Cases2 cases

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1. IL-0299 About 60 indices of Wikimedia's CirrusSearch OpenSearch clusters went red and unwritable during a host decommission

   Wikimedia Foundation2025

   Symptoms

   From 21:08 on 28 May 2025, about 60 indices of Wikimedia's OpenSearch clusters for CirrusSearch were in red, unwritable state. At 21:27 CirrusStreamingUpdaterFlinkJobUnstable alerts began to fire because the search update pipeline could not write to those indices.

   Writes rejected

   Trigger

   A Puppet patch that removed cluster access for the hosts elastic/cirrussearch10[53-67] while some of them still held primary shards.

   Root cause

   Configuration change

   Component

   Elasticsearch or OpenSearch

   Contributing factor

   Unverified assumptionNo runbookUntested backup

   Detection

   CirrusStreamingUpdaterFlinkJobUnstable alerts fired from 21:27; the review records the detection as human, by the engineer who started troubleshooting the next morning, and many other alerts were suppressed for maintenance.

   Remediation

   • Deleted the red indices, which brought the cluster back to green
   • Restored the broken indices

   Action items

   • Re-enable the snapshot repositories in production CirrusSearch
   • Create an operations-focused OpenSearch dashboard
   • Verify or fix the Logstash pipeline for Search Platform-owned OpenSearch clusters

   Lessons

   Banning nodes before removing them is a best-effort action and did not work as expected. A large operation such as decommissioning many servers at once had no maintenance plan or runbook, and the snapshots, dashboards and logging that recovery needed were not available or not documented well enough.

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2. IL-0215 Twilio's billing Redis master lost all balances and went read-only, and auto-recharge charged customer cards repeatedly

   Twilio2013

   Symptoms

   At 2:39 AM PDT (9:39 UTC) on 18 July 2013, services that relied on the redis-master of Twilio's billing system began to fail because the load on its host had become extreme. At 2:42 AM PDT monitoring alerted the on-call engineers to a failure in the Redis cluster.

   Database overloadData lossWrites rejected

   Trigger

   At 1:35 AM PDT on 18 July a loss of network connectivity disconnected all billing redis-slaves from the redis-master at once, and all of them requested a full synchronization at the same time. On-call engineers then restarted the overloaded redis-master.

   Root cause

   Configuration change

   Component

   In-memory cacheDatabase replication

   Contributing factor

   Retry stormHarmful manual actionUnsafe failure behaviour

   Detection

   At 2:42 AM PDT monitoring alerted the on-call engineers to a failure in the Redis cluster. At 3:28 AM PDT the billing system monitoring reported the anomalous charges, and at 12:36 PM PDT monitoring detected the recurrence.

   Remediation

   • Restarted the redis-master to relieve the high load, which loaded a wrong configuration
   • Took the billing system offline to stop further credit card charges
   • Restored service to all suspended accounts
   • Restored the lost account balances from the independent double-bookkeeping record in the relational datastore
   • Took billing offline again after the recurrence and reactivated the newly suspended accounts
   • Refunded all erroneous credit card charges
   • Gave all affected accounts a credit of 10% of their last 30 days of spend
   • Replaced the original Redis cluster and corrected the redis-master configuration
   • Activated the billing system progressively for groups of customer accounts

   Action items

   • Disable Redis restarts on the redis-master and recover the master by pivoting a slave instead
   • Add fail-safes so the billing system neither suspends accounts nor charges cards when balances are missing or cannot be written
   • Validate the billing system against the double-bookkeeping databases in real time

   Lessons

   Losing the in-flight balances and the ability to write balances at the same time exposed a flaw in the auto-recharge system: it failed in a dangerous way, charging cards and suspending accounts instead of stopping.

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