^{Golang stdlib-only}

FlowState is a dynamic L7 Reverse Proxy built in Go using only the stdlib.

• Build a stdlib-only Layer 7 reverse proxy to understand HTTP internals, connection reuse, and request lifecycle.
• Implement explicit routing, backend selection, and middleware composition without framework abstractions.
• Make timeouts, cancellation, health state, and observability first-class so behavior under failure is understandable.
• Favor clear control flow and measurable correctness over feature count.

• Not a production replacement for Envoy, NGINX, or Traefik.
• No dynamic control plane, distributed config, or service discovery at first.
• No HTTP/3, gRPC-native balancing, or WASM/plugin ecosystem.
• No external metrics/tracing libraries.

• Ingress path: net/http server accepts request.
• Router: matches host/path/method to a route entry.
• Middleware chain: request ID, logging, rate limiting, recovery, optional auth later.
• Load balancer: selects a healthy backend for the route.
• Proxy layer: rewrites request, injects forwarding headers, strips hop-by-hop headers, forwards via custom http.RoundTripper.
• Health subsystem: active probes + passive failure observation update backend state.
• Metrics/admin endpoints: expose counters, latency buckets, backend health.

Internal flow: server -> router -> middleware -> lb -> proxy transport -> response filters -> metrics

• If a backend times out, connection fails, or returns repeated 5xx responses, it may be marked unhealthy and removed from rotation.
• If all backends are unavailable, return 502 or 503 consistently.
• Client cancellation propagates to backend requests through context.Context.
• On shutdown, stop accepting new requests and allow in-flight requests to drain for a bounded time.
• No guarantee of zero request loss during process crash or forced termination.

Measure with testing.B, httptest, and simple load tools.

• Requests/sec through proxy with 1, 10, 100 concurrent clients.
• Latency: p50/p95/p99 under healthy backends.
• Tail latency under one slow backend.
• Cost of middleware stack.
• Effect of connection reuse vs disabled keep-alive.
• Load-balancer distribution fairness across backends.

• Unit tests for router matching, backend selection, health transitions, middleware behavior.
• httptest.Server integration tests for end-to-end proxying.
• Timeout/cancellation tests with slow handlers and canceled contexts.
• Race-detector runs for backend state, counters, and health updates.
• Fuzz tests for route parsing, config parsing, and header rewriting.
• Deterministic failure injection for connection errors and backend flapping.

• Static config loader
  • Use: encoding/json or encoding/gob/custom parser, os, flag
  • Learn: config modeling, validation, immutable runtime config
• HTTP server bootstrap
  • Use: net/http, context, os/signal
  • Learn: server lifecycle, graceful shutdown, deadline propagation
• Basic route matching
  • Start simple: exact path + prefix match, host/method aware
  • Use: slices/maps, longest-prefix match
  • Learn: request dispatch, matching semantics
• Custom reverse proxy path
  • Use: net/http, net/url, io, custom handler instead of magic wrappers
  • Learn: request cloning, upstream URL rewriting, response copying
• Custom transport
  • Use: http.Transport
  • Learn: keep-alives, MaxIdleConns, MaxIdleConnsPerHost, dial timeout, TLS handshake timeout, response header timeout
• Forwarding and hop-by-hop header handling
  • Learn: X-Forwarded-For, X-Forwarded-Proto, RFC hop-by-hop header stripping
• Weighted round robin
  • Learn: stateful load balancing, atomic counters vs mutexes, fairness

• Request-scoped context and timeouts
  • Use: context.WithTimeout
  • Learn: cancellation propagation, bounded resource usage
• Structured logging
  • Use: log/slog
  • Learn: request lifecycle logging, correlation IDs
• Request ID middleware
  • Use: headers + context values
  • Learn: cross-cutting concerns, observability basics
• Recovery middleware
  • Use: defer, recover
  • Learn: panic boundaries in servers
• Active health checks
  • Use: time.Ticker, net/http or net.DialTimeout
  • Learn: health probe design, state transitions
• Passive health checks
  • Learn: consecutive failure counting, decay/reset, backend ejection
• Consistent unhealthy-backend policy
  • Learn: fail-open vs fail-closed, cooldowns
• Graceful shutdown and backend draining
  • Use: http.Server.Shutdown
  • Learn: lifecycle management under load
• Metrics endpoint
  • Use: expvar or custom text format via net/http
  • Learn: counters, gauges, latency buckets

• Trie router only if route count justifies it
  • Learn: radix tree / trie, path parameter trade-offs
• Power of Two Choices
  • Learn: queue-aware balancing, herd effect mitigation
• Token bucket rate limiter
  • Use: time, sync, monotonic refill math
  • Learn: admission control, backpressure
• Connection pool tuning benchmarks
  • Learn: transport tuning under concurrency
• Latency histograms / approximate percentiles
  • Learn: buckets vs exact percentiles, tail latency measurement
• Zero-allocation hot-path cleanup where useful
  • Learn: buffer reuse, avoiding needless header/value copies

• Circuit breaker
  • Learn: closed/open/half-open FSM, rolling failure windows
• Hot config reload
  • Use: atomic config swap, signal-driven reload
  • Learn: read-copy-update style config management
• Per-route middleware policies
  • Learn: policy composition and precedence
• Retry policy for idempotent requests only
  • Learn: retry safety, duplicate side effects
• Outlier detection
  • Learn: passive latency/error-based backend suppression