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Local Domains Native Daemon (C++) Spec

This document defines the native local-domains path for Flow.

Goal:

  • keep stable *.localhost names,
  • remove docker/nginx runtime dependency,
  • provide low-overhead local routing suitable for agent-heavy workflows.

Scope

This is an incremental migration path.

Phase 1 (implemented now):

  • opt-in engine: f domains --engine native ...
  • experimental C++ daemon (domainsd-cpp) built by Flow with clang++
  • host-based HTTP/1.1 routing from ~/.config/flow/local-domains/routes.json
  • WebSocket upgrade passthrough
  • request-side chunked transfer-encoding decode
  • upstream keepalive connection pooling for safe HTTP/1.1 reuse
  • bounded active handler slots with overload shedding (503)
  • upstream connect/read/write timeouts (504 for upstream connect timeout)
  • health endpoint: /_flow/domains/health
  • macOS launchd socket-activation installer for native privileged :80 bind without Docker

Phase 2:

  • better connection pooling and backpressure controls
  • optional HTTP/2/TLS frontend mode

Phase 3:

  • optional HTTPS + HTTP/2
  • structured trace export for agent context

Non-goals (for current phase)

  • system DNS changes
  • packet filter / firewall manipulation
  • replacing .localhost conventions

Control plane

Flow CLI remains the control plane:

f domains list
f domains add app.localhost 127.0.0.1:3000
f domains rm app.localhost
f domains --engine native up
f domains --engine native down
f domains --engine native doctor

Engine selection:

  • CLI flag: --engine docker|native
  • env fallback: FLOW_DOMAINS_ENGINE=native
  • default: docker

State

Current state remains:

  • routes: ~/.config/flow/local-domains/routes.json

Native runtime artifacts:

  • pid: ~/.config/flow/local-domains/domainsd.pid
  • log: ~/.config/flow/local-domains/domainsd.log
  • built daemon binary: ~/.config/flow/local-domains/domainsd-cpp
  • macOS launchd plist (optional): /Library/LaunchDaemons/dev.flow.domainsd.plist

Native tuning env vars (read by f domains --engine native up and passed to daemon):

  • FLOW_DOMAINS_NATIVE_MAX_ACTIVE_CLIENTS (default 128)
  • FLOW_DOMAINS_NATIVE_UPSTREAM_CONNECT_TIMEOUT_MS (default 10000)
  • FLOW_DOMAINS_NATIVE_UPSTREAM_IO_TIMEOUT_MS (default 15000)
  • FLOW_DOMAINS_NATIVE_CLIENT_IO_TIMEOUT_MS (default 30000)
  • FLOW_DOMAINS_NATIVE_POOL_MAX_IDLE_PER_KEY (default 8)
  • FLOW_DOMAINS_NATIVE_POOL_MAX_IDLE_TOTAL (default 256)
  • FLOW_DOMAINS_NATIVE_POOL_IDLE_TIMEOUT_MS (default 15000)
  • FLOW_DOMAINS_NATIVE_POOL_MAX_AGE_MS (default 120000)

Native daemon protocol (current)

Listen address:

  • 127.0.0.1:80

macOS launchd mode:

  • daemon can be started with --launchd-socket domainsd (socket inherited from launchd)
  • launchd owns privileged bind; daemon runs as local user

Routing:

  • Read Host header (strip :port)
  • Lookup host -> target from routes.json
  • Forward request to target (host:port)

Special endpoint:

  • GET /_flow/domains/health
  • returns header X-Flow-Domainsd: 1

Errors:

  • 404 when host route is not configured
  • 502 on upstream connection/forward failures
  • 503 when proxy is saturated and sheds overload
  • 504 on upstream connect timeout
  • 400 for malformed HTTP requests

Reliability guardrails

  • Keep Docker engine as default during hardening.
  • Native engine is explicit opt-in.
  • f domains doctor should always show effective owner on port 80.
  • Any startup failure must surface log path directly.

Performance targets

  • added local proxy latency p99 < 1ms for tiny responses
  • idle CPU near zero
  • route update visibility < 100ms (mtime-based reload)

Validation checklist

f domains add myflow.localhost 127.0.0.1:3000
sudo ./tools/domainsd-cpp/install-macos-launchd.sh   # macOS when direct bind is denied
f domains --engine native up
f domains --engine native doctor
curl -H 'Host: myflow.localhost' http://127.0.0.1/
sudo ./tools/domainsd-cpp/uninstall-macos-launchd.sh # optional teardown

Next implementation work

  1. Add per-upstream timeouts (connect, first-byte, total) with explicit 502/504 mapping.
  2. Add end-to-end trace summary output for agents (route misses, upstream failures, latency buckets).
  3. Add launchd install/uninstall tasks for persistent local startup.
  4. Add optional HTTPS + HTTP/2 frontend mode.