Cases14 cases

6 to 10 of 14

1. IL-0106 A malformed Traefik IngressRoute triggered a Traefik v2 bug once a node rollover restarted every Traefik pod, and all Kubernetes services returned 404

   DFDS2021

   Symptoms

   At 08:00 UTC on 2021-09-06 every service in DFDS's Kubernetes cluster answered with 404. Investigation started five minutes later.

   Service outageHigh error rateProbe failures

   Trigger

   The automated Kubernetes node rollover on 2021-09-06 restarted all the Traefik v2 pods, five days after a malformed IngressRoute manifest had been applied.

   Root cause

   Software defect

   Component

   Ingress controllerCloud load balancer

   Contributing factor

   Change with a delayed effectNo configuration testsExcessive blast radius

   Detection

   All services returned 404 from 08:00 UTC and root cause investigation started at 08:05. Time to detection was 5 minutes.

   Remediation

   • changed the ALB health check path, which made the targets healthy but did not stop the 404s
   • rolled back by hand to Traefik v1 as the main ingress controller
   • deleted the malformed IngressRoute by hand and confirmed the Traefik v2 bug
   • applied the changes upstream

   Action items

   • Upgrade Traefik to v2.5, where the bug is fixed, and verify that it removes the bug
   • Change the Traefik v2 health check path to something more robust
   • Set up an external liveness probe on the ingress controller
   • Consider workload segmentation to limit the blast zone
   • Look into monitoring the Traefik v2 API for rapid changes
   • Look into smoke testing during and after node rollover

   Lessons

   A malformed IngressRoute did nothing visible for five days and then took down all ingress when every Traefik v2 pod had restarted during a node rollover. Because Traefik v2 failed gracefully, the first signals pointed at the ALB health checks instead of at Traefik. On incident handling, the team concluded that appointing an incident manager could have helped, that the war room meeting should be recorded and a log book kept, that people not involved should only make suggestions in writing, and that a short first round of sharing thoughts and gut feelings is useful.

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2. IL-0010 A templating change that dropped its {{ }} delimiters made Argo CD delete all 478 Skyscanner services in every region

   Skyscanner2021

   Symptoms

   On 25 August 2021, skyscanner.net was unavailable and the Skyscanner mobile apps could not work, for travellers and business partners everywhere at the same time. The full global outage lasted four and a half hours.

   Service outageResources deleted

   Trigger

   A peer-reviewed change to a rarely changed root-level file of the cells templating configuration, intended as a no-op, that left out the {{ }} delimiters; it was submitted around 15:30 GMT, merged to master and rolled out automatically to every region at once.

   Root cause

   Configuration change

   Component

   Argo CDConfiguration templating

   Contributing factor

   Missed in reviewNo staged rolloutNo configuration testsUntested backupAutomation amplificationExcessive blast radius

   Remediation

   • restored the correct configuration for each cluster and let Argo CD reconcile the cluster back to it
   • restored services through the backup and restoration process
   • brought one region back first with the critical services and served all traffic from it from around 20:30 GMT
   • restored and verified the remaining regions and non-critical services over Thursday and Friday

   Action items

   • Avoid global configuration deploys, including rare change paths like this one
   • Treat templated configuration as code and add testing and linting
   • Plan and wargame more drastic failure scenarios in runbooks
   • Verify backup and restore processes, including access to the backups and the loss of a region
   • Refactor runbooks so they are clear and, where appropriate, idempotent for a stressed engineer in the early morning
   • Reconsider how much configuration is templated across regions, weighing configuration drift against automated global rollout

   Lessons

   Global configuration changes had been prevented in many cases, but not in this rarely used path. Configuration that gains templating and logic has become code and needs tests and at least linting. The failure scenarios and runbooks were not pessimistic enough, a backup only counts once it has been restored, and runbooks need care so they work for a stressed engineer at night. Automation that templates configuration across regions trades drift for the risk of rolling one mistake out everywhere. An experienced incident commander who kept the response calm and blameless made a large difference.

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3. IL-0086 A deployment meant for a CircleCI test Kubernetes cluster also removed Traefik and Calico resources from the production cluster, taking down the CircleCI UI and runner.circleci.com

   CircleCI2021

   Symptoms

   At 15:35 EST on 11 March 2021 the CircleCI web UI and runner.circleci.com became unavailable. The API, which uses a separate ingress point, stayed reachable, and builds kept running.

   Service outageResources deleted

   Trigger

   An update deployed to a test Kubernetes cluster also removed Traefik and Calico resources from CircleCI's primary production cluster.

