Cases2 cases

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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.

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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

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