Cases3 cases

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1. IL-0226 Azure Front Door edge servers crashed worldwide on incompatible configuration metadata, causing timeouts and DNS errors

   Microsoft Azure2025

   Symptoms

   From 15:41 UTC on 29 October 2025, customers and Microsoft services using Azure Front Door (AFD) and Azure Content Delivery Network (CDN) got connection timeout errors and DNS resolution issues. Monitoring alerts started the investigation at 15:48 UTC.

   Intermittent timeoutsDNS resolution failuresService outageHigh latency

   Trigger

   At 15:35 UTC a specific sequence of valid customer configuration changes, made across two different AFD control plane build versions, produced incompatible customer configuration metadata that was deployed to the edge sites.

   Root cause

   Software defect

   Component

   CDN and edge networkAuthoritative DNS servers and records

   Contributing factor

   Gap in automated testsChange with a delayed effectIneffective rollbackExcessive blast radius

   Detection

   The configuration protection system detected the data plane impact and blocked configuration changes at 15:43 UTC, and monitoring alerts started the investigation at 15:48 UTC.

   Remediation

   • The configuration protection system automatically blocked all new and in-flight configuration changes worldwide
   • Removed the problematic customer configurations from the latest LKG snapshot by hand
   • Blocked all customer configuration changes from propagating to the data plane
   • Deployed the updated LKG configuration across the global fleet
   • Moved Azure Portal away from Azure Front Door
   • Rebalanced traffic by hand to a few healthy edge sites, then switched back to automatic traffic management
   • Blocked all AFD customer configuration changes at the Azure Resource Manager level until safeguards were in place

   Action items

   • Fix the control plane incompatibility and the data plane bug
   • Process each customer configuration completely and synchronously before production stages
   • Add stages to the phased configuration rollout, with longer bake time between them
   • Move configuration processing on data plane servers into isolated worker processes
   • Reduce configuration propagation time from 45 minutes to about 15 minutes
   • Test backwards compatibility with configurations generated by earlier control plane builds
   • Move critical first-party infrastructure to an active-active setup with fail away
   • Isolate customer configuration and traffic with micro cell segmentation of the AFD data plane
   • Restore customer configurations within one hour by using the local configuration cache
   • Cut data plane recovery further, to about 10 minutes
   • Remove delays in Azure Service Health alerts by raising resource thresholds
   • Extend automated Azure Service Health alerts to similar classes of service degradation
   • Fail over support request systems faster so Premier and Unified customers can always open requests

   Lessons

   Microsoft describes configuration propagation risk as inherent to a global Content Delivery Network, in which route, WAF and origin changes must be deployed quickly worldwide. Both this incident and the 9 October AFD incident came from that risk. This time the incompatible metadata passed a protection system that advances only on healthy data plane signals, because the crash came later from delayed asynchronous processing. Some learnings and repair items from the earlier incident applied here as well. Microsoft advises customers to build a resilient global HTTP ingress layer that stays available during regional or network disruptions, to manage resources through the REST API or PowerShell when management portals are unavailable, and to configure Azure Service Health alerts so the right people hear about service issues.

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2. IL-0338 Google Cloud HTTP(S) and TCP/SSL Proxy load balancers returned 502s or reset connections on 33% to 87% of requests

   Google Cloud2018

   Symptoms

   From 12:17 to 12:49 PDT on 17 July 2018, Google Cloud HTTP(S) Load Balancers answered a share of requests with 502 return codes, and connections to TCP/SSL Proxy Load Balancers were reset. The share of failed requests moved between 33% and 87% during those 32 minutes, and automated monitoring alerted Google engineers at 12:19.

   High error rateConnection resetsHigh latency

   Trigger

   A configuration change in the production environment intermittently set off a bug in a new feature of the second-layer Google Front Ends.

   Root cause

   Software defect

   Component

   Cloud load balancerCDN and edge network

   Contributing factor

   Gap in automated testsExcessive blast radius

   Detection

   Automated monitoring alerted Google's engineering team at 12:19 PDT, two minutes after the 502s began.

   Remediation

   • Reverted the configuration change at 12:44 PDT, which stopped the GFE restarts

   Action items

   • Add safeguards that disable features not yet in service
   • Harden the GFE testing stack against latent bugs in production binaries that make a task restart
   • Isolate the shards of GFE pools further from each other to reduce the scope of failures
   • Build a consolidated dashboard of all configuration changes for GFE pools

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3. IL-0388 A Google Cloud live migration took a Discord Redis primary offline, and the failover broke API instances and then guilds

   Discord2017

   Symptoms

   At about 14:01 PDT on 13 October 2017, Discord engineers were alerted to anomalies by real-time metrics and monitoring. A Redis instance that was the primary of a highly available cluster used by the API services had dropped offline after Google Cloud Platform migrated it automatically, and some API instances did not handle the failover to the secondary node correctly.

   Failover failureHigh latencyService outage

   Trigger

   Google Cloud Platform automatically migrated the Redis instance that was the primary of a highly available cluster used by the API services, and the node incorrectly dropped offline.

   Root cause

   Software defect

   Component

   In-memory cacheDatabaseCDN and edge network

   Contributing factor

   Unpatched known bugOverlapping failuresMonitoring blind spot

   Detection

   Real-time metrics and monitoring alerted engineers at 14:01 PDT. The second phase was seen at 15:41 as another anomaly in monitoring, and end user reports of servers appearing offline were escalated at 15:42.

   Remediation

   • Ran a rolling restart of all API instances to clear the Redis connection issues
   • Corrected the misconfigured edge caching rule for the expensive API route
   • Disabled the problem API endpoint on the edge network
   • Recovered the Cassandra cluster
   • Ran a second rolling restart of all API instances for the latency left by the Cassandra issues
   • Removed the misbehaving guilds node from the service hashring
   • Forcefully restarted the misbehaving guilds node
   • Restarted the whole service, forcing all users to reconnect
   • Blocked traffic that relied on the failing push component to reduce its load
   • Corrected the API setting that lets users reconnect quickly and safely

   Action items

   • Fix the known issues with Redis failover within the coming weeks
   • Change the cached route so this type of error does not degrade other services
   • Monitor the signals that would have warned of the cascading failure
   • Improve the reliability and failure handling of the affected services
   • Roll out an update of the library with the suspected flaws

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