Cases19 cases

7 to 11 of 19

1. IL-0254 Square's internal DNS servers failed and Square products, services and payments were unavailable for about 15 hours

   Square2023

   Symptoms

   From 1:54 PM ET on 7 September 2023, Square products and services were unavailable, and at 2:02 PM ET engineers were notified of infrastructure issues and began investigating DNS. Square Online websites stayed up, but their customers could not make payments.

   DNS resolution failuresService outage

   Trigger

   At 11:04 AM ET a host-based firewall rule change was deployed to enable region communication, which increased the size of the firewall rules on nodes. Combined with a DNS service upgrade, it put unexpected load on the internal DNS servers. The timeline also lists a DNS zone change at 1:56 PM ET.

   Root cause

   Configuration change

   Component

   Authoritative DNS servers and recordsNode iptables rules

   Contributing factor

   No staged rolloutCache-masked failureCircular dependency

   Detection

   Engineers were notified of infrastructure issues at 2:02 PM ET, eight minutes after services became unavailable.

   Remediation

   • Shed network traffic to the DNS servers and started bringing up new DNS servers by hand
   • Deployed networking changes by hand to re-enable the authorization and access services
   • Deployed the networking changes by hand to all data centers
   • Rebuilt the DNS servers
   • Built a new configuration from the reverted firewall ruleset and pushed it to the DNS hosts

   Action items

   • Move the DNS infrastructure onto isolated infrastructure
   • Add monitoring and optimizations for critical networking infrastructure
   • Optimize dependencies between the deploy and platform infrastructure where feasible
   • Expand support for Offline Mode and improve communication about when it is available

   View case
2. IL-0223 A connection surge congested the devices between AWS's internal and main networks in us-east-1, breaking EC2 APIs

   Amazon Web Services2021

   Symptoms

   From 7:30 AM PST on 7 December 2021, AWS services in us-east-1 that communicate between AWS's internal network and its main network saw rising latency and errors. The AWS operations teams lost real-time monitoring data at once, and the first thing found in the logs was elevated internal DNS errors.

   Control plane lagHigh latencyHigh error rateMonitoring outageDNS resolution failures

   Trigger

   At 7:30 AM PST an automated activity that scales the capacity of one AWS service hosted in the main AWS network triggered unexpected behaviour from a large number of clients inside the internal network.

   Root cause

   Software defect

   Component

   Transit gateway or network interconnectAutomated capacity manager

   Contributing factor

   Retry stormCircular dependency

   Detection

   The congestion immediately cut off real-time monitoring data for the AWS operations teams, who turned to logs and first identified elevated internal DNS errors.

   Remediation

   • Moved internal DNS traffic away from the congested network paths
   • Isolated the top sources of traffic onto dedicated network devices
   • Disabled some heavy network traffic services, including EventBridge event delivery
   • Brought additional networking capacity online
   • Disabled the automated scaling activities that triggered the event

   Action items

   • Keep the triggering scaling activities off until all remediations are deployed
   • Fix the latent issue that kept networking clients from backing off
   • Deploy network configuration that protects the affected networking devices in a similar congestion event
   • Keep API Gateway servers healthy when the internal network is unavailable and speed up their recycling
   • Release a new Service Health Dashboard that makes service impact easier to understand
   • Run the support system actively across multiple AWS regions

   View case
3. IL-0221 A backbone maintenance command disconnected all Facebook data centers, and its DNS servers withdrew their BGP routes

   Meta2021

   Symptoms

   On 4 October 2021 Facebook's platforms went down worldwide: every connection in Facebook's global backbone network was lost, and the data centers were cut off from each other and from the internet. The rest of the internet could no longer find Facebook's servers, because Facebook's authoritative DNS servers had become unreachable.

   Service outageDNS resolution failuresNetwork unreachable

   Trigger

   During routine maintenance of the backbone, a command was issued to assess the availability of global backbone capacity, and it unintentionally took down all connections in the backbone network.

   Root cause

   Operator error

   Component

   Backbone network (WAN)BGP routingAuthoritative DNS servers and records

   Contributing factor

   Automation amplificationCircular dependencyLimited debug access

   Remediation

   • Sent engineers onsite to the data centers to debug the issue and restart systems
   • Brought the backbone network back online across the data center regions
   • Brought services back gradually, managing the rising load to avoid crashes from a traffic surge

   Action items

   • Find ways to simulate the global backbone being taken offline in storm drills
   • Strengthen testing, drills and overall resilience

   Lessons

   The hardening that protects Facebook's data centers against unauthorized access slowed recovery from an outage caused by Facebook's own error, and the author judges that tradeoff worth keeping. Storm drills prepared the teams to bring load back safely, although no drill had yet simulated the global backbone going offline.

