Incidents/2023-11-15 esams unreachable

== Summary ==
ESAMS DC was unreachable by users. Globally, we experienced a request drop from ~150k req/s to ~91k req/s as outlined in the Grafana dashboard. Users that were trying to reach Amsterdam DC experienced network errors and delays.
The incident doc still mentioned "We’re still investigating why some Grafana panels shows ~1hr of requests drop instead of ~10m (the actual incident duration)"
==Timeline==
* 14:03 <XioNoX> !log reboot fpc0 on cr1-esams - T346779 (SAL)
* 14:04 <XioNoX> I restarted the linecard on the wrong router… Outage starts
* 14:05 Start receiving alerts, e.g. <+icinga-wm> PROBLEM - Host ncredir3003 is DOWN: PING CRITICAL - Packet loss
* 14:06 <vgutierrez> let's depool esams?
* 14:07 <jynus> vgutierrez: it is recovering now
* 14:07 <XioNoX> looks like it's back faster than a depool is needed
* 14:08 <vgutierrez> I can't reach text-lb.esams.wikimedia.org from here FWIW
* 14:08 depool esams https://gerrit.wikimedia.org/r/c/operations/dns/+/974537/
* 14:08 DEPOOL FAILED due to authends-update issues
* 14:09 <+icinga-wm> started to see recoveries RECOVERY
* 14:09 <sukhe> /tmp/dns-check.rudydea2/zones/netbox/4.64.10.in-addr.arpa
* 14:09 <sukhe> we need to fix this to fix authdns-update
* 14:10 <topranks> let's just merge https://gerrit.wikimedia.org/r/c/operations/dns/+/974534
* ??:?? <XioNoX> text-lb is reachable again
* 14:11 <jynus> NEL is still high
* 14:16 marostegui updates the status page
* 14:17 <jynus> http traffic volume is still elevated, though
* 14:18 <jynus> others parameters look back to normal, including NEL timeouts
* 14:21 <+jinxer-wm> (NELHigh) resolved: (2) Elevated Network Error Logging events Outage ends
* 14:25 Updated status page to monitoring
* 14:26 Incident opened; fabfur becomes IC.
* 14:35 Status page set to resolved
2023-11-15 esams unreachable - NEL.png
2023-11-15 esams unreachable - varnish.png
2023-11-15 esams unreachable - webreqs.png
* Network Error Logging
* Webrequests live dashboard
==Detection==
The outage was detected by an SRE shortly after an accidental change. Fired alerts confirmed the outage as well:
<+icinga-wm> PROBLEM - Host ncredir3003 is DOWN: PING CRITICAL - Packet loss
Did the appropriate alert(s) fire? Was the alert volume manageable? Did they point to the problem with as much accuracy as possible?
==Conclusions ==
OPTIONAL: General conclusions (bullet points or narrative)
===What went well?===
* …
OPTIONAL: (Use bullet points) for example: automated monitoring detected the incident, outage was root-caused quickly, etc
===What went poorly?===
* …
OPTIONAL: (Use bullet points) for example: documentation on the affected service was unhelpful, communication difficulties, etc
===Where did we get lucky?===
* …
OPTIONAL: (Use bullet points) for example: user's error report was exceptionally detailed, incident occurred when the most people were online to assist, etc
==Links to relevant documentation==
* Depooling servers
==Actionables==
* …
Create a list of action items that will help prevent this from happening again as much as possible. Link to or create a Phabricator task for every step.
Add the #Sustainability (Incident Followup) and the #SRE-OnFire Phabricator tag to these tasks.
==Scorecard==
Incident Engagement ScoreCard
 	Question 	Answer
(yes/no)	Notes
People 	Were the people responding to this incident sufficiently different than the previous five incidents? 	 	
People 	Were the people who responded prepared enough to respond effectively 	yes	
People 	Were fewer than five people paged? 	 	
People 	Were pages routed to the correct sub-team(s)? 	 	
People 	Were pages routed to online (business hours) engineers?  Answer “no” if engineers were paged after business hours.	 	
Process 	Was the "Incident status" section atop the Google Doc kept up-to-date during the incident? 	 	
Process 	Was a public wikimediastatus.net entry created? 	yes	Status page entry
Process 	Is there a phabricator task for the incident? 	 	
Process 	Are the documented action items assigned? 	 	
Process 	Is this incident sufficiently different from earlier incidents so as not to be a repeat occurrence? 	 	
Tooling 	To the best of your knowledge was the open task queue free of any tasks that would have prevented this incident? Answer “no” if there are
open tasks that would prevent this incident or make mitigation easier if implemented.	no 	
Tooling 	Were the people responding able to communicate effectively during the incident with the existing tooling? 	yes	
Tooling 	Did existing monitoring notify the