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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.
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;
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 Behaviors: 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.joinRoomreturns 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
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.
/**
* 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
/** 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
/**
* 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$`.
* Before the transport is connected the request is remembered and applied
* once it is. Rejects where the device could not be used, including the
* user closing the picker. One publication per source: publishing a source
* again unmutes it.
*/
publish(
request: PublishRequest,
): Promise<LocalAudioMediaTrack | LocalVideoMediaTrack>;
/** Removes one of our tracks; a screen share takes its audio with it. */
unpublish(id: string): Promise<void>;
}
Transports
import { type Transport } from "matrix-js-sdk/lib/matrixrtc";
/**
* One transport advertised in a membership. Transport independent: `type` and
* `id` are all the SDK needs; `raw` and `resolved$` 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:
/** 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$, replayed on subscribe; the js-sdk session never
does. A host supplies its own backend through MatrixRTCClientOptions.backend;
without one, LiveKit is built from the LiveKit-flavoured client options. The
reasoning behind each of these choices is in
MediaBackendPlan.md.
Media: tracks$ 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$, a list of MediaTracks, so that nothing outside
the SDK imports Participant, TrackPublication, Track or TrackReference.
The adapter in sdk/src/media-backend/livekit/ is the only place that reads a
participant. The backend supplies the same two fields through mediaFor$, so
there is one shape from the LiveKit participant to the host and nobody has to
subscribe through one Behavior to reach another.
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$: Behavior<boolean>;
/** False when the SFU reports the track as unencrypted. Fixed for the life of the track. */
encrypted: boolean;
/**
* Inbound statistics for a remote track, outbound for a local one, polled
* every `intervalMs` while the observable is subscribed and not at all
* otherwise, so a statistics panel costs nothing while closed.
*/
stats$(intervalMs: number): Observable<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$: Behavior<boolean>;
/**
* How loud the track is right now, 0 to 1, as the backend measures it. 0
* while muted. Not what decides `isActive$`: the backend applies its own
* threshold and hysteresis to that.
*/
audioLevel$: Behavior<number>;
/** Route playback through Web Audio, for earpiece pan and gain. Undefined resets. */
setAudioContext(ctx: AudioContext | undefined, plugins?: AudioNode[]): void;
/** No-op for our own audio, which is never played back. */
setVolume(volume: number): void;
}
export interface VideoMediaTrack extends MediaTrack {
kind: "video";
}
/** The controls a member has over a track it publishes itself. */
export interface LocalMediaTrack {
/** Mutes or unmutes. Resolves with the state that resulted. */
setEnabled(enabled: boolean): Promise<boolean>;
/** Captures from another device. Rejects for a screen share, which has none. */
setDevice(deviceId: string): Promise<void>;
}
export interface LocalAudioMediaTrack
extends AudioMediaTrack, LocalMediaTrack {}
export interface LocalVideoMediaTrack extends VideoMediaTrack, LocalMediaTrack {
/** For mirroring; undefined where the camera does not say which way it faces. */
facingMode$: 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/ 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$; toggle the microphone and camera; leave.
Every media element carries the track's state as data-* attributes (muted,
encrypted, active, frameWidth, frames), which is what the tests read to
check mute propagation, encryption, adaptive resolution and paused subscriptions
(the speaker detection check is a fixme: the fake microphones do not reliably
register at the SFU). That is more
than the smallest consumer needs, and the price of testing the SDK through its
public API only.
Open decisions
- Publishing before
join(). The initial requests are published at the join; creating the tracks earlier, for a lobby preview, is still missing. applicationDataafter the join. The option is read once; there is no way to update it on a live membership yet.- Raw membership state. The Matrix side's own state machine
(
MembershipStateinLocalMembership.ts) stays internal;state$on the participation is its public view combined with the media connection. A developer panel may want it asLocalRTCMember.membershipState$. - Developer panel.
resolved$on each connected transport covers what the panel shows today. Anything beyond that (LiveKit room state, connection quality) needs an opaquedebug$per transport. - Data beside the media.
sendData/data$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.