   View case
4. IL-0229 bunny.net's DNS servers crashed in a loop on a corrupted routing database file and could not fetch a fix from its CDN

   bunny.net2021

   Symptoms

   On 22 June 2021, within minutes of an update released at 8:25 AM UTC, bunny.net's global fleet of close to 100 DNS servers kept crashing and restarting, more than 60% of traffic was lost, hundreds of Gbits of throughput disappeared, and over 750,000 websites were affected. After 10 minutes the team realised that the DNS servers were restarting and dying and that nothing could bring them back up in that state.

   Container crash loopDNS resolution failuresService outageMisrouted requestsLoad imbalance

   Trigger

   At 8:25 AM UTC bunny.net released an update meant to reduce the download size of the SmartEdge optimization database, and it uploaded a corrupted file to Edge Storage.

   Root cause

   Faulty deployment

   Component

   Authoritative DNS servers and recordsCDN and edge networkObject storage

   Contributing factor

   Circular dependencySingle point of failureAutomation amplificationIneffective rollbackHarmful manual action

   Detection

   About 10 minutes after the release, the team realised that the DNS servers were restarting and dying.

   Remediation

   • Deployed by hand an update that disabled the SmartEdge system on the DNS nodes
   • Routed all requests through Frankfurt instead of Madrid on a couple of DNS servers
   • Migrated the deployment systems and files to a third-party cloud storage service
   • Rewired the deployment system and DNS software to the new storage and deployed
   • Synced the databases and deployed the newest file sets through the third-party service

   Action items

   • Phase out the BinaryPack library as a hot-fix
   • Test third-party libraries more extensively
   • Migrate the internal APIs to an independent third-party service
   • Find how to stop non-critical software from becoming a single point of failure across clusters
   • Make the DNS system keep a local copy of all backup data with automatic failure detection

   Lessons

   bunny.net had built its systems to rely on each other, including the critical parts of its internal infrastructure, so one corrupted file brought down several layers of redundancy with no way back. A part of the infrastructure treated as non-critical turned out to be a single point of failure for several clusters at once, which canary deployments did not catch. Building infrastructure inside its own ecosystem lets one failure cascade through all of it. In the rush to fix the outage the team made typos and mistakes, and the final deployment came after it calmed down and double-checked everything.

   View case
5. IL-0220 An overloaded AWS Transit Gateway dropped packets on 4 January 2021, and Slack's web tier saturated and went down

   Slack2021

   Symptoms

   During the Americas' morning of 4 January 2021, an external monitoring service paged Slack because error rates were creeping up. While the investigation was starting, Slack's own dashboarding and alerting service became unavailable, so responders had to work without their usual dashboards and alerts.

   High error rateMonitoring outagePacket lossService outageScale-up failureFile descriptor exhaustionCloud API rate limit exhausted

   Trigger

   On the first working day after the holidays, Slack's traffic went from the quietest time of the year to one of its biggest days overnight, with cold client caches pulling more data than usual, and one of Slack's AWS Transit Gateways did not scale fast enough for it.

   Root cause

   Cloud provider outage

   Component

   Transit gateway or network interconnectAutomated capacity manager

   Contributing factor

   Circular dependencyAutomation amplification

   Detection

   An external monitoring service paged Slack when error rates started creeping up during the Americas' morning. AWS engineers were separately alerted to the packet drops by AWS's internal monitoring.

   Remediation

   • Disabled automated downscaling of the web tier to keep serving capacity and allow investigation
   • Cleared up the broken, partly provisioned instances that filled the autoscaling groups
   • Repaired provision-service so new web tier instances could be provisioned and enter service
   • AWS engineers increased the Transit Gateway capacity manually across all Availability Zones

   Action items

   • AWS reviews its Transit Gateway scaling algorithms for large packet-per-second increases
   • Request a preemptive upscaling of the Transit Gateways at the end of the next holiday season
   • Run the dashboard service instances in the same VPC as their database to remove the Transit Gateway dependency
   • Load test provision-service regularly
   • Reevaluate health-checking and autoscaling configurations so they cannot overload provision-service again

   Lessons

   Monitoring has to stay as independent as possible of the infrastructure it watches, or it fails when it is needed most. Here the dashboard and alerting services failed because they ran in a different VPC from their databases and so depended on the Transit Gateways. provision-service had been load tested before, but this event exceeded the earlier scale-ups.

   View case

By root cause8