Cases2 cases

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1. IL-0081 API requests timed out during deployment rollouts because the ingress-nginx enable-serial-reloads option held back endpoint updates while a long NGINX reload was running

   Cresta2025

   Symptoms

   During a deployment, traffic into the Kubernetes cluster started hitting dead ends: some API requests timed out and requests were slow and disrupted while the rollout ran.

   Intermittent timeoutsStale endpoint routingHigh latency

   Trigger

   A deployment rollout changed backend endpoints while a long-running NGINX full reload was in progress, with the enable-serial-reloads option enabled on the ingress-nginx controller.

   Root cause

   Software defect

   Component

   Ingress controller

   Contributing factor

   –

   Remediation

   • restarted the NGINX Ingress controller as an attempt to resolve the problem

   Action items

   • Open an issue in kubernetes/ingress-nginx describing how enable-serial-reloads blocks dynamic endpoint updates

   Lessons

   A flag meant only to serialise full reloads also delayed endpoint updates, which opened a window during rollouts in which the ingress routed to pods that no longer existed. Options that change how the ingress controller applies configuration need to be checked against what happens to endpoint updates during a long reload.

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2. IL-0011 HAProxy ingress piled requests onto the same pods because a Go template visited the backend map in sorted pod IP order

   Skyscanner2019

   Symptoms

   On a Sunday, a team using one of Skyscanner's Kubernetes clusters manually paged the out-of-hours Kubernetes support because their service showed elevated latency, worst at the p90 and maximum levels.

   High latencyLoad imbalanceOOMKilledIntermittent connection failures

   Trigger

   Upgrading the HAProxy ingress controller to v0.6 and going back from Service virtual IPs to individual pod IPs as backends, on the understanding that the sort-backends flag introduced in v0.5, false by default, meant backends were randomly shuffled.

   Root cause

   Software defect

   Component

   Ingress controllerHAProxyConfiguration templating

   Contributing factor

   Unrepresentative stagingUnverified assumptionIgnored early signal

   Detection

   A cluster user manually paged the out-of-hours Kubernetes support about elevated latency; Prometheus queries then showed the uneven request distribution across pods.

   Remediation

   • switched the affected service from pod IPs back to the Service virtual IP through a config change in the internal deployment tool Slingshot
   • patched the controller to keep each backend's servers in an insertion-ordered slice and changed the template to iterate over that slice
   • tested the patch in sandbox clusters and rolled it out to the production clusters

   Action items

   • Run the internal controller patch in production and return to pod IPs as the default backends
   • Raise the problem upstream with a GitHub issue and pull request
   • Test changes to cluster components against the specific behaviour and conditions that matter

   Lessons

   The team already knew that sorted backends cause hot-podding and still did not test the new flag well enough to see that it did not work; changes to cluster components need tests aimed at the exact behaviour that matters, under conditions like production. A keyword can behave differently in different parts of a language: range over a map is unordered in Go code but sorted in text/template. A small cluster component can do outsized harm to every user of the cluster and, in the end, to customers.

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