Cases7 cases

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1. IL-0430 Mackerel's Redis stopped answering during a replication setup, keepalived failed over by mistake, and Mackerel degraded

   Hatena2018

   Symptoms

   On 26 September 2018 at 10:51 JST the whole Mackerel monitoring service stopped working properly and its alive monitoring stopped. The application servers could not connect to Redis, which stores Mackerel's monitoring data, and could not return correct responses.

   Monitoring outageHigh latencyIntermittent timeouts

   Trigger

   Redis memory use had been growing, so operators set up replication in order to scale Redis up. Building the replication took a long time and Redis stopped answering for a while, and keepalived treated this as a node failure and started a failover nobody intended.

   Root cause

   Configuration change

   Component

   In-memory cacheDatabase replicationDatabase failover manager

   Contributing factor

   Automation amplificationOverlapping failures

   Remediation

   • Switched back to the correct Redis and briefly saw the application return
   • Restarted the application servers one after another, which did not improve the symptoms
   • Detected the organizations posting inappropriate metrics and blocked their requests
   • Adjusted the timeout interval of the application servers
   • Temporarily added capacity to Diamond, the TSDB
   • Increased the number of Kinesis shards and Redis Cluster shards
   • Scaled out the API application servers
   • Put Mackerel in maintenance mode, checked it internally and let outside requests back gradually

   Action items

   • Raise the number of keepalived health checks for Redis so a brief load increase does not trigger a failover
   • Scale Redis up
   • Add more application servers
   • Change the application to store only the monitoring data it needs in Redis, reducing Redis memory use
   • Build a way to detect inappropriate API requests quickly and show them on a dashboard
   • Build a function to block organizations making inappropriate requests quickly
   • Keep improving the accuracy of detecting inappropriate requests and consider adding features such as an API limit
   • Strengthen monitoring of the application servers' internal processing so problems are handled before they become outages

   Lessons

   Hatena did not find why recovery took so long, but expects that keeping failover from reacting to short load spikes and giving the application more performance headroom would prevent an outage this long. It also aims to keep problems contained to a small part of the service when a failure does happen.

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2. 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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3. IL-0520 logs.cloud.gov trapped users in a login redirect loop because its full Redis session store stopped saving sessions

   cloud.gov2016

   Symptoms

   At 7:12pm Eastern Time on 5 July 2016, logs.cloud.gov, the cloud.gov log search service, sent users into a login redirect loop, so they could not log in to the service.

   Service outageWrites rejected

   Root cause

   Capacity exhaustion

   Component

   In-memory cacheElasticsearch or OpenSearch

   Contributing factor

   Overlapping failuresMonitoring blind spot

   Remediation

   • Moved Redis on the logsearch service to a much larger memory instance so it could keep logs while Elasticsearch was down
   • Corrected the Elasticsearch memory limits and the /tmp noexec hardening setting and redeployed the cluster
   • Redeployed the Kibana instance and verified the logs once the cluster deployment was complete

   Action items

   • Add a failure alert that checks whether logs are coming through
   • Make Redis auto-scaling or highly available so that failures in more fragile downstream components do not cause data loss
   • Extend the incident response plan to start with events that are not security-specific
   • Create a chaos bot for the logs.cloud.gov service

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4. IL-0278 A weekly update put a Redis Cloud management process into an infinite loop, and most nodes of a GCP cluster failed

   Redis2016

   Symptoms

   On 8 February 2016 at 17:45 UTC, four nodes of the Redis Cloud cluster us-central1-1-1.gce.garantiadata.com on Google Cloud Platform stopped responding, four minutes after a first node had failed. The databases whose endpoints end in us-central1-1-1.gce.garantiadata.com went down.

   Service outageFailover failure

   Trigger

   The weekly upgrade of Redis Cloud's non-critical cluster components reached its first Google Cloud Platform cluster, us-central1-1-1.gce.garantiadata.com, at 17:30 UTC.

   Root cause

   Faulty deployment

   Component

   In-memory cacheDatabase failover managerAuthoritative DNS servers and records

   Contributing factor

   Overlapping failures

   Remediation

   • Shut down and terminated all nodes of the cluster
   • Deleted all of the cluster's endpoints from its DNS zone, removing by hand the entries left by throttled deletes
   • Provisioned new cluster nodes from scratch without the weekly update
   • Reconnected the original nodes' storage to the new nodes so persistence-enabled databases could be recovered
   • Bootstrapped and launched the databases, recovering the ones with persistence from storage

   Action items

   • Suspend all automatic weekly upgrades until a procedure that prevents this kind of outage is in place
   • Revalidate the QA processes for DevOps code deployments
   • Improve the tooling so it handles throttling of DNS requests

   View case

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