Cases3 cases

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1. IL-0514 PyPI's backends stopped responding for 7 minutes as rescheduled pods could not start while Consul and Vault were down

   Python Software Foundation2022

   Symptoms

   At 18:23 UTC on 28 July 2022, PSF Infrastructure got an alert that at least one Vault container in PyPI's Kubernetes cluster was down and needed unsealing. At 18:46 UTC PyPI's backends became unresponsive, and they stayed down until 18:53 UTC.

   Service outagePods stuck in Terminating

   Trigger

   From 17:24 UTC the cloud provider automatically replaced several instances of PyPI's Kubernetes cluster because of an unknown failure. All replacements were healthy by 18:20 UTC, but the event reshuffled many pods and restarted Consul and Vault on new nodes.

   Root cause

   Cloud provider outage

   Component

   Coordination and key-value storeSecrets store

   Contributing factor

   Automation amplification

   Detection

   An alert at 18:23 UTC told PSF Infrastructure that at least one Vault container was down and needed unsealing.

   Remediation

   • Forcefully removed the Consul pod stuck in Terminating so it was recreated and Consul could elect a leader
   • Unsealed the new Vault containers

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2. IL-0588 AppOptics metrics APIs returned 5xx as Kafka partitions stayed leaderless after an AWS power event hit their ZooKeeper

   AppOptics2018

   Symptoms

   On May 31, 2018 from 21:53 UTC the metrics APIs of AppOptics and Librato returned a 5xx for a majority of incoming metric data. APM metrics and traces sent from AppOptics APM were affected, and many alert notifications were delayed.

   High error rateFailover failureMonitoring outage

   Trigger

   A power event in one of AWS's availability zones in us-east-1 cut off nodes in several critical parts of the AppOptics and Librato infrastructure, including the ZooKeeper node that was then the leader of the ensemble coordinating the Kafka brokers.

   Root cause

   Cloud provider outage

   Component

   Message brokerCoordination and key-value store

   Contributing factor

   Single point of failureExcessive blast radiusMonitoring blind spot

   Remediation

   • Restarted one ZooKeeper node by hand to force a leader election, after which the ensemble regained quorum
   • Restarted every Kafka broker in a rolling restart until all partitions were back online
   • Worked through the backlog, including the backlog of alerts, and resumed normal operations

   Action items

   • Isolate the Kafka clusters better across availability zones and regions
   • Stop sharing one ZooKeeper ensemble across Kafka clusters
   • Run game days on this failure and check whether newer Kafka versions fix it
   • Put in place more checks that notify the team earlier about subsystems that need extra help to recover

   Lessons

   Running services across several availability zones and more than one Kafka cluster did not isolate the pipeline from the failure of one zone, because partitions of different clusters sat in the same zones and the clusters shared one ZooKeeper ensemble. More operational monitoring of the subsystems that needed help to recover would have led to a faster recovery.

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3. IL-0362 PagerDuty could not dispatch notifications for 18 minutes when a peering point shared by two AWS regions degraded

   PagerDuty2013sev1

   Symptoms

   On Saturday 13 April 2013 at 8:11am Pacific Time, PagerDuty's on-call engineer was paged about one node of the notification dispatch system, and an attempt at 8:13 to bring the node back failed. Customers saw delayed notifications and 500 errors on PagerDuty's API endpoints.

   High latencyHigh error rateIntermittent connection failures

   Trigger

   At 7:57am Pacific Time, according to AWS, a peering point in Northern California began to degrade, causing connectivity problems for the two AWS regions that PagerDuty used as two of its three datacenters.

   Root cause

   Cloud provider outage

   Component

   Transit gateway or network interconnectCoordination and key-value store

   Contributing factor

   Single point of failureUnverified assumptionMonitoring blind spot

   Detection

   The PagerDuty on-call engineer was paged at 8:11am about an issue with one node of the notification dispatch system, and at 8:18 monitoring detected multiple-provider failures for notifications.

   Remediation

   • Restarted most of the failed coordinator processes by hand
   • The notification dispatch system reached quorum again and processed all queued notifications

   Action items

   • Add more logging and aggregate logs into a single searchable source
   • Add a process watcher that restarts failed coordinator processes automatically
   • Build a dashboard that shows inter-host connectivity
   • Train engineering staff on Cassandra and ZooKeeper
   • Investigate moving off one of the AWS regions to a new hosting provider and datacenter that avoids a single point of failure

   Lessons

   Datacenters chosen to be independent and physically separate can still share a network peering point, and a design built to survive the loss of one datacenter did not survive losing two at once.

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