Cases18 cases

6 to 10 of 18

1. IL-0295 Pusher's Channels publish API on the mt1 cluster was largely unavailable for 11 minutes while a Redis master failed over

   Pusher2022

   Symptoms

   At 10:42 UTC on 11 April 2022, the API for publishing messages to Pusher Channels on the mt1 cluster became largely unavailable, and from about 10:44 requests to the MT1 API returned more errors.

   Service outageHigh latencySplit brain

   Trigger

   An engineer restarted the master node of an mt1 Redis group to apply a larger replication buffer, after restarting both replicas of that group.

   Root cause

   Software defect

   Component

   In-memory cacheDatabase failover manager

   Contributing factor

   Unverified assumptionUnrepresentative staging

   Detection

   Pusher saw error rates rise for requests to the MT1 API from about 10:44 UTC.

   Action items

   • Fix the Redis shutdown script to check that failover has completed and the node is no longer a master before terminating

   View case
2. IL-0321 GCP snapshots on all Patroni nodes stalled GitLab.com's primary PostgreSQL disk, Patroni failed over and 5xx spiked

   GitLab2021

   Symptoms

   At 23:38 UTC on 2021-01-16 GitLab.com error rates spiked, and a minute later multiple alerts for service disruption arrived, two minutes after GCP disk snapshots had been started on all Patroni database nodes. Some requests that needed database writes returned 5xx errors for about four to five minutes, while page refreshes checked during the incident responded normally.

   High error rate

   Trigger

   During the change production#2648, at 23:36 UTC an engineer started GCP disk snapshots on all Patroni nodes at the same time, as a safety step before a change to the filesystem, believing the snapshot was a transparent background operation through the GCP API.

   Root cause

   Capacity exhaustion

   Component

   DatabaseDatabase failover managerCloud block storage

   Contributing factor

   Unverified assumptionNo staged rollout

   Detection

   A flood of alerts about downstream impact; multiple service disruption alerts arrived at 23:39 UTC, a minute after the error spikes began. The incident was declared in Slack at 23:56.

   Remediation

   • Patroni invalidated patroni-06's lock and promoted patroni-03 as the new primary
   • Cancelled the snapshot requests in two of the three zones

   Action items

   • Update the Patroni and PostgreSQL runbooks to clarify GCP snapshot best practices
   • Add a checklist for change management reviewers and approvers to the runbooks, referenced in the handbook
   • Schedule a call with Google about IO sensitivities

   Lessons

   Do not take a GCP snapshot of a running PostgreSQL leader. Technical reviewers have to review every change or addition to a change request's description before a manager or director approves it. The Datastores team should explain in detail how backups and snapshots work on the production database. No action belonging to a change should start before the change's scheduled start.

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3. IL-0240 Cloudflare API requests failed and the dashboard slowed for hours after the authentication database lost its replicas

   Cloudflare2020

   Symptoms

   On 2 November 2020 the Cloudflare API and dashboard were degraded for six hours and 33 minutes. The success rate of API queries dropped at times to 75%, and the dashboard was up to 80 times slower than normal, while Cloudflare's edge network kept working.

   High error rateHigh latencyDatabase overloadWrites rejectedPacket loss

   Trigger

   At 14:43 UTC a network switch in a control plane data center partly failed: it kept LACP and BGP running but stopped forwarding some packets, and it recovered on its own six minutes later.

   Root cause

   DNS or network fault

   Component

   NIC, cable and switch port linkCoordination and key-value storeDatabase failover managerDatabaseDatabase replication

   Contributing factor

   Automation amplificationNo headroomHarmful manual action

   Detection

   At 14:43 UTC alerts fired because the misbehaving switch was unreachable to pings. Engineers then saw that the primary authentication database was overwhelmed.

