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Example Postmortem from SRE book, pp. 487-491

Shakespeare Sonnet++ Postmortem (incident #465)

Date

2015-10-21

Authors

  • jennifer
  • martym
  • agoogler

Status

Complete, action items in progress

Summary

Shakespeare Search down for 66 minutes during period of very high interest in Shakespeare due to discovery of a new sonnet.

Impact

Estimated 1.21B queries lost, no revenue impact.

Root Causes

Cascading failure due to combination of exceptionally high load and a resource leak when searches failed due to terms not being in the Shakespeare corpus. The newly discovered sonnet used a word that had never before appeared in one of Shakespeare's works, which happened to be the term users searched for. Under normal circumstances, the rate of task failures due to resource leaks is low enough to be unnoticed.

Trigger

Latent bug triggered by sudden increase in traffic.

Resolution

Directed traffic to sacrificial cluster and added 10x capacity to mitigate cascading failure. Updated index deployed, resolving interaction with latent bug. Maintaining extra capacity until surge in public interest in new sonnet passes. Resource leak identified and fix deployed.

Detection

Borgmon detected high level of HTTP 500s and paged on-call.

Action Items

Action Item Type Owner Bug
Update playbook with instructions for responding to cascading failure mitigate jennifer n/a DONE
Use flux capacitor to balance load between clusters prevent martym Bug 5554823 TODO
Schedule cascading failure test during next DiRT process docbrown n/a TODO
Investigate running index MR/fusion continuously prevent jennifer Bug 5554824 TODO
Plug file descriptor leak in search ranking subsystem prevent agoogler Bug 5554825 DONE
Add load shedding capabilities to Shakespeare search prevent agoogler Bug 5554826 TODO
Build regression tests to ensure servers respond sanely to queries of death prevent clarac Bug 5554827 TODO
Deploy updated search ranking subsystem to prod prevent jennifer n/a DONE
Freeze production until 2015-11-20 due to error budget exhaustion, or seek exception due to grotesque, unbelievable, bizarre, and unprecedented circumstances other docbrown n/a TODO

Lessons Learned

What went well

  • Monitoring quickly alerted us to high rate (reaching ~100%) of HTTP 500s
  • Rapidly distributed updated Shakespeare corpus to all clusters

What went wrong

  • We're out of practice in responding to cascading failure
  • We exceeded our availability error budget (by several orders of magnitude) due to the exceptional surge of traffic that essentially all resulted in failures

Where we got lucky

  • Mailing list of Shakespeare aficionados had a copy of new sonnet available
  • Server logs had stack traces pointing to file descriptor exhaustion as cause for crash
  • Query-of-death was resolved by pushing new index containing popular search term

Timeline

2015-10-21 (all times UTC)

Time Description
14:51 News reports that a new Shakespearean sonnet has been discovered in a Delorean's glove compartment
14:53 Traffic to Shakespeare search increases by 88x after post to /r/shakespeare points to Shakespeare search engine as place to find new sonnet (except we don't have the sonnet yet)
14:54 OUTAGE BEGINS -- Search backends start melting down under load
14:55 docbrown receives pager storm, ManyHttp500s from all clusters
14:57 All traffic to Shakespeare search is failing: see http://monitor/shakespeare?end_time=20151021T145700
14:58 docbrown starts investigating, finds backend crash rate very high
15:01 INCIDENT BEGINS docbrown declares incident #465 due to cascading failure, coordination on #shakespeare, names jennifer incident commander
15:02 someone coincidentally sends email to shakespeare-discuss@ re sonnet discovery, which happens to be at top of martym's inbox
15:03 jennifer notifies shakespeare-incidents@ list of the incident
15:04 martym tracks down text of new sonnet and looks for documentation on corpus update
15:06 docbrown finds that crash symptoms identical across all tasks in all clusters, investigating cause based on application logs
15:07 martym finds documentation, starts prep work for corpus update
15:10 martym adds sonnet to Shakespeare's known works, starts indexing job
15:12 docbrown contacts clarac & agoogler (from Shakespeare dev team) to help with examining codebase for possible causes
15:18 clarac finds smoking gun in logs pointing to file descriptor exhaustion, confirms against code that leak exists if term not in corpus is searched for
15:20 martym's index MapReduce job completes
15:21 jennifer and docbrown decide to increase instance count enough to drop load on instances that they're able to do appreciable work before dying and being restarted
15:23 docbrown load balances all traffic to USA-2 cluster, permitting instance count increase in other clusters without servers failing immediately
15:25 martym starts replicating new index to all clusters
15:28 docbrown starts 2x instance count increase
15:32 jennifer changes load balancing to increase traffic to nonsacrificial clusters
15:33 tasks in nonsacrificial clusters start failing, same symptoms as before
15:34 found order-of-magnitude error in whiteboard calculations for instance count increase
15:36 jennifer reverts load balancing to resacrifice USA-2 cluster in preparation for additional global 5x instance count increase (to a total of 10x initial capacity)
15:36 OUTAGE MITIGATED, updated index replicated to all clusters
15:39 docbrown starts second wave of instance count increase to 10x initial capacity
15:41 jennifer reinstates load balancing across all clusters for 1% of traffic
15:43 nonsacrificial clusters' HTTP 500 rates at nominal rates, task failures intermittent at low levels
15:45 jennifer balances 10% of traffic across nonsacrificial clusters
15:47 nonsacrificial clusters' HTTP 500 rates remain within SLO, no task failures observed
15:50 30% of traffic balanced across nonsacrificial clusters
15:55 50% of traffic balanced across nonsacrificial clusters
16:00 OUTAGE ENDS, all traffic balanced across all clusters
16:30 INCIDENT ENDS, reached exit criterion of 30 minutes' nominal performance

Supporting Information

@steve-dupuis

This comment has been minimized.

steve-dupuis commented Sep 27, 2016

Hi guys ..
Some quick questions - Where can I get a copy of Example Postmortem from SRE book
Do you have a tool that creates the dated items in the Timeline table?
Thanks,
Steve Dupuis, Ottawa Canada

@mlafeldt

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Owner

mlafeldt commented Sep 30, 2016

@steve-dupuis There's https://github.com/etsy/morgue, but I haven't used it myself.

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