January 2, 2026
Eventually™ is now provable
Proving Liveness with TLA
Math finally promises your messages won’t ghost you
TLDR: TLA’s Toolbox now proves liveness—showing sent data will eventually arrive—and the author demos it on Xen’s vchan. Commenters celebrated the “eventually true” vibe and laughed at an ironic NullPointerException note, seeing formal proofs inch closer to everyday reliability.
Formal verification just pulled a power move: the TLA Toolbox can now prove “liveness” — in plain English, if you send something, it won’t be stuck forever. The author test-drives it on Xen’s vchan (a messaging pipe between virtual machines) and shows how careful steps and rules guarantee that sent bytes eventually show up. Think math as a delivery guarantee, not just wishful thinking. The crowd’s vibe? A mix of swoon and snark. User MobiusHorizons loved the romance of “x will eventually be y,” wishing for a programming type that starts as null but can never fall back — commitment, but for variables. Then came the punchline: a help note about the toolbox failing to launch due to a NullPointerException. Yes, the proof tool tripped over the very error it’s meant to banish. Cue the memes about “eventually” meaning “after you restart the app.” Jokes aside, fans see this as a legit milestone: TLAPS (the TLA Proof System) finally speaks enough temporal logic to aim at availability, not just integrity. For anyone new here, TLA+ is a method for specifying systems so you can check they do what they claim, and it’s getting sharper by the minute. Curious? Peek at TLA+ and Xen vchan for context.
Key Points
- •TLA Toolbox now supports proving liveness properties in TLA+.
- •The author revisits the Xen vchan protocol, previously specified in TLA+ and tested with TLC, with an integrity proof completed in 2018.
- •Temporal logic support in newer TLAPS versions enables formal liveness (Availability) proofs previously not possible.
- •A simple one-way channel model is defined with Sent and Got counters, buffer usage, and a liveness property using the leads-to (~>) operator.
- •The system specification uses Init, Send and Recv actions, Next, and a weak fairness condition WF_vars(Recv), with invariants ensuring type correctness, non-overflow, and Sent ≥ Got.