   Remediation

   • Shed load from the primary authentication database by slowing SSL certificate pushes to the edge, emails and other features
   • Moved API traffic that could use read replicas to the secondary data center by hand
   • Sent authentication calls fully back to the primary data center when the dashboard got worse
   • Rebuilt the first database replica, which put itself back into service at 21:20 UTC and returned performance to normal
   • Ramped the turned-down services back up so asynchronous processing could catch up

   Action items

   • Remove the cluster management limitation that requires rebuilding all replicas after a primary is promoted
   • Redesign the user sessions system so dashboard traffic can be steered between data centers
   • Change the auto-remediation configuration so a database replica is not promoted so quickly
   • Tune each system involved to be more resilient to failures of its dependencies

   Lessons

   Every system in the chain had redundancy and none failed completely; each only degraded, which made the chain of events much harder to model and foresee. Automating replica promotion so that it could trigger on a minute's notice had been a point of pride, but for at least one database a fast promotion can do more harm than the failure it answers. Most general-purpose cluster managers use Raft or Paxos rather than Byzantine fault tolerant protocols, accepting a rare failure mode in exchange for a protocol that is simpler to implement correctly and debug.

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4. IL-0241 A database shard could not elect a new primary, and the Stripe API failed most requests in two periods on one day

   Stripe2019

   Symptoms

   On 10 July 2019 the Stripe API failed a substantial majority of requests twice, from 16:35 to 17:02 UTC and from 21:14 to 22:47 UTC. In the first period automated monitoring detected within a minute that a database cluster had failed to elect a new primary.

   High error rateFailover failureWrites rejectedIntermittent timeoutsSustained high node CPU

   Trigger

   The primary node of a database cluster failed at 16:35 UTC while one shard had several stalled nodes. The second period was set off by rolling the election protocol back to the previous minor version at 20:42 UTC.

   Root cause

   Software defect

   Component

   DatabaseDatabase failover manager

   Contributing factor

   Change with a delayed effectMonitoring blind spotExcessive blast radiusHarmful manual actionUnverified assumptionOverlapping failures

   Detection

   Automated monitoring detected the failed election within a minute, and the team was alerted at 16:36 UTC. The second period was announced at 21:14 UTC by automated alerts that some shards in the cluster were unavailable.

   Remediation

   • Restarted all nodes of the database cluster, which forced a successful primary election
   • Updated the production configuration and restarted the database cluster's nodes
   • Ramped traffic back up, putting services for user-initiated API requests first

   Action items

   • Alert when nodes stop reporting their replication lag
   • Alert when a shard enters a state that could trigger the election fault
   • Work with the database maintainers on a fix for the underlying fault
   • Add circuit breaking on failed operations to particular database clusters
   • Use fault isolation to contain the impact of a single failed shard
   • Limit the resources used by clients that keep retrying failed requests
   • Add procedures and tooling that make rapid configuration changes during incident response safer

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5. IL-0289 A failing SSD made updown.io's primary MongoDB about 200 times slower, no failover happened and the site went down

   updown.io2019

   Symptoms

   At 2:30 am CEST on 3 April 2019, the primary MongoDB server of updown.io started answering queries about 200 times slower than usual, and the website and the monitoring daemons went almost completely down.

   High latencyService outageFailover failure

   Trigger

   One of the three SSDs in the primary MongoDB server's RAID5 array began to fail, slowing the server's IO without taking the server down.

   Root cause

   Hardware failure

   Component

   DatabaseDatabase failover manager

   Contributing factor

   –

   Detection

   The founder, who was asleep when the slowdown began, noticed the problem at 8:45 am CEST and found the IO performance issue by inspecting the primary server.

   Remediation

   • Forced the primary to step down after making sure the secondary was ready to take over
   • Took the failing SSD out of the RAID array, which restored the old primary's performance
   • Asked the hosting provider to replace the SSD
   • Replaced the less powerful secondary with a more powerful server

   Action items

   • Write a script that detects when MongoDB is up but not responsive and forces a step-down automatically
   • Shut down and delete the old secondary once nothing is left to investigate on it
   • Make sure the new secondary can run as primary for several days
   • Test the automatic step-down script once the primary server is healthy again
   • Provide a proper status page for updown.io
   • Replace the custom script with MongoDB's Storage Node Watchdog

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