideoCaptureSettings;
    }
  | {
      source: "screenShare";
      /** Whether to capture the screen's audio too. Default true. */
      audio?: boolean;
      capture?: VideoCaptureSettings;
    };
```

Device enumeration, permission prompts and the lobby preview stay with the
host: it hands over publish requests naming the devices it chose.

```ts
/**
 * Takes the whole MatrixClient, not a narrow pick, and finds the MatrixRTC
 * session itself. The js-sdk is the MatrixRTC implementation today; when the
 * rust-rtc crate replaces it, the SDK has to bridge the crate to the js-sdk
 * client (events, state, to-device, delayed events, OpenID), and only the SDK
 * knows what that bridge needs. Holding the client keeps that change inside
 * the SDK. The js-sdk session is not part of the public API.
 *
 * The room has to be the one the client's sync loop maintains; see "What a
 * host must provide".
 */
export function createMatrixRTCClient(
  scope: ObservableScope,
  client: MatrixClient,
  room: Room,
  options: MatrixRTCClientOptions,
): MatrixRTCClient;
```

### The client

```ts
/** The homeserver link that is down, in the order they depend on each other. */
export type MatrixDisconnectReason = "sync" | "membership" | "probablyLeft";

/** Why the participation is not connected: the first failing link, in dependency order. */
export type DisconnectReason = MatrixDisconnectReason | "media";

export type ParticipationState =
  | { kind: "waitingForTransport" }
  | { kind: "connecting" }
  | { kind: "connected" }
  | { kind: "reconnecting"; reason: DisconnectReason }
  /** A failed publication is not fatal: the member can still receive, so it stays on the local member's media. */
  | { kind: "failed"; error: MatrixRTCError }
  /** `leave()` was called. Terminal. */
  | { kind: "left" };

/**
 * An error raised by the client. `code` and `category` say what went wrong in
 * a form a host can translate; `cause` holds the backend's error.
 */
export class MatrixRTCError extends Error {
  code: ErrorCode;
  category: ErrorCategory;
}

export interface MatrixRTCClient {
  join(): void;
  leave(): void;
  /**
   * The Matrix membership combined with the local media connection. One
   * value: a reason exists only while `reconnecting`, an error only once
   * `failed`.
   */
  state$: Behavior<ParticipationState>;

  /** Null until our own membership has been seen in the room. */
  localMember$: Behavior<LocalRTCMember | null>;
  remoteMembers$: Behavior<RemoteRTCMember[]>;
  /** `remoteMembers.length`, plus one for the local member once it exists. One per membership, so one user may count several times. */
  memberCount$: Behavior<number>;

  /**
   * Whether the session has grown large enough that MatrixRTC has stopped
   * rotating the media encryption key.
   */
  keyRotationSuppressed$: Behavior<boolean>;

  /**
   * Transports the client currently holds a live connection to. The
   * connections themselves stay internal; this is what a developer panel
   * needs.
   */
  connectedTransports$: Behavior<TransportMetadata[]>;

  /**
   * Plays the members' audio on this device from now on. Rejects where the
   * browser cannot switch to it. A host that routes audio itself, or leaves
   * the browser's choice, never calls it.
   */
  setAudioOutputDeviceId(deviceId: string): Promise<void>;

  /**
   * Sends a short text to every member on the local transport, over a
   * reliable data channel beside the media. A packet holds about 15 KiB;
   * anything larger belongs in a room event. Rejects while the local
   * transport is not connected. Encrypted on the wire like the media, but
   * not end to end with the media key.
   */
  sendData(topic: string, text: string): Promise<void>;
  /**
   * What remote members sent with `sendData`, on every transport the client
   * is connected to. A message from an identity that is not a member is
   * dropped, so a host only ever hears from attested members.
   */
  data$: Observable<DataMessage>;
}