   Root cause

   Faulty deployment

   Component

   Ingress controllerCalico

   Contributing factor

   Excessive blast radiusNo runbookMonitoring blind spot

   Detection

   CircleCI engineers were alerted and began investigating. The status page posted its first notice at 20:45 UTC.

   Remediation

   • restored Traefik in the production cluster
   • restored Calico in the production cluster, after which pods served customer requests again

   Action items

   • Improve network-level documentation and runbooks to cover every layer of the Kubernetes networks
   • Improve monitoring and alerting for the network infrastructure
   • Add guardrails that isolate the production and staging clusters so they cannot interfere with each other

   Lessons

   A deployment path that can reach both a test cluster and production can remove shared networking components from production. When the ingress controller and the CNI are both affected, restoring the ingress alone is not enough: engineers had to find the link between Calico and Traefik and restore Calico before pods served traffic again. Time went to DNS and AWS red herrings because nothing linked the outage to the test deployment; CircleCI's answer is better network runbooks, network alerting and guardrails between clusters.

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4. IL-0172 An AWS Tokyo data centre cooling failure took down 6 of 60 CockroachDB nodes on Kubernetes within 22 minutes, and ranges lost quorum because the Helm-deployed cluster had no availability zone locality configured

   Devsisters2021

   Symptoms

   During a team dinner, a Slack alert reported that one node of the CockroachDB cluster behind Cookie Run: Kingdom had gone down. The DevOps engineers were not worried at first, because CockroachDB is built to survive the loss of some nodes.

   Node NotReadyData lossService outage

   Trigger

   A cooling system failure in an AWS Tokyo region data centre overheated the servers in one availability zone, which held CockroachDB nodes.

   Root cause

   Cloud provider outage

   Component

   Application databaseHelm

   Contributing factor

   Unverified assumptionExcessive blast radius

   Detection

   A Slack alert reported that a database node had gone down.

   Remediation

   • copied the data outside the broken ranges into new tables with INSERT INTO ... SELECT queries that skipped the affected ranges
   • ran debug unsafe-remove-dead-replicas together with Cockroach Labs engineers, recovering one of the two lost data ranges
   • restored the last data range with the storage-layer recovery method from the previous incident

   Action items

   • Configure CockroachDB Locality so data is spread evenly across availability zones
   • Deploy with one chart and a separate values file per availability zone to set Locality
   • Replace nodes by adding Locality-configured nodes, confirming replicas in every zone, then retiring the old nodes

   Lessons

   A replication factor sized for random instance retirements does not protect against a whole availability zone failing; for a game of this scale a multi-AZ placement is required, not optional. Keeping one uniform Helm chart made zone topology hard to set, which is part of why it was left out. A repetitive chart with a values file per zone sets it reliably.

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5. IL-0094 Student pods filled the shared nodes of a JupyterHub cluster and starved the hub's core pods, freezing the hub

   2i2c2020

   Symptoms

   At 08:52 PM IST a report came in from WER that many students could not access the JupyterHub and that it was frozen; pages were stuck and nothing was usable. A rise in activity was visible by 09:02 PM, but the usual fixes, such as opening an incognito window and restarting servers, did not help.

   Service outageSustained high node CPU

   Trigger

   WER students' pods used a good chunk of their memory limit (tests had shown they almost always use just under 1G of their 2G limit, against a 512M guarantee) on nodes shared with the hub's core pods.

   Root cause

   Capacity exhaustion

   Component

   Node poolContainer memory limit

   Contributing factor

   No resource requests or limitsUnverified assumptionExcessive blast radius

   Detection

   WER reported stuck pages and students unable to reach the hub in a GitHub issue (2i2c-org/docs#27).

   Remediation

   • removed memory overcommit for the WER students
   • raised resource guarantees for all core pods, which restarted them and moved some to a new node

   Action items

   • Put user pods in a separate pool so they do not create pressure on the core pods (infrastructure#89)
   • Set limits on the support infrastructure (Prometheus, Grafana, ingress) as well (infrastructure#90)
   • Document and think about overcommit ratios for memory usage (infrastructure#91)
   • Set up better Grafana dashboards to monitor resource usage (infrastructure#92)
   • Document how people can get kubectl access to the cluster so others can look into issues too (infrastructure#87)

   Lessons

   Core pods without resource guarantees get whatever is left on a node, so when user pods on the same nodes fill the CPU and memory, the hub that serves everyone freezes. The memory overcommit ratio has to come from measured use: WER students used just under 1G against a 512M guarantee, which a 4x overcommit could not hold.

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