PFAC: Pre-Flight Admission Control System for High-Throughput Distributed Services

This document specifies PFAC (Pre-Flight Admission Control), a predictive, cost-aware admission layer for Rust web services. Rather than reacting to overload after resources have already been consumed, PFAC estimates per-request resource demand before handler execution, atomically reserves capacity in a shared store (Redis via Lua), and adapts an effective budget through a health-scaled safety factor. The specification formalizes the cost model, relates PFAC to classical and modern admission/overload work (RED, CoDel, SEDA, Google SRE adaptive throttling, Breakwater, Prequal), and provides implementation sketches for Tower/Axum, Redis/Lua, and batched MySQL writes. Twelve CI-reproducible k6 scenarios on GitHub Actions runners record end-to-end behavior against a Rust/Axum server with three admission modes (PFAC, plain token bucket, and disabled) and two controllers (three-band linear and PI). This repository contains the LaTeX source for the paper (pfac-spec.tex), the benchmark server with Redis-backed admission control, the device API implementation and examples, and the k6 load test scripts.