/** One `sendData` call as it arrives at a remote member. */
export interface DataMessage {
  member: RemoteRTCMember;
  topic: string;
  text: string;
}
```

`state$` is two state machines, one per box in the diagram, combined by one pure
function. `LocalMembership` knows only the Matrix side: `waitingForTransport`,
`joining`, `joined`, `reconnecting` with a `MatrixDisconnectReason`, `failed`,
`left`. The client folds the local media connection in: `joined` with the media
up is `connected`, a media drop after that is `reconnecting: "media"`, a
connection error is `failed`. Each step is computed in priority order from the
previous state, so `left` and `failed` absorb everything after them, and
`reconnecting` is only ever reached from `connected`; a membership that drops
before the media ever came up stays `connecting`. One reason, not a set, because
the links are a chain: without sync the membership is unknowable, and the media
is worth little while the Matrix side is down, so the first broken link in that
order is what a host shows and what Element Call's analytics count. A failed
publication is not in here at all: the member can still receive, so it stays on
the local member's media.

### Members

```ts
/**
 * What a membership says, as hosts read it. The js-sdk `CallMembership` is
 * behind it; anything beyond this is the js-sdk's API, not the SDK's.
 */
export type RTCMembership = Pick<
  CallMembership,
  | "userId"
  | "deviceId"
  | "memberId"
  | "rtcBackendIdentity"
  | "application"
  | "applicationData"
  | "getTransport"
  | "transports"
  | "createdTs"
  | "getAbsoluteExpiry"
>;

export interface RTCMember {
  local: boolean;
  /**
   * The identity the media backend knows this member by: the membership's
   * `rtcBackendIdentity`, which is `${userId}:${deviceId}` before sticky
   * events and a hash of the member id in Matrix 2.0 mode.
   */
  id: string;
  userId: string;
  deviceId: string;
  membership$: Behavior<RTCMembership>;
  /**
   * Matrix room state rather than MatrixRTC, but the SDK already holds the
   * room and every consumer wants them next to the membership. Disambiguated
   * among the members of the session.
   */
  displayName$: Behavior<string>;
  avatarUrl$: Behavior<string | undefined>;
  /** Which transport this member is on; undefined when the membership names none. */
  transport$: Behavior<TransportMetadata | undefined>;
  /**
   * The member's tracks, in publication order, once it has shown up on its
   * transport; null until then ("waiting for media"). An entry stays the same
   * object for as long as the same publication is behind it. Which track is
   * which is in its `source`; `trackBySource$` picks one out. Hand raise and
   * reactions are room events keyed by member and stay with the host for now;
   * they are the obvious next fields here.
   */
  tracks$: Behavior<(AudioMediaTrack | VideoMediaTrack)[] | null>;
  /** Emits when the SFU reports a key problem for this member. */
  encryptionError$: Observable<EncryptionError>;
}

export interface RemoteRTCMember extends RTCMember {
  local: false;
}

export interface LocalRTCMember extends RTCMember {
  local: true;
  tracks$: Behavior<(LocalAudioMediaTrack | LocalVideoMediaTrack)[] | null>;
  /**
   * Publishes a source and resolves with its track once it is in `tracks$`.
   * Before the transport is connected the request is remembered and applied
   * once it is. Rejects where the device could not be used, including the
   * user closing the picker. One publication per source: publishing a source
   * again unmutes it.
   */
  publish(
    request: PublishRequest,
  ): Promise<LocalAudioMediaTrack | LocalVideoMediaTrack>;
  /** Removes one of our tracks; a screen share takes its audio with it. */
  unpublish(id: string): Promise<void>;
}
```

### Transports

```ts
import { type Transport } from "matrix-js-sdk/lib/matrixrtc";

