alknet/docs/architecture/crates/tty

status, last_updated

status	last_updated
draft	2026-07-07

alknet-tty

Terminal session protocol handler for the ALPN-as-service architecture: a ProtocolHandler on alknet/tty that pumps a bidirectional byte stream (stdin/stdout/stderr) with a JSON control channel (resize, signal, eof, exit) over a framed bidi stream, decoupled from the backend that allocates the PTY (docker, SSH, local process) via a TtyBackend trait.

Documents

Document	Status	Description
overview.md	draft	Crate purpose, the two-carriage model in brief, dependencies, ALPN, backend location map, feature gates
tty-wire.md	draft	The wire format: negotiation frame (JSON carriage), raw chunk codec ([stream_type: u8][length: u32 be][payload]), control channel (stream_type 3, JSON control messages), sentinels
tty-backend.md	draft	TtyBackend trait, TtyParams, TtyHandle, TtyControl — the inversion point between the wire-format adapter and the backends. Carries REQ-TTY-01 (backends need not be natively async)
tty-adapter.md	draft	TtyAdapter (ProtocolHandler on alknet/tty): session lifecycle, three-pump bidirectional driver, negotiation errors, exit-chunk ordering (ADR-055), access control
tty-local.md	draft	alknet-tty-local sibling crate: LocalTtyBackend via portable_pty (PTY) and std::process::Command (pipe/runner). Carries REQ-TTY-02 (signal forwarding to the process group)

Applicable ADRs

ADR	Title	Relevance
001	ALPN-Based Protocol Dispatch	TtyAdapter registers on alknet/tty
002	ProtocolHandler Trait	TtyAdapter implements ProtocolHandler
003	Crate Decomposition	alknet-tty depends on alknet-core; backends depend on alknet-tty (Amendment 1: alknet-call as protocol-foundation, framing utility reuse)
006	ALPN String Convention and Connection Model	alknet/tty is the custom ALPN; one ALPN per connection; new ALPN for incompatible versions
007	BiStream Type Definition	TtyAdapter receives a Connection, accepts bidi streams, pumps per-session
009	One-Way Door Decision Framework	Wire format is one-way; local backend placement is two-way (decided, not deferred)
012	Call Protocol Stream Model	The call protocol's stream model — which tty's raw carriage is not using for the body, by design
032	Forwarded-For Identity	forwarded_for for proxied terminal sessions (hub→worker)
040	WebTransport ALPN-Stream-Proxy	Parked per ADR-044; the alknet/tty ALPN is reachable over WebTransport's stream proxy when WebTransport revives
044	Defer h3/WebTransport; Browsers Use WebSocket	WebTransport deferred; the browser terminal case revives with WebTransport
049	Streaming Handler for Subscriptions	The StreamingHandler path tty explicitly does not use for the byte body — raw carriage, not call.responded events
050	Dynamic Resource Ownership	Terminal sessions are runtime-spawned resources; AccessControl shape declares against this model
052	alknet-tty Wire Format and Two-Carriage Model	The chunk codec, control channel, negotiation frame, fixed channel set
053	TtyBackend Trait and TtyHandle	The backend trait, handle shape, blocking-backend accommodation (REQ-TTY-01)
054	Local TTY Backend as a Sibling Crate	alknet-tty-local behind a local feature re-export; PTY vs pipe per-session
055	Exit Code on a Control Chunk	Exit code on stream_type 3; "exit chunk is last" invariant; adapter owns the ordering

Relevant Open Questions

OQ	Title	Status	Relevance
OQ-43	TtyControl trait object Clone constraint	resolved	control: Option<Box<dyn TtyControl + Send + Unpin + Clone>> via an Arc-backed Clone newtype; confirmed by the POC's concrete PtyControl
OQ-44	Terminal modes (TTY modes)	deferred(scope)	TerminalParams.modes reserved; default terminal modes suffice for current scope; blocked on a concrete mode-control use case
OQ-45	Flow control for high-throughput stdout	resolved	QUIC per-stream flow control is the backpressure mechanism (chain complete by construction); no application-level windowing. Reversal is an additive ControlMessage variant, not a wire-format change
OQ-46	Runner API surface	deferred(scope)	The runner mechanism (pipe mode) is in alknet-tty; runner policy (job management, log persistence, task graph) is a downstream crate, not in scope here
OQ-47	Stdin closure canonical signal	resolved	Either a zero-length stdin chunk or a {"type":"eof"} control chunk; both are accepted; the spec recommends eof for explicitness

Key Design Principles

1. A terminal session is a terminal concern, not an SSH or Docker concern. SSH and Docker are two backends that can allocate a PTY. alknet-tty owns the terminal session lifecycle; the backends (DockerTtyBackend, SshTtyBackend, LocalTtyBackend) implement a TtyBackend trait. This dissolves the PTY hedge in the alknet-ssh research (DP-5): PTY is not an SSH feature delegated to a separate crate, it's a tty feature that SSH happens to be able to provide. See overview.md and ADR-053.

