--- status: draft last_updated: 2026-07-12 --- # channel-client.md — ChannelClient The client side of a channels connection. ADR-080 is the decision; this doc specifies the API. ## What `ChannelClient` is the symmetric counterpart to `ChannelsAdapter` (ADR-075). The server side is a `ProtocolHandler` (`ChannelsAdapter::handle`); the client side takes over an established transport `Connection`, runs the demux/mux, and exposes `open_channel(alpn, params) -> Channel` to the application. This is the channels analogue of `CallClient` (server: `CallAdapter`; client: `CallClient`) in the call protocol. ## API ```rust pub struct ChannelClient { manager: ChannelManager, // The transport-side demux/mux, running in a background task. ... } impl ChannelClient { /// Construct a `ChannelClient` over a pre-established transport /// `Connection` on ALPN `alknet/channels`. This is the /// transport-agnostic primary constructor: the caller (or a /// transport-specific dial helper) produces the `Connection` — /// via `Connection::from_stream`/`from_bidi` (TCP+TLS, /// WebTransport, SSH `direct-tcpip`), a quinn connection, or any /// other `AsyncRead + AsyncWrite` source — and this method takes /// over: installs channel 0 (`alknet/call`), spawns the demux/mux, /// and returns the client. Mirrors the server side's /// transport-agnostic `ChannelsAdapter::handle(Connection)` and /// `CallClient::spawn_dispatch(Connection)`. /// /// This is the one-way-door API surface (ADR-080). It must not be /// coupled to a transport — the channels protocol is /// transport-agnostic (ADR-071, ADR-065), and the client side is /// half of that protocol. pub async fn from_connection(connection: Connection) -> Result; /// QUIC convenience constructor. Dials a QUIC connection to `addr` /// on ALPN `alknet/channels` (using `credentials` for the TLS /// handshake — ADR-034 verifier selection), then calls /// `from_connection`. This is the "I just want QUIC" one-liner; /// it is additive over `from_connection` and is a two-way door — /// `connect_tcp_tls`, `connect_webtransport`, etc. can be added /// alongside it without touching the one-way-door surface. pub async fn connect_quic( addr: SocketAddr, credentials: CallCredentials, ) -> Result; /// Open a data channel with the given ALPN and params. Sends /// `channel/open` on channel 0, waits for the response, and returns /// the channel. pub async fn open_channel( &self, alpn: &str, stream_types: &[u8], params: Value, direction: ChannelDirection, ) -> Result; /// Subscribe to the peer's resource updates. Returns a stream of /// resource-set events (ADR-073 channel/resources/subscribe). Each /// event carries the JSON `output.resources` array from ADR-073's /// `channel/resources/subscribe` response shape. pub async fn subscribe_resources(&self) -> Result, ChannelError>; /// The call-protocol connection on channel 0, for invoking channel /// lifecycle operations and any other call ops the peer exposes. pub fn call(&self) -> &CallConnection; } pub enum ChannelDirection { InitiatorToResponder, ResponderToInitiator, } pub struct Channel { pub channel_id: u32, pub stream_types: Vec, /// The sub-streams, accessible via accept_bi() (ADR-074 generic path) /// or into_sub_streams() (ADR-074 typed path). pub source: ChannelBidiStreamSource, } /// One event from `channel/resources/subscribe`. Wraps the JSON `output` /// object from ADR-073's subscribe response — the `resources` array /// describing what ALPNs the peer exposes and with what `access` preview. /// The channels crate maps the JSON to this typed struct; the fields mirror /// ADR-073's response shape. pub struct ResourceEvent { pub resources: Vec, } pub struct ResourceEntry { pub alpn: String, pub backends_or_targets: Vec, // ALPN-specific enumeration pub access: Value, // preview of AccessControl (advisory) } ``` ## Transport-agnostic by construction `ChannelClient` is the client side of the channels protocol. The channels protocol is transport-agnostic (ADR-071 substrate modes; `Connection::from_stream`/`from_bidi`/`from_source` from ADR-065/070 take any `AsyncRead + AsyncWrite`). The client side must not be welded to a transport — that would repeat the server-side welding ADR-065 explicitly unwound. `from_connection(connection: Connection)` is the primary constructor and the one-way-door API surface. It takes a pre-established `Connection` and takes over channels establishment. The transport is the caller's concern: `Connection::from_bidi(tls_stream, ...)