Cases7 cases

4 to 7 of 7

1. IL-0256 Basecamp was fully down twice in two days as hardware load balancers crashed and routers lost sync after failover

   37signals2020

   Symptoms

   On Tuesday 1 September 2020 Basecamp could not be loaded at all for 17 minutes, from 9:51am to 10:08am Central Time (14:51–15:08 UTC), and nothing seemed to work. On Wednesday 2 September it happened again for 15 minutes, from 10:50am to 11:05am Central Time (15:50–16:05 UTC).

   Service outageLink flappingHigh latency

   Trigger

   On 1 September the network links between the data centers went offline, and during the recovery a hardware load balancer crashed. On 2 September the primary load balancer in Virginia crashed again, and later its secondary crashed too.

   Root cause

   Software defect

   Component

   On-premises load balancerBackbone network (WAN)

   Contributing factor

   Overlapping failuresNo headroom

   Remediation

   • On 1 September, intervened by hand to get the network routers working with the standby load balancer
   • On 2 September, fixed the same router and load balancer desynchronization by hand

   Action items

   • Find the cause of the load balancer crash with the vendor and apply its bugfix
   • Replace the hardware load balancers
   • Reroute the redundant cross-datacenter network paths for proper circuit diversity
   • Evaluate moving from hardware to software load balancers, which can be deployed in minutes
   • Evaluate decentralizing the load balancer architecture to limit the impact of one failure
   • Assess the systems for resilience, fragility and risk, and review the assessment process itself

   Lessons

   Basecamp plans for failures like these and practises responses to catastrophic failures in its live systems, but three obscure issues in five days chained into overlapping failures. What caught the team by surprise were cascading failures that exposed unexpected fragility and hard paths to recovery, which Basecamp says it can prepare for.

   View case
2. IL-0255 Basecamp's two redundant links between data centers dropped together and a Redis replica resync loop stalled chat

   37signals2020

   Symptoms

   At 11:59AM CDT on Friday 28 August 2020, Basecamp's Ops team received a flood of alerts from PagerDuty, Nagios and Prometheus. Monitoring showed that the newer of the two metro links between the data centers in Ashburn, VA and Chicago, IL had been lost for about 20-30 seconds, and BC3 kept responding slowly after the link came back.

   Link flappingHigh latencyReplication lag

   Trigger

   Both redundant metro links between Basecamp's Ashburn and Chicago data centers dropped for short periods; monitoring showed the newer link down for about 20-30 seconds.

   Root cause

   Dependency failure

   Component

   Backbone network (WAN)BGP routingIn-memory cacheDatabase replication

   Contributing factor

   Single point of failureRetry stormSignal buried in noise

   Detection

   A flood of alerts from PagerDuty, Nagios and Prometheus, with monitoring showing the lost metro link. A lingering alert for one Redis DB instance later pointed to the replication problem.

   Remediation

   • Turned chat off at the load balancer layer so the rest of BC3 could stabilise
   • Restarted Redis on the replica that was stuck in the resynchronization loop

   Action items

   • Make the documented Redis configuration changes for the known replication issue
   • Make concrete plans to remove the shared physical path under the two metro links
   • Prove the failover systems in production at peak load
   • Make explicit room for post-incident follow-up work and push other work out

   Lessons

   Planned redundancy and live testing had given Basecamp a false sense of security, especially at peak load: moving between redundant network links or shifting load across data centers should work without issue, and Basecamp would rather find out in a controlled test with the full team ready. The follow-up from an outage needs urgency and explicitly cleared time, not just a place on a list.

   View case
3. IL-0222 Google Cloud US regions had heavy packet loss and some went unreachable after the network control plane was descheduled

   Google Cloud2019

   Symptoms

   On Sunday 2 June 2019, with user impact beginning between 11:47 and 11:49 US/Pacific, Google Cloud projects in several US regions saw elevated packet loss from network congestion, and some regions became unreachable. Customers saw higher latency, intermittent errors and lost connectivity to instances in us-central1, us-east1, us-east4, us-west2, northamerica-northeast1 and southamerica-east1, while us-west1, Europe and Asia had no regional congestion.

   Packet lossNetwork unreachableHigh latencyHigh error rateMonitoring outage

   Trigger

   At 11:45 US/Pacific a relatively rare type of datacenter maintenance event started in one physical location, and the automation that runs it descheduled logical clusters that also held network control plane jobs in other physical locations.

   Root cause

   Software defect

   Component

   Network control plane (SDN)BGP routingBackbone network (WAN)

   Contributing factor

   Excessive blast radiusCircular dependencyLimited debug accessIneffective rollback

   Detection

   Google engineers were alerted to the failure two minutes after it began and started the incident process used for the most serious production incidents. The first customer notice appeared on the status page at 12:53 US/Pacific.

   Remediation

   • Lowered the priority of less critical network traffic classes to reduce congestion
   • Halted the datacenter automation software responsible for the maintenance event
   • Rescheduled the network control plane and its supporting infrastructure in each location
   • Rebuilt and redistributed the lost network control plane configuration, rolling it out from 14:03
   • Directed service traffic away from the affected regions
   • Moved the Cloud VPN control plane in three regions out of its fail-safe state

   Action items

   • Keep the maintenance automation off until safeguards stop it descheduling jobs in several physical locations at once
   • Harden the cluster management software to reject such requests whatever their origin
   • Reconfigure the network control plane to reject maintenance requests of the type involved
   • Make the network control plane in each location persist its configuration
   • Let the network stay in fail static mode longer when the control plane is lost
   • Review, update and test emergency response tooling and procedures, including customer communication tooling, for such network failures
   • Extend continuous disaster recovery testing to this and similarly catastrophic failures

   View case
4. IL-0358 Packet Public Cloud locations had high latency and unreachable paths from routing loops and blackholed traffic

   Packet2019

   Symptoms

   At about 08:49 UTC on 2 June 2019, Packet's internal network monitoring alerted on serious reachability and performance problems in several of Packet's global datacenter locations. The first investigation found routing loops and traffic being blackholed to specific internal and Internet destinations.

   Network unreachablePacket lossHigh latency

   Trigger

   A trans-continental US backbone link in the network of Packet's upstream provider Zayo had a hardware failure and blackholed IP traffic routed over it.

   Root cause

   Dependency failure

   Component

   Backbone network (WAN)BGP routingNetwork control plane (SDN)

   Contributing factor

   –

   Detection

   Packet's internal network monitoring alerted at 08:49 UTC on reachability issues in several global datacenter locations, and engineers confirmed them at 08:52.

   Remediation

   • Migrated traffic to an alternative network and removed Zayo from the provider mix
   • Applied route filters to return service to normal
   • Optimised specific routing policies to improve latency across the backbone with the new traffic mix

   Action items

   • Expand internal connectivity monitoring and alerting of on-call operations teams
   • Expand network automation and routing so traffic can be balanced across all available backbone paths and an operator can trigger automated re-routes across all Packet datacenters in a major provider incident
   • Make Packet's provisioning systems less dependent on third party network providers and inter-datacenter connectivity

   View case

By root cause4