Cases7 cases

6 to 7 of 7

1. 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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2. IL-0274 GitHub's code search Elasticsearch cluster hit near-100% CPU and rapid master elections; a restart found lost shards

   GitHub2013

   Symptoms

   On Thursday 24 January 2013 GitHub's exception tracking and monitoring systems detected a large spike in exceptions. Most of them were timeouts in code search queries and in the background jobs that update the code search indexes with data from new pushes, and the outage of the newly launched Code Search lasted into Friday 25 January.

   Intermittent timeoutsSustained high node CPUSplit brainData loss

   Trigger

   On 17 January GitHub upgraded the code search cluster from Elasticsearch 0.19.9 to 0.20.2 before launching Code Search publicly; the outage began on 24 January, after the public launch.

   Root cause

   Software defect

   Component

   Elasticsearch or OpenSearchLanguage runtime settings

   Contributing factor

   Unreviewed defaultsDeferred upgradesUntested changeIgnored early signalHarmful manual action

   Detection

   GitHub's exception tracking and monitoring systems detected a large spike in exceptions, mostly timeouts from code search queries and from the indexing background jobs.

   Remediation

   • Stopped the whole cluster and brought it back in maintenance mode, with shard allocation and rebalancing disabled
   • Brought in engineers from elasticsearch, the company, to review the cluster and its unallocatable shards
   • Upgraded Java and Elasticsearch and applied the recommended configuration through a Puppet topic branch and environment

   Action items

   • Provision a staging environment for the code search cluster to test infrastructure changes
   • Have the elasticsearch team audit the code search cluster configuration again in future

   Lessons

   The Elasticsearch upgrade was not tested enough on GitHub's infrastructure before it reached the code search cluster, partly because the cluster had no proper staging environment. For large clusters the elasticsearch team recommended equal minimum and maximum JVM heap through ES_HEAP_SIZE, at 50% of system RAM, starting recovery immediately instead of after recover_after_time, and configuring minimum_master_nodes so that paused nodes do not form their own clusters.

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