alknet/docs/architecture/questions/045-flow-control-for-high-throughput-stdout.md

OQ-45: Flow Control for High-Throughput stdout

• Origin: docs/research/alknet-tty/phase-0-findings.md OQ-TTY-03; crates/tty/tty-wire.md (no windowing in the chunk format).

• Status: resolved

• Door type: Two-way

• Priority: low

• Resolution: No application-level windowing. QUIC's per-stream flow control is the backpressure mechanism — the chunk format carries no window. This is decided, not a default assumption.

  The backpressure chain is complete by construction — every link awaits its producer, so slowness at the client's read end propagates all the way back to the child process's stdout write without any unbounded buffering link:

  1. Client reads slowly → QUIC flow control. The server's write_chunk writes into a quinn SendStream, which respects per-stream flow control. When the client's receive window fills, the send awaits. (Standard, mature quinn behavior — not something alknet-tty inventes.)
  2. Send awaits → drainer channel backpressures. The drainer task writes chunks to the client one at a time. When write_chunk awaits, the drainer stops pulling from the bounded writer_rx channel (depth 64 in the POC's session.rs).
  3. Drainer channel fills → stdout pump backpressures. The stdout→client pump does writer_tx.send(chunk).await; when the drainer isn't pulling, the send awaits.
  4. Stdout pump awaits → backend stdout channel backpressures. The pump stops receiving from the backend's stdout channel (the local backend's mpsc::Receiver<Bytes>, also bounded at 64).
  5. Stdout channel fills → reader thread backpressures. The backend's reader thread (portable_pty master reader, or a piped Child::stdout) does blocking_send; when the channel is full, the std thread blocks and stops reading from the kernel.
  6. Reader thread stops → OS pipe/PTY buffer fills → process blocks. The kernel PTY buffer (or Stdio::piped() buffer, typically 64 KiB) fills, and the child process's write() to stdout blocks. The process is throttled.

  Every link in the chain awaits its producer. The chain is the standard composition of QUIC flow control + bounded tokio channels + OS pipe buffers — the same pattern the docker POC's logs subscription and the tty POC's PTY pump already use. A high-throughput stdout workload (e.g., cargo build output) throttles at the process when the client can't keep up; no unbounded buffer breaks the chain.

  The two-way-door reversal — a per-stream window-update control message on TTY's STREAM_CTRL_IN (stream_type 3) — is an additive extension to the control channel (a new ControlMessage variant), not a wire-format header change. It is not the expected path; it is noted in ADR-052's consequences as the cheap reversal if a flow-control problem ever surfaces that QUIC's defaults cannot handle (e.g., a pathological stream that needs sub-QUIC-window backpressure signaling). Tuning concerns (read buffer size, channel depth) are implementation-level, not architectural, and don't warrant an ADR.

• Cross-references: ADR-052, tty-wire.md, tty-adapter.md, /workspace/alknet-tty-poc/src/session.rs (the three-pump driver — the bounded channels at every link), /workspace/alknet-tty-poc/src/local_pty.rs (the blocking→async bridge — the reader thread that backpressures into the kernel).