An engineering notebook. Whatever's on the workbench.

A deterministic Java comparison of five admission strategies under identical 2x overload. Reject-fast policies (token bucket, rate limiter) hold p99 at 10ms; a bounded queue accepts fewer requests at a 500ms p99, because under sustained overload a buffer is the worst of both.

A deterministic Java measurement harness where a closed-loop benchmark reports a whole-run p99 of 10ms for a service that froze for 500ms, while open-loop scheduling and HdrHistogram's coordinated-omission correction both report 460ms from the same samples.

A deterministic Java hedging simulation where a p95 threshold cuts p99 from 200ms to 43ms for 3.7% extra load, while a p99 threshold improves nothing and burns the most wasted work. Hedging is a latency technique you buy with capacity.

Most of the migration was mechanical. ShutdownOnFailure became a Joiner, throwIfFailed disappeared, and StructuredTaskScope.open replaced the constructor. Two things were not mechanical, and those are the ones worth reading.

Before a fan-out service can be trusted under load, four things need to be true: outcomes counted per scope, deadlines propagated, bulkheads in place, and pinning watched in JFR. What each one looks like in code.

Structured concurrency patterns are worth the complexity only when the cancellation policy is decided before the first fork, not after the first timeout. The three we run: aggregation on quorum, bulkheading per tenant, and a deadline shape that protects against one slow upstream.

The Loom migration from Java 21 to Java 25 is one real change plus two cleanups: Joiner-based scopes replace ShutdownOnFailure, scoped values lose the preview flag, and virtual threads stay exactly where you left them.

Loom roadmap planning is mostly a timing problem: adopting preview features too early and waiting until they settle both ship nothing in the end.

Most thread-pool dashboards go quiet once a service moves to virtual threads, because a small carrier count hides millions of virtual threads and high CPU usually means pinning, not business load.