Ao Li

Deterministic Simulation Testing for Distributed Systems: The Good, the Bad, and the Ugly

Sun, 31 May 2026 00:00:00 +0000

Deterministic Simulation Testing for Distributed Systems: The Good, the Bad, and the Ugly

During my PhD, I built Fray, a deterministic simulation testing framework for concurrent programs written in Java. Fray was quite successful and has found many bugs in mature concurrent programs. As an academic, a natural question for me was: can we use a similar idea to test distributed systems? This led to the last chapter of my PhD thesis: Diorama, a deterministic simulation testing framework for distributed systems.

Can a Data Race Happen on a Single-Core Machine?

Thu, 18 Sep 2025 00:00:00 +0000

Can a Data Race Happen on a Single-Core Machine?

1. The Question

I woke up wondering: Can a data race happen on a single-core machine?.

A data race occurs when a program contains two conflicting accesses (write/write or read/write) that are not ordered by a happens-before relationship. This is the definition from Java Language Specification.

My first instinct was to ask Claude. The answer I received confidently showed a counter++ example and called it a “data race.”

Discovering a JDK Race Condition, and Debugging it in 30 Minutes with Fray

Sat, 07 Jun 2025 00:00:00 +0000

Discovering a JDK Race Condition, and Debugging it in 30 Minutes with Fray

I’ve been adding more integration tests for Fray recently. To ensure Fray can handle different scenarios, I wrote many creative test cases. Many of them passed as expected, while some failures led to epic fixes in Fray. Then something unexpected happened: Fray threw a deadlock exception while testing the following seemingly innocent code:

 1private void test() {
 2    ScheduledThreadPoolExecutor executor = new ScheduledThreadPoolExecutor(1);
 3    // Shutdown thread.
 4    new Thread(() -> {
 5        executor.shutdown();
 6    }).start();
 7    try {
 8        ScheduledFuture<?> future = executor.schedule(() -> {
 9            Thread.yield();
10        }, 10, TimeUnit.MILLISECONDS);
11        try {
12            future.get();
13            Thread.yield();
14        } catch (Throwable e) {}
15    } catch (RejectedExecutionException e) {}
16}

This code creates a ScheduledThreadPoolExecutor, schedules a task, and shuts down the executor in another thread. Initially, I suspected a bug in Fray, but after investigation, I discovered that the deadlock was actually caused by a bug in the JDK itself.

Evil Scheduler: Mastering Concurrency Through Interactive Debugging

Tue, 11 Mar 2025 00:00:00 +0000

Evil Scheduler: Mastering Concurrency Through Interactive Debugging

TLDR

Watch the video below to see how Fray debugger works! You may also want to check the Fray repo.

I enjoy the concept of Deadlock Empire, an interactive game that teaches the semantics of locks and other concurrency primitives. The core idea of this game is for you to act as an evil scheduler. Your goal is to guide the execution of threads into a bad state, such as a deadlock or two threads entering a critical section simultaneously. However, you cannot do this arbitrarily; you must adhere to the semantics of locks and other concurrency primitives. To win the game, you need a deep understanding of these concepts.

Chrome Returns 206 when the Server Returns 403

Mon, 03 Mar 2025 00:00:00 +0000

Chrome Returns 206 when the Server Returns 403

Postmortem

It is a feature not a bug! I got detailed explanation from chrome developers which is very helpful. You may find the complete response in the bug ticket. Also this blog post draws a lot of attention after I post it on Hackers News. You may find comments here.

Updates: I did know the people who replied my issue are not the developers as pointed out by a HN comment.