Cases3 cases

1 to 3 of 3

1. IL-0441 A ZOZO SQL Server database stalled and applications hit timeout storms whenever batch executions peaked in sales

   ZOZO2021

   Symptoms

   From around mid-2020, a SQL Server database behind one ZOZO feature made the applications throw many timeout errors whenever batch executions per second passed a certain threshold.

   Intermittent timeoutsDatabase overloadLock contention or deadlock

   Trigger

   Traffic increases during sales and lucky-bag launches pushed batch executions per second on the database above a threshold.

   Root cause

   unknown

   Component

   Database

   Contributing factor

   –

   Remediation

   • Fixed the plan guides of the four stored procedures whose compilation waits caused blocking
   • Disabled automatic statistics updates on the affected tables during periods of expected high traffic
   • Enabled automatic statistics updates again once traffic calmed down
   • Compressed the large indexes held in the data cache to use memory more efficiently

   Lessons

   When the root cause of a failure cannot be found, choosing among the things observed during the failure those that can be removed or reduced, and acting on them, can still create a state in which the failure no longer happens.

   View case
2. IL-0387 Mailgun MongoDB secondaries were overloaded by rising connections, and a rollback then moved reads onto the primary too

   Mailgun2017

   Symptoms

   On 12 January 2017 at 14:09 UTC, Mailgun engineers began responding to alerts for the click and open tracking services and the primary MongoDB clusters in one production region. Within ten minutes the engineers found that the MongoDB secondary servers, which serve most reads, were under heavy load and slow to answer, which also caused the tracking errors. The status page reported intermittent timeouts on the Control Panel, API, SMTP and tracking links.

   Database overloadIntermittent timeoutsHigh error rate

   Trigger

   A gradual increase in connections to the MongoDB secondary servers, which coincided with several MongoDB configuration changes made the day before. A rollback of those changes then also introduced, by mistake, a change that moved the API and SMTP services from the secondaries to the primary.

   Root cause

   unknown

   Component

   Database

   Contributing factor

   Harmful manual actionSingle point of failure

   Detection

   Alerts for the click and open tracking services and the primary MongoDB clusters at 14:09 UTC.

   Remediation

   • Rolled back the previous day's MongoDB configuration changes through several pull requests from 15:15 UTC
   • Resized the MongoDB servers to handle more connections
   • Deployed connection limits at the MongoDB server level
   • Limited the number of connections the services could open to the MongoDB primary
   • Redeployed the configuration that makes services prefer secondary servers for reads

   Action items

   • Run the tracking infrastructure across two independent regions
   • Limit the number of connections every service can open to the database
   • Permanently increase the size of the primary Mongo cluster and roll the change out to the other data centers
   • Let click redirection keep working when downstream services fail

   View case
3. IL-0393 Load on the BBC programme metadata database surged and metadata requests failed, leaving BBC iPlayer down over a weekend

   BBC2014

   Symptoms

   At 9.30 on Saturday 19 July 2014, the load on the database of the BBC system that provides programme and clip metadata rose sharply, and many requests for metadata to its application servers started to fail. BBC iPlayer, BBC iPlayer Radio and audio and video playback on other parts of bbc.co.uk were affected.

   Database overloadHigh error rateService outage

   Root cause

   unknown

   Component

   Database

   Contributing factor

   No cachingOverlapping failures

   Remediation

   • Restored the failed pool of the caching layer first
   • Switched the BBC homepage to its emergency mode until the cache was stable
   • Did significant remedial work on the metadata system from Saturday afternoon into the evening
   • Ran the service in a reduced state through the weekend and did the full restoration on Monday morning

   Action items

   • Complete the forensics on the root causes and put measures in place to reduce the chance of similar interruptions

   View case

By root cause1