/**
 * One transport advertised in a membership. Transport independent: `type` and
 * `id` are all the SDK needs; `raw` and `resolved$` are there for a
 * backend-specific developer panel and for connection diagnostics. One object
 * per transport, however many memberships name it.
 */
export interface TransportMetadata {
  /** `"livekit"` today. */
  type: string;
  /** Stable key, unique per transport in the session: the raw transport, serialised with sorted keys. */
  id: string;
  /** The transport object as it appears in the membership, e.g. `{ type, livekit_service_url }`. */
  raw: Transport;
  /**
   * What the backend had to fetch before it could connect. Undefined until
   * the connection has resolved it, and again after the connection stops or
   * fails. The local transport resolves before the join, since the membership
   * needs it; remote ones when their connection starts.
   */
  resolved$: Behavior<ResolvedTransport | undefined>;
}

export type ResolvedTransport =
  | {
      type: "livekit";
      /** The SFU websocket url, as opposed to the JWT service url in `raw`. */
      url: string;
      /** A secret: fit for a developer panel, never for a log line. */
      token: string;
      roomAlias: string;
      identity: string;
    }
  | { type: string; [key: string]: unknown };
```

### Call-specific things kept out of the client

Checked against `CallViewModel` and its options. These stay in Element Call because
they only make sense for a call:

| Today                                                                                                        | Why it is call-specific                                                                              | Where it lives instead                                                                              |
| ------------------------------------------------------------------------------------------------------------ | ---------------------------------------------------------------------------------------------------- | --------------------------------------------------------------------------------------------------- |
| `autoLeave$` / `AutoLeaveReason` (`"allOthersLeft" \| "timeout" \| "decline"`)                               | hanging up because the other side declined or everyone left is call etiquette, not session semantics | `CallViewModel.leave$`, built from `remoteMembers$` and the room timeline                           |
| `ringAttempts$`, `createCallNotificationLifecycle$`, `createSentCallNotification$`, `createReceivedDecline$` | ringing, pickup timeouts and decline events are MSC4075 call-notify UX                               | `src/state/CallViewModel/CallNotificationLifecycle.ts`, fed by `remoteMembers$` and the Matrix room |
| `waitForCallPickup`, `autoLeaveWhenOthersLeft` options                                                       | inputs to the above                                                                                  | `CallViewModelOptions`                                                                              |
| `hideScreensharing` option                                                                                   | a UI policy; the session can always share a screen if the platform can                               | `CallViewModel` offers no screen share control                                                      |
| `ringingVm$`, `ringingStatusLocation`                                                                        | view models                                                                                          | `CallViewModel`                                                                                     |
| `hostBridge.hangUp$`, `hangup()`, `userHangup$`                                                              | widget / host integration                                                                            | `CallViewModel`                                                                                     |
| `ElementCallError`                                                                                           | carries Element Call translation keys                                                                | the view model wraps `MatrixRTCError`, reading `cause`                                              |
| `participantCount$` sound limits, join/leave sounds                                                          | call UX                                                                                              | `CallViewModel`                                                                                     |
| `callIntent`                                                                                                 | what a membership says about the call                                                                | `applicationData["m.call.intent"]`, set by the host                                                 |

Kept despite the name: `sendNotificationType`, because the js-sdk join sends the
notification as part of entering the session. Only _reacting_ to it is
call-specific.

## Media backend: `MediaBackend`

The client speaks MatrixRTC: memberships, the transports named in them, the
delayed leave, the keys. What carries the media is a backend behind one
interface, `MediaBackend` in `sdk/src/media-backend/api.ts`, created once per client.
LiveKit under `sdk/src/media-backend/livekit/` is the only backend today, and the only
place in the SDK that imports `livekit-client`; the import boundary lint rule
enforces that outside `sdk/src/media-backend/` the LiveKit packages appear in type
imports only.

The seam sits above the connection manager on purpose. One connection per
transport url, with a subscribe-only connection to every remote SFU, is the
LiveKit multi-SFU strategy, not a MatrixRTC concept: a cascading SFU keeps the
membership transport as it is and only changes which SFUs the client connects to,
and full mesh has one peer connection per member and no token service. So a
backend is handed the whole session's view and gives back media:

