This week, Heroku made Router 2.0 generally available, bringing features like HTTP/2, performance improvements and reliability enhancements out of the beta program!

Throughout the Router 2.0 beta, our engineering team has addressed several bugs, all fairly straight-forward with one exception involving Puma-based applications. A small subset of Puma applications would experience increased response times upon enabling the Router 2.0 flag, reflected in customers’ Heroku dashboards and router logs. After thorough router investigation and peeling back Puma’s server code, we realized what we had stumbled upon was not actually a Router 2.0 performance issue. The root cause was a bug in Puma!...

For quite some time we've received reports from our larger customers about a mysterious H13 - Connection closed error showing up for Ruby applications. Curiously it only ever happened around the time they were deploying or scaling their dynos. Even more peculiar, it only happened to relatively high scale applications. We couldn't reproduce the behavior on an example app. This is a story about distributed coordination, the TCP API, and how we debugged and fixed a bug in Puma that only shows up at scale.

Connection closed

First of all, what even is an H13 error? From our error page documentation:

This error is thrown when a process in your web dyno accepts a connection, but then...