PFAC Payments: Implementation of Pre-Flight Admission Control for Payment Processing Systems

This document specifies the implementation of PFAC (Pre-Flight Admission Control) for payment processing systems. Building upon the foundational PFAC whitepaper, this technical note details the application of predictive, cost-aware admission control to payment workflows. The implementation covers per-request resource estimation before handler execution, atomic capacity reservation via Redis/Lua, and health-scaled adaptive budgeting tailored for high-throughput payment services. This repository includes the PDF specification, and the implementation source code in Rust with Axum/Tower, demonstrating PFAC applied to payment transaction admission, rate limiting, and overload protection in financial service contexts.