```ts
/** The fields of a member the backend supplies, as the member exposes them. */
type MemberMediaFields = Pick<RTCMember, "tracks$" | "encryptionError$">;

export interface MediaBackend {
  readonly transportType: string; // "livekit"
  /** Authenticate with the transport the client found; the probe for delegation runs here. */
  prepareLocalTransport(
    transport: Transport,
  ): Promise<{ canDelegateDelayedLeave: boolean }>;
  /** Hand over the delayed leave, each time the membership manager has a new delay id. */
  delegateDelayedLeave(delayId: string): Promise<void>;
  readonly local: LocalMediaBackend; // tracks$ encryptionError$ connectionState$ setPublishing publishError$ publish unpublish
  /** A remote member's media; also how the backend learns which transports to connect to. */
  mediaFor$(
    scope: ObservableScope,
    membership$: Behavior<RTCMembership>,
  ): MemberMediaFields;
  readonly connections$: Behavior<BackendConnection[]>; // transport, state, resolved; diagnostics
  sendData(topic: string, text: string): Promise<void>;
  readonly data$: Observable<DataPacket>;
}
```

What stays on the client's side: discovery of the local transport from the
homeserver's list (a homeserver endpoint, so not a backend's), the membership
state machine in `LocalMembership.ts` and its combination with
`local.connectionState$` into `state$`, the choice of delayed leave timings,
mapping a data packet's sender to a member, and the transport registry, which keys on the raw
transport serialised with sorted keys and derives `connectedTransports$` and each
`TransportMetadata.resolved$` from `connections$`.

The backend's context carries no Matrix client and no membership list. A backend
that needs the client takes a `Pick` of exactly what it calls in its own options
(`createLivekitBackend(scope, context, { client, mediaQuality, tokenEndpoint })`),
and learns of remote members only through `mediaFor$`, registering each
membership's transport for as long as the member's scope lives. Media keys reach
it as a stream, `mediaKeys$`, replayed on subscribe; the js-sdk session never
does. A host supplies its own backend through `MatrixRTCClientOptions.backend`;
without one, LiveKit is built from the LiveKit-flavoured client options. The
reasoning behind each of these choices is in
[`MediaBackendPlan.md`](./MediaBackendPlan.md).

## Media: `tracks$` and `MediaTrack`

Every member has media. Today, in Element Call, that media _is_ a `livekit-client`
participant, handed straight through to the view models. In the SDK the participant
becomes the member's `tracks$`, a list of `MediaTrack`s, so that nothing outside
the SDK imports `Participant`, `TrackPublication`, `Track` or `TrackReference`.
The adapter in `sdk/src/media-backend/livekit/` is the only place that reads a
participant. The backend supplies the same two fields through `mediaFor$`, so
there is one shape from the LiveKit participant to the host and nobody has to
subscribe through one Behavior to reach another.

```ts
export type MediaSource =
  | "microphone"
  | "camera"
  | "screenShare"
  | "screenShareAudio"
  /** Published without a source; the application knows what it is. */
  | "unknown";

export type MediaStreamStats =
  RTCInboundRtpStreamStats | RTCOutboundRtpStreamStats | undefined;

/** One published track of a member. */
export interface MediaTrack {
  source: MediaSource;
  kind: "audio" | "video";
  /** Stable for the life of the track. */
  id: string;
  muted$: Behavior<boolean>;
  /** False when the SFU reports the track as unencrypted. Fixed for the life of the track. */
  encrypted: boolean;
  /**
   * Inbound statistics for a remote track, outbound for a local one. Polled
   * once a second, but only while someone is subscribed, so a statistics
   * panel costs nothing while closed.
   */
  stats$: Behavior<MediaStreamStats>;
  /**
   * Rendering. The view hands its <video> or <audio> element over; the SDK
   * sets its stream and, for video, registers the size and on-screen
   * observers that pick a simulcast layer and pause the subscription while
   * the element is hidden. attach is idempotent per element; detach has to be
   * called before the element leaves the DOM so those observers are released.
   */
  attach(element: HTMLMediaElement): void;
  detach(element: HTMLMediaElement): void;
}

