Wrangling monotonic systems in TLA⁺
Tractable modeling of replicated logs & CRDTs
Posted on November 1, 2023
| Andrew Helwer
TLA⁺ sees a lot of use modeling distributed systems. The ability to explore all possible interleavings of events makes concurrency simple to reason about. For this TLA⁺ uses something called finite model-checking, which is really just a breadth-first search through the entire state space. The key here - and this really must be emphasized - is that the model is finite. There can’t be an infinite number of states, or of course the model checker will run forever.
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FOSS I Love
Local game streaming with Sunshine and Moonlight
Posted on April 29, 2023
| Andrew Helwer
Here I’ll write an ode to two closely-linked FOSS projects that have, recently, absolutely floored me with their competence & quality: Sunshine and Moonlight. These are used to stream video games over a (usually local) network. Sunshine is a game streaming server: it runs on your PC as it chugs away doing all the heavy lifting of running the game itself, while Moonlight is a game streaming client that runs on whatever thin hardware exists where you want to play the game!
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Inlining SVGs for Dark Mode
Posted on April 6, 2023
| Andrew Helwer
I will here indulge in the traditional practice of using my blog to talk about how I’m using my blog. This page is built with the Hugo static site generator. I recently updated it to use the latest version of the beautifulhugo theme, which unbeknownst to me included a dark mode colorscheme. Recent browsers use the prefers-color-scheme option to automatically choose light or dark mode CSS styles, if the website supports it.
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Using TLA⁺ at Work
Designing a snapshot coordination system
Posted on April 5, 2023
| Andrew Helwer
Here’s a short report of a time I used TLA⁺ at work, with interesting results. TLA⁺ is a formal specification language that is particularly effective when applied to concurrent & distributed systems. TLA⁺ made it tractable for an ordinary software engineer to reason about a tricky distributed systems problem, and it found a bug introduced by an “optimization” I tried to add (classic). The bug required 12 sequential steps to occur and would not have been uncovered by ordinary testing.
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Pseudocode Showdown
Python vs. PlusCal & TLA⁺
Posted on March 30, 2023
| Andrew Helwer
Last weekend I had a conversation with an undergraduate student new to computer science, who was reading CLRS. “I wish” they said, “that all the pseudocode in my algorithms textbook was just written in Python.” “Ah” I said, “but textbook authors sometimes want their work to endure beyond a decade.” “But Python’s been around for a long time” came the reply, “and it’s very readable, and you can’t execute pseudocode anyway so what’s the harm?
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Google Groups has been left to die
Where should the formal methods community move?
Posted on March 8, 2023
| Andrew Helwer
An unusual topic for this blog, but worth boosting to a larger audience: Google Groups is dying. Its epitaph is not yet inscribed on the Killed by Google website, but the end is easily seen from here (although it should also be noted its death was called as early as 13 years ago). The deficiencies in Google Groups search, supposedly Google’s forte, have long been noted. Lately though basic features have just stopped working.
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Can sanitizers find the two bugs I wrote in C++?
Posted on February 7, 2023
| Andrew Helwer
A few days ago I published a short post about two bugs I wrote while developing the C++ external scanner for my TLA⁺ tree-sitter grammar. Reactions were mixed! Many people were supportive, but there were of course the usual drive-by claims by developers that the bugs were trivial, they would’ve found & fixed them inside of 20 minutes, and I was laughably incompetent for having written them in the first place.
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Two C++ bugs I wrote
Posted on February 4, 2023
| Andrew Helwer
Here’s a short post about two bugs I wrote while writing C++ code for the external scanner of my TLA⁺ tree-sitter grammar. External scanners use handwritten C or C++ code to parse the non-context-free parts of your language. I’ll try not to dump on C++ too hard but both of the bugs are highly ridiculous and exemplify why I hope to write as little of the language as possible for the rest of my career.
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Writing a TLA⁺ tree-sitter grammar
My foray into free software
Posted on January 11, 2023
| Andrew Helwer
2021 saw the completion of my first substantial free software project: a TLA⁺ grammar for tree-sitter, the error-tolerant incremental parser generator. The project stabilized & found users over the course of 2022, then over the holidays I used it to build the TLA⁺ Unicode Converter. The new year is a time to reflect on the past and look to the future, so here in early 2023 seems ideal to publish my experience.
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What's the difference between a computer and a rock?
Posted on October 19, 2022
| Andrew Helwer
Author’s note: I wrote this in 2017 and it languished in the drafts since then because I couldn’t think of a good conclusion. Really this is typical of the subject matter and all this piece offers is something to chew on, if you have the interest in it or have never before considered the question. At the time I was helping tutor a TLA⁺ seminar and had the opportunity to ask Leslie Lamport (who has thought about computation quite a bit!
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