Cases8 cases

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1. IL-0339 Azure Front Door and Azure CDN users saw intermittent connection errors, timeouts and latency spikes for hours

   Microsoft Azure2024

   Symptoms

   From 11:45 UTC on 30 July 2024, a subset of customers got intermittent connection errors, timeouts and latency spikes when connecting to Microsoft services that use Azure Front Door (AFD) and Azure Content Delivery Network (CDN). The Azure Portal team saw the first service degradation at 11:47 UTC.

   Intermittent connection failuresIntermittent timeoutsHigh latencyPacket loss

   Trigger

   Around 11:45 UTC the Azure network DDoS protection service was disengaging after a routine DDoS mitigation and returning traffic to its default routing to AFD, but the routes at one site in Europe could not be updated because the network control plane could not reach that site during a local power outage.

   Root cause

   Configuration change

   Component

   CDN and edge networkDDoS protection serviceNetwork control plane (SDN)Data center power and cooling

   Contributing factor

   Overlapping failuresExcessive blast radiusMonitoring blind spot

   Detection

   The Azure Portal team detected service degradation at 11:47 UTC and began to investigate. At 12:10 UTC network monitoring correlated the Portal incident to a network issue at one site in Europe, and networking engineers joined.

   Remediation

   • Rerouted traffic under the standard congestion playbook, which removed most of the impact by 13:58 UTC
   • Ordered the network control plane to reissue its commands to the misbehaving device, which could not be reached
   • Removed the device that kept attracting traffic from the network and scanned the network for other issues

   Action items

   • Add the missing configuration on network devices so a DDoS mitigation issue in one geography cannot spread to another
   • Improve validation and monitoring in the Azure network to detect invalid configurations
   • Improve monitoring for when the DDoS protection service is unreachable from the control plane but still serving traffic

   View case
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

   View case
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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