export interface AudioMediaTrack extends MediaTrack {
  kind: "audio";
  /**
   * Whether the track carries sound right now, as the backend measures it.
   * False while muted. A call reads this on the microphone track and calls it
   * "speaking". LiveKit measures it per member, so every audio track of a
   * member reports the same value.
   */
  isActive$: Behavior<boolean>;
  /** Route playback through Web Audio, for earpiece pan and gain. Undefined resets. */
  setAudioContext(ctx: AudioContext | undefined, plugins?: AudioNode[]): void;
  /** No-op for our own audio, which is never played back. */
  setVolume(volume: number): void;
}

export interface VideoMediaTrack extends MediaTrack {
  kind: "video";
}

/** The controls a member has over a track it publishes itself. */
export interface LocalMediaTrack {
  /** Mutes or unmutes. Resolves with the state that resulted. */
  setEnabled(enabled: boolean): Promise<boolean>;
  /** Captures from another device. Rejects for a screen share, which has none. */
  setDevice(deviceId: string): Promise<void>;
}

export interface LocalAudioMediaTrack
  extends AudioMediaTrack, LocalMediaTrack {}

export interface LocalVideoMediaTrack extends VideoMediaTrack, LocalMediaTrack {
  /** For mirroring; undefined where the camera does not say which way it faces. */
  facingMode$: Behavior<"user" | "environment" | undefined>;
  /** Restarts the camera facing the other way; resolves with the device now in use. */
  switchFacingMode(): Promise<string | undefined>;
  /** Background blur and the like; undefined removes the processor. */
  setProcessor(processor: VideoProcessor | undefined): Promise<void>;
}

export type EncryptionError = "MissingKey" | "InvalidKey";

/** The member's first track of a source; undefined while there is none, or no media yet. */
export function trackBySource$<S extends MediaSource>(
  scope: ObservableScope,
  tracks$: Behavior<MediaTrack[] | null>,
  source: S,
): Behavior<TrackOfSource<S> | undefined>;
```

### Rendering: `attach` and `detach`

The view passes its DOM element into the SDK and the SDK does the rest; there is no
`MediaStreamTrack` on the interface. The reason is adaptive stream: Element Call
runs with `adaptiveStream: true`, which means LiveKit decides the simulcast layer
from the size of the attached video element and pauses the subscription at the
SFU while no attached element is on screen. Those size and visibility observers are
set up inside LiveKit's `attach`. Handing the element over is the only way to keep
that without reimplementing the observers outside the SDK. The e2e tests check both
effects: a larger tile receives a higher resolution, a hidden tile stops receiving
frames.

What `attach` covers for free, so the host does not have to: `srcObject` and
`autoplay` setup with the Safari and Firefox black-video workarounds; `play()` and
the autoplay-blocked signal; following the track through mute, reconnect and
re-subscription into every attached element; the Web Audio graph for earpiece pan
and gain; the adaptive stream observers.

The interface is web-only. A non-DOM host would need a different media surface; if
one shows up, a `MediaStreamTrack` accessor is added next to `attach`, not instead
of it.

## Packaging

`sdk/` is packaged exactly as `component/` is: a `package.json` with peer
dependencies (`matrix-js-sdk`, `livekit-client`, `rxjs`), `exports` with types,
type emission through `sdk/tsconfig.build.json`, an externals list in
`vite-sdk.config.ts` that `pnpm lint:externals` enforces, and installation from the
repository as a git dependency with a `path:` suffix. `pnpm build:sdk` produces
`sdk/dist`. It is not a pnpm workspace package yet; the root is not a workspace, and
moving to one later carries the package name, entry point and externals over
unchanged.

Element Call consumes the SDK by its package name from day one, so the swap to a
published dependency is one line: `tsconfig.json` `paths` and the `resolve.alias`
in `vitePluginsConfig` both map `@element-hq/matrixrtc-sdk` to `sdk/index.ts`. The
alias has to live in `vitePluginsConfig`, which every config is built on. Put only
into the app's own config, the harness resolves the name through `sdk/package.json`
to whatever stale `sdk/dist` is lying around.

Layout:

```
sdk/
  index.ts            the entry point: re-exports the API and the primitives
  src/api.ts          the public types above: client, members, transports
  src/media-api.ts    the public media types: publish requests, tracks
  src/errors.ts       MatrixRTCError and its codes
  src/config.ts       MatrixRTCMode, session timings, media quality
  src/encryption.ts   E2eeType, EncryptionSystem
  src/MatrixRTCClient.ts  createMatrixRTCClient: wires the two folders below
  src/participationState.ts  the membership state combined with the media connection
  src/reactive/       Behavior, ObservableScope, the observable operators
  src/matrixrtc/      the MatrixRTC side: the local membership, memberships, member
                      metadata, transports, discovery, the js-sdk join
  src/media-backend/  the MediaBackend interface in api.ts; livekit/ is the one
                      backend: connections, publisher, JWT, keys, the tracks
  src/utils/          LazyBehavior, mapScoped, network retry, display names, test helpers
  dev/                the harness (below)
  SdkArchitecture.md  this document
  SdkMigration.md     how Element Call gets from CallViewModel to the client
```

The lint rule `element-call/sdk-import-boundary` keeps it that way: nothing under
`sdk/` may import a file outside `sdk/` (that is `src/`, `component/`,
`playwright/`) or React, i18n or Compound. The harness under `sdk/dev/` is a host
and exempt.

## Dev harness: `sdk/dev/`

A web app that uses the SDK and nothing else: plain DOM, no React, no Compound, no
i18n, imports only `@element-hq/matrixrtc-sdk`, `matrix-js-sdk` and `rxjs`. It is
where the SDK is run during development and what the e2e tests in `playwright/sdk`
drive. If something cannot be done in the harness without reaching into `src/`, the
SDK API is missing something.

What it does: log in with a password, or with an access token the URL hands it (the
tests register users through the admin API and skip `/login`, which the dev
homeserver rate-limits); join the room and wait for the sync to hold it; create the
client with microphone and camera enabled; show one tile per member with the display
name, a `<video>` attached from the member's `"camera"` track and, for remote
members, an `<audio>` from the `"microphone"` track, both picked out of
`tracks$` with `trackBySource$`; toggle the microphone and camera; leave.

Every media element carries the track's state as `data-*` attributes (`muted`,
`encrypted`, `active`, `frameWidth`, `frames`), which is what the tests read to
check mute propagation, encryption, adaptive resolution and paused subscriptions
(the speaker detection check is a `fixme`: the fake microphones do not reliably
register at the SFU). That is more
than the smallest consumer needs, and the price of testing the SDK through its
public API only.

## Open decisions

- **Publishing before `join()`.** The initial requests are published at the
  join; creating the tracks earlier, for a lobby preview, is still missing.
- **`applicationData` after the join.** The option is read once; there is no
  way to update it on a live membership yet.
- **Raw membership state.** The Matrix side's own state machine
  (`MembershipState` in `LocalMembership.ts`) stays internal; `state$` on the
  participation is its public view combined with the media connection. A
  developer panel may want it as `LocalRTCMember.membershipState$`.
- **Developer panel.** `resolved$` on each connected transport covers what the
  panel shows today. Anything beyond that (LiveKit room state, connection
  quality) needs an opaque `debug$` per transport.
- **Data beside the media.** `sendData`/`data$` ride on LiveKit's reliable data
  packets: topic-agnostic, matched to members by identity, received on every
  connection but sent only on the local transport, so a member on another
  transport does not hear us. They are transport-encrypted, not end to end:
  LiveKit can encrypt data packets with the media key, but that is not wired
  through the key provider yet. A host that needs either reach or end-to-end
  encryption uses room events.
- **Unencrypted rooms with per-participant keys.** Keys only reach devices the
  crypto tracks, which are the members of encrypted rooms. If the SDK is to work
  in an unencrypted room, tracking the members' devices becomes its job.