Cases2 cases

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1. IL-0472 Node-to-pod traffic and DNS failed on Tinder's 605-node Flannel cluster when every node's ARP cache passed gc_thresh3

   Tinder2019

   Symptoms

   In the early morning of 8 January 2019 Tinder's platform had a persistent outage. Communication from nodes to pods failed, and so did DNS lookups, because DNS ran inside the Kubernetes cluster.

   Service outageNetwork unreachableDNS resolution failures

   Trigger

   Earlier that morning, pod and node counts on the cluster were scaled up in response to an unrelated increase in platform latency.

   Root cause

   Capacity exhaustion

   Component

   Linux kernelCNI plugin

   Contributing factor

   Unreviewed defaults

   Remediation

   • Raised gc_thresh1, gc_thresh2 and gc_thresh3 on the nodes
   • Restarted Flannel so it re-registered the missing networks

   Lessons

   Tinder lists the outage among its learnings about network fabric limits: with Flannel over VXLAN, every node, and every pair of nodes that exchange pod traffic, adds entries to each node's ARP table, so at 605 nodes the default gc_thresh3 hard cap was exceeded on all nodes.

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2. IL-0431 During Mackerel's move to AWS, PostgreSQL failed over to a replica that had stopped replicating, losing an hour of data

   Hatena2017

   Symptoms

   On 7 August 2017, while checking the application after the maintenance that moved Mackerel from an on-premises data center to AWS, the team found that data updated during a specific time window was missing. Everything written to the PostgreSQL database between 14:30 and 15:30 JST was gone.

   Data lossReplication lagNetwork unreachable

   Trigger

   One hour after the application was switched to the newly promoted primary db001, db001 lost network connectivity and the database failed over to the standby db002.

   Root cause

   Operator error

   Component

   DatabaseDatabase replicationDatabase failover manager

   Contributing factor

   Overlapping failuresUnreviewed defaultsUnrepresentative stagingAutomation amplification

   Detection

   The team found the missing data while checking how the application behaved after the migration work.

   Remediation

   • Tried to restore the lost updates by hand from the table data left on db001 and gave up
   • Tried point-in-time recovery from the base backup and WAL and gave up
   • Built a new replica db003 with db002 as primary to restore redundancy
   • Kept db001 in maintenance state for investigation
   • Watched db002 for some time to confirm the problem did not recur, then completed the migration

   Action items

   • Change the PostgreSQL replica creation script to set recovery_target_timeline = 'latest' in recovery.conf by default
   • Add a check to a server configuration test tool such as Serverspec that recovery.conf contains recovery_target_timeline = 'latest'

   Lessons

   A setting needed only for a rare layout, such as a grandchild replica during a migration, was applied by hand through a script option and was forgotten. Because recovery_target_timeline = 'latest' is harmless on ordinary standbys too, Hatena now sets it by default whenever a replica is created. The staging rehearsal used the same layout and steps but had the setting right, so it could not catch the mistake. Hatena notes that a similar layout may be used again for a future PostgreSQL upgrade or a move to another region.

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