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1. IL-0310 An Aurora PostgreSQL failover to an upsized reader reversed twice and backend writes failed in read-only transactions

   Hightouch2025

   Symptoms

   At 16:39 EDT on 23 October 2025, Hightouch triggered a failover of its Aurora PostgreSQL cluster in us-east-1 to a newly upsized reader. The AWS Console showed the usual parameter adjustments and instance restarts, but after the page refreshed the original writer was still the primary, and backend services could not run write queries although AWS showed the cluster as healthy.

   Failover failureWrites rejected

   Trigger

   A planned upsize of the Aurora cluster's instances, done without a maintenance window, after the AWS us-east-1 outage of 20 October had left an event backlog that strained the small Postgres instance. The manual failover to the upsized reader at 16:39 EDT started the failure.

   Root cause

   Software defect

   Component

   DatabaseDatabase failover manager

   Contributing factor

   Unrepresentative staging

   Detection

   The AWS Console showed after a refresh that the original writer was still primary, and backend application logs showed write errors on a read-only transaction. Per-instance query monitoring in Datadog and the RDS database logs showed the brief writer advertisement.

   Remediation

   • Restarted the backend services, which cleared the write errors after each failed failover
   • Scaled down every service that writes to the database
   • Triggered the failover again with no concurrent writes, which completed, and upsized the cluster in us-east-1

   Action items

   • Pause writers before any intentional failover, as written into the internal playbooks
   • Monitor for unexpected flips of the instance that advertises the writer role

   Lessons

   Hightouch's takeaways are that a migration can end in the desired state, the starting state or somewhere in between even with services one trusts, so traffic redirection and brief dependency outages should be planned for. The brief writer advertisement could only be found because queries to each instance were monitored in Datadog and the RDS database logs were available. Limiting how much any single component can affect the system helps both uptime and maintenance, and a staging load test, even of the exact procedure, did not reproduce the conditions that triggered the Aurora race condition.

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