2. Two-carriage model: JSON negotiation, then raw chunks. The bidi stream opens with a single length-prefixed JSON negotiation frame (terminal params, backend selector, command), then switches to a raw chunk format ([stream_type: u8][length: u32 be][payload]) for the life of the session. The call protocol's JSON-RPC shape handles the structured request; raw bytes handle the body, which is what a terminal actually is. No per-chunk EventEnvelope overhead, no base64. See tty-wire.md and ADR-052.

3. Fixed channel set, not extensible. Four stream types (0=stdin, 1=stdout, 2=stderr, 3=control), no negotiation. A 5th channel type is a wire-format change (one-way door); the ALPN model handles extensibility at the protocol level (a new ALPN is cheap, a wire-format change is not). The impoverishment vs SSH channels is the feature: alknet-tty multiplexes one service (a terminal session) with a fixed channel structure, not arbitrary services. See tty-wire.md.

4. The backend trait is the inversion point. alknet-tty defines TtyBackend; the backend crates implement it. alknet-tty depends on alknet-core; backends depend on alknet-tty for the trait; alknet-tty does not depend on any backend. This preserves ADR-003's no-handler-depends-on-another-handler rule (Amendment 1 for the alknet-call framing utility reuse). See tty-backend.md and ADR-053.

5. Backends need not be natively async (REQ-TTY-01). The trait's adapter-facing types (AsyncWrite, Stream<Item = Bytes>, BoxFuture, TtyControl) are the adapter's contract. A backend may expose blocking handles internally and bridge them via std threads + tokio mpsc/oneshot (the pattern portable_pty requires, and the local-PTY POC validated). The bridging pattern is a documented, supported implementation strategy. See tty-backend.md and tty-local.md.

6. Exit code on a control chunk, last before stream close (ADR-055). {"type":"exit","code":N} rides on the control channel (stream_type 3) and is the last chunk before the server closes the write half. This gives coordinators deterministic completion notification — no polling, no plugin state. The adapter owns the ordering; backends resolve exit_code and the adapter awaits, sends the chunk, closes. See tty-adapter.md and ADR-055.

7. The runner pattern is preserved, not specialized. The local backend in pipe mode (terminal: None) is a process-streaming endpoint — the same shape as GitHub/Gitea Actions runners, just over alknet's transport instead of HTTP polling. alknet-tty provides the mechanism (framed byte stream + exit code); runner policy (job management, log persistence, task graph) is a downstream crate's job. See tty-local.md and ADR-054.

References

• docs/research/alknet-tty/phase-0-findings.md — Phase 0 research (wire format, backend trait, REQ-TTY-01/02, decision points DP-1 through DP-6, open questions OQ-TTY-01 through OQ-TTY-05)
• /workspace/alknet-tty-poc/ — Phase 0 local-PTY validation POC (src/raw.rs chunk codec, src/control.rs JSON control schema, src/local_pty.rs blocking→async bridge, src/session.rs session pump, tests/integration.rs + tests/signal.rs round-trip tests)
• /workspace/alknet-docker-poc/src/raw.rs — the seed codec (stream_type 0/1/2) the tty POC extended with stream_type 3
• docs/research/alknet-docker/poc-summary.md — the POC that seeded this crate (two-carriage model, raw chunk format, validated targets)
• docs/research/alknet-ssh/phase-0-findings.md DP-5 — the PTY hedge this crate dissolves (PTY is a tty feature, not an SSH feature)
• /workspace/@alkdev/dispatch/ — the reverse-runner prior art (currently requires SSH; LocalTtyBackend removes that requirement)
• portable-pty 0.9 source — the blocking-API constraint that drives REQ-TTY-01 and the signal-delivery contract (REQ-TTY-02)