` for TCP+TLS, a quinn `Connection`, a WebTransport `BiStream`, an SSH `direct-tcpip` channel wrapped via `from_stream`, a WebSocket carrying `alknet/channels` (the browser path per ADR-044) — all produce a `Connection` that `from_connection` accepts unchanged. This mirrors the server side's `ChannelsAdapter::handle(Connection)`, which is substrate-agnostic by the same mechanism. `connect_quic(addr, credentials)` is a **convenience** constructor — dial QUIC, then `from_connection`. It is additive and two-way-door: transport-specific dial helpers (`connect_tcp_tls`, `connect_webtransport`, …) join it as transports are added, none of which touch the `from_connection` contract. The dial helper set is open-ended by design. The credential/verifier-selection rule (ADR-034) lives in the transport's own dial path, not in `from_connection` — `from_connection` receives an already-established, already-authenticated `Connection`, exactly as `ChannelsAdapter::handle` does on the server side. The ~20 lines of verifier-selection boilerplate each dial helper rebuilds is the known duplicated cost of not having `AlknetClient` (OQ-55) extracted yet; `from_connection` keeps that boilerplate on the *transport-specific dial* side, not on the channels protocol's one-way-door surface. ## Bidirectionality preserved The channels protocol is bidirectional — either side can open a channel (ADR-073 §direction semantics). `ChannelClient::open_channel` supports both `ChannelDirection::InitiatorToResponder` and `ChannelDirection::ResponderToInitiator`. The client is not "the client side" in the request/response sense — it can also receive `channel/open` requests from the peer (the peer initiates, the client's `ChannelManager` responds). This mirrors the call protocol's operation overlay (each side populates what operations they expose). `ChannelClient` is one endpoint of a bidirectional channels connection. The name follows the `CallClient` convention (the side that dialed), not a request/response role. ## Relationship to `AlknetClient` (OQ-55 — deferred) `ChannelClient`'s *API* is transport-agnostic — `from_connection` takes a pre-established `Connection`. What is deferred (OQ-55) is the shared *dial+TLS* seam (`AlknetClient`): the transport-specific work each dial helper does — open a socket, run the TLS handshake, apply ADR-034's verifier-selection rule, produce a `Connection`. That dial is genuinely transport-specific (QUIC, TCP+TLS, WebTransport, raw TCP, SSH), and we have one shape implemented (QUIC, in `connect_quic`). Extracting a QUIC-shaped connector now and naming it `AlknetClient` would bake QUIC in as *the* establishment shape — the same welding ADR-065 unwound on the server side. This is why `from_connection` is the one-way-door surface and `connect_quic` is a two-way-door convenience over it. `AlknetClient` (when extracted, after a second transport's dial exists) becomes the shared *dial*; `from_connection` stays the shared *channels-take-over*. The two concerns are separated now, before the one-way-door API is cast. The friction while `AlknetClient` is deferred is duplicated verifier-selection boilerplate across dial helpers (~20 lines each) — not duplicated capability and not a QUIC-welded client API. ## Design Decisions All design decisions are documented as ADRs in [decisions/](../../decisions/). | ADR | Decision | Summary | |-----|----------|---------| | [080](../../decisions/080-channelclient.md) | ChannelClient | Client side; transport-agnostic `from_connection` primary, `connect_quic` convenience; `AlknetClient` dial-seam deferred (OQ-55) | ## Open Questions - **OQ-55** (deferred(scope)): `AlknetClient` core **dial+TLS seam** extraction — blocked on a second *transport's* dial. `ChannelClient`'s API is transport-agnostic (`from_connection`); the deferred part is the shared *dial* across transports, not the channels protocol. ## References - ADR-080: ChannelClient (the decision) - ADR-073: channel lifecycle operations (`open_channel` sends `channel/open`) - ADR-074: ChannelBidiStreamSource (what `Channel.source` wraps) - ADR-075: ChannelManager (the shared state `ChannelClient` holds) - OQ-55: AlknetClient / client establishment extraction - `docs/architecture/crates/call/client-and-adapters.md` — `CallClient` (the shape `ChannelClient` mirrors)