Router 2.0 Common Runtime Router: The Road to Beta

Summary

Heroku’s Router 2.0 is a beta replacement for the Common Runtime’s Erlang-based “Hermes” router, which is planned to be replaced in 2024. The project addresses the difficulty of safely delivering upgrades from a legacy custom HTTP server and aims to share routing code with the Go-based Private Spaces router; common functionality has been extracted into an internal HTTP library. Architecturally, Router 2.0 is an L7 router running in memory behind the existing L4 proxy, alongside Hermes, allowing app-level traffic to move from the old path without a big-bang cutover. The team added continuous load testing that exercises scaling policies and blocks deployments missing internal service-level objectives, and introduced Network Error Logging to measure browser-observed failures. Feature parity is still incomplete, with HTTP/2 planned next; apps can opt into the beta using a Heroku Labs command.

Takeaways

1. 1

   Router 2.0 is written in Go to match team expertise and share routing code with the Go-based Private Spaces router through an internal HTTP library.

2. 2

   The L4 proxy sends default-routing traffic through Hermes, while apps enabled for Router 2.0 use an in-memory listener to reach the new router, supporting a gradual strangler-pattern migration.

3. 3

   Continuous load tests push staging routers to their limits, exercise scaling policies, and feed results into the deployment pipeline, which blocks releases that miss internal service-level objectives.