Cases2 cases

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1. IL-0197 Moving a three-node Elasticsearch StatefulSet to new persistent volumes split the cluster in two, because the Helm chart hardcoded cluster.initial_master_nodes to elasticsearch-0

   Palark2021

   Symptoms

   After the last old Elasticsearch pod, elasticsearch-0, was deleted and came back on a new, empty persistent volume, `_cat/nodes` run inside that pod listed a single node: elasticsearch-0 itself, marked as master. The same command run inside elasticsearch-1 listed the other five nodes, elasticsearch-1 to elasticsearch-5, with elasticsearch-2 as master.

   Split brain

   Trigger

   Deleting the elasticsearch-0 pod and its PVC so that it restarted on an empty volume of the new StorageClass.

   Root cause

   Operator error

   Component

   Elasticsearch or OpenSearchPersistent volumes and stateful setsHelm

   Contributing factor

   Unverified assumption

   Detection

   The engineer running the migration checked `_cat/nodes` after elasticsearch-0 restarted and saw it alone in a cluster of its own.

   Remediation

   • scaled the three deployments of workers writing to Elasticsearch down to zero
   • listed elasticsearch-0, elasticsearch-1 and elasticsearch-2 under initial_master_nodes in the elasticsearch ConfigMap and removed the cluster.initial_master_nodes environment variable from the StatefulSet
   • saved and deleted the elasticsearch discovery Service and deleted the PVC of elasticsearch-0 while the rolling update ran
   • set initial_master_nodes to elasticsearch-3, elasticsearch-4 and elasticsearch-5, created an empty PVC for elasticsearch-0 and deleted all six pods
   • recreated the discovery Service from the saved manifest, after which the six nodes formed one cluster with every index green

   Action items

   • Restore the original initial_master_nodes values in the ConfigMap and restart all pods again
   • Move all shards to nodes 0, 1 and 2 and scale the cluster down from six nodes to three
   • Commit all the manual changes to the repository

   Lessons

   A setting that only matters when a cluster first starts, such as Elasticsearch's cluster.initial_master_nodes, can sit in a Helm chart template rather than in the ConfigMap that was reviewed. When a pod named in it starts with an empty data directory, it can bootstrap a new cluster beside the existing one. The author's conclusion is that a migration plan like this one breaks on any surprise and needs a plan B ready. Before running it in a more important environment, rehearse it in a test environment configured like production, and either schedule downtime or move the load to another cluster: stop the writers, take a fresh backup, then move the data.

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2. IL-0196 Adding a second Redis replica and lowering memory limits in one RedisFailover change restarted the only Redis pod that held data, and Sentinel promoted the empty new replica, erasing the data

   Palark2020

   Symptoms

   Right after an updated RedisFailover resource was rolled out, the large Redis database lost all its data and the service went down; the data had to be restored from a backup taken just before the change.

   Data lossService outage

   Trigger

   A RedisFailover change that raised the number of Redis replicas from one to two and, in the same rollout, lowered the containers' memory requests and limits from 22 GB to 19 GB.

   Root cause

   Software defect

   Component

   Application databasePersistent volumes and stateful setsContainer memory limit

   Contributing factor

   No staged rolloutSingle point of failureUnverified assumption

   Remediation

   • restored the Redis data from the backup prepared before the rollout

   Action items

   • Report the readiness probe flaw to the Redis Operator project and test the developers' fix
   • Reconsider the use of the Spotahome Redis Operator
   • Clean up the Redis databases and fix the application code that created the excess keys
   • Consider Redis built-in clustering and sharding

   Lessons

   Changing the replica count and the pod resources in one rollout turned a seemingly harmless change into a rolling restart of the only pod with data while the new replica was still empty. A readiness probe that only checks that Redis answers a ping does not tell whether a replica has finished syncing. Operators for critical stateful components need to be evaluated for maturity before production use, and the dump.rdb file should be backed up before any change to the Redis cluster.

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