Cases2 cases

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1. IL-0352 Heroku Router 2.0 spread HTTP requests unevenly across dynos for four days, causing latency and timeouts

   Heroku2025

   Symptoms

   From about 22:00 UTC on 18 July 2025, some apps on Heroku's Common Runtime that used Router 2.0 had their HTTP requests balanced unevenly across their web dynos. The dynos that got more requests were overloaded, and the apps saw higher HTTP request latency, timeouts, reduced availability and various Heroku error codes in application logs and dashboards.

   Load imbalanceHigh latencyIntermittent timeouts

   Trigger

   From 22:00 UTC on 18 July 2025 router instances began hitting a code condition that broke the entropy metadata Router 2.0 uses to choose dynos. The condition had been in the code since the Router 2.0 beta and became more likely as more HTTP requests went through the new router, with about 94% of Common Runtime apps migrated.

   Root cause

   Software defect

   Component

   Cloud load balancer

   Contributing factor

   Monitoring blind spotUnrepresentative staging

   Detection

   No alert fired. Heroku Support received the first customer reports of uneven request balancing at 16:46 UTC on 20 July 2025, and an internal investigation was declared on 22 July.

   Remediation

   • Advised affected customers to disable Router 2.0 and fall back to the legacy router
   • Cycled the production router fleets, which improved load balancing for affected apps
   • Deployed the router fix to staging, then rolled it out to all Common Runtime partitions by 00:12 UTC on 23 July

   Action items

   • Introduced gating and notifications so that load balancing behaviour regressions do not occur
   • Communicate the timeline and send reminders to apps still on the legacy router before automated migrations resume

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2. IL-0351 Heroku apps in the EU and US had slow and failed requests as DNS round-robin sent traffic to single router hosts

   Heroku2017

   Symptoms

   Between 3 and 7 June 2017, Heroku customers in both the EU and US regions had periods of high latency: some requests failed and others took an excessive time to pass through the Heroku Routing Layer. All customers in the EU and US were affected at various times.

   High latencyHigh error rateLoad imbalance

   Root cause

   DNS or network fault

   Component

   Authoritative DNS servers and recordsCloud load balancer

   Contributing factor

   Unverified assumption

   Remediation

   • Found the sources of high-volume traffic and worked with those apps' owners to distribute their requests better
   • Replaced the legacy DNS records with new, corrected DNS records

   Action items

   • Make monitoring alert engineers sooner to unevenly distributed traffic
   • Investigate how to configure DNS and load balancers to reduce the effect of high-volume and bursting traffic
   • Work with caching Add-on partners on the DNS configuration issues that affect them, to achieve proper load balancing

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