Cases2 cases

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1. IL-0461 Every rollout of Datadog's metrics query service on Kubernetes raised errors that the logs blamed on DNS

   Datadog2022

   Symptoms

   From September 2021, each rollout of an update to Datadog's metrics query service, which serves real-time and historical metric data to the web frontend and to monitor evaluation, came with a rise in errors. Clients retried failed queries, so few errors reached users, but the retries slowed dashboards and monitors. The service's logs showed DNS errors stopping it from connecting to the data stores it depends on.

   High error rateDNS resolution failuresHigh latencyOOMKilledPacket lossStale endpoint routing

   Trigger

   Rollouts of the metrics query service, which delete the old pods. The client behaviour behind the failures came from an August 2021 rollout of the metrics query clients that applied round_robin as the new default gRPC load balancing policy.

   Root cause

   Configuration change

   Component

   conntrack tableCiliumLinux kernelCoreDNSClient-side DNS resolver and cacheexternal-dnsHTTP and RPC client library

   Contributing factor

   Change with a delayed effectRetry stormUnknown limitMonitoring blind spot

   Detection

   Errors rose during rollouts of the metrics query service, and the service's logs showed DNS errors when connecting to its data stores.

   Remediation

   • Raised the memory of the node-local-dns pods from 64 MB to 256 MB, which stopped the OOM kills but not the DNS errors
   • Tested larger instance types and confirmed that scaling up the service's infrastructure resolved the errors
   • Removed the custom gRPC reconnect parameters from the clients to restore the defaults

   Action items

   • Contribute a change to Cilium that adds an unreachable route for a deleted pod's IP so clients get an ICMP error

   Lessons

   The errors looked like DNS but were not. ENA metrics and VPC Flow Logs gave the visibility into VPC connection tracking that host metrics did not, and Cilium, Kubernetes and AWS networking interact in ways that can make reverse path filtering drop packets it should allow. DNS propagation time depends on many settings. The causes were changes Datadog had made itself, to work around a dependency bug, to resolve earlier incidents and to override default reconnect parameters, which looked safe at the time and showed their effects much later, when the link was hard to see.

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2. IL-0252 Spotify users who were logged out could not log back in during a global outage of service discovery

   Spotify2022

   Symptoms

   At 18:12 UTC on 8 March 2022, reports began to surface of users being logged out of the different Spotify client apps. Anyone who was logged out of a Spotify app could not log back in, and the outage was global.

   High error rate

   Trigger

   Google Cloud Traffic Director, the xDS-based traffic control plane and discovery system that some Spotify services use, had an outage.

   Root cause

   Cloud provider outage

   Component

   Service discoveryHTTP and RPC client library

   Contributing factor

   Overlapping failures

   Detection

   Reports of users being logged out of the client apps started to surface at 18:12 UTC.

   Remediation

   • Rolled out configuration changes that moved the affected systems back to the DNS-based service discovery

   Action items

   • Work with Google Cloud to understand how the Traffic Director issues led to a large outage for Spotify users
   • Add monitoring and alerting that catches similar service discovery problems earlier
   • Add safety nets for monitoring, automatic error detection and self-recovery

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