Cases2 cases

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1. IL-0432 AutoReserve notifications silently went unsent as Memorystore for Redis 3.2 rejected writes with OOM below its limit

   Hello Inc.2026

   Symptoms

   A bug report said that some reservation notifications in the AutoReserve restaurant reservation service never arrived. Of the several kinds of notification sent as a reservation progresses, only one kind sometimes arrived and sometimes did not.

   Writes rejectedMemory pressure

   Root cause

   Capacity exhaustion

   Component

   In-memory cache

   Contributing factor

   Monitoring blind spotUnsafe failure behaviourUnverified assumption

   Detection

   A bug report from AutoReserve said that some reservation notifications were not arriving.

   Remediation

   • Scaled the Memorystore for Redis instance from 8 GiB to 16 GiB, which did not stop the write rejections
   • Upgraded Redis from 3.2 to 7.2, which removed the fragmentation and stopped the write rejections
   • Moved the notification sent status from Redis to the relational database

   Action items

   • Consider enabling active defrag if physical memory use rises above 1.3 times logical memory use

   Lessons

   The logs and the instance page metrics did not show the cause; it took looking inside Redis with INFO memory to see physical memory far above logical memory. A cache is a place for values that can be rebuilt if lost and whose writes may fail, so the sent status, which had to persist and be decided once under concurrent processing, belonged in the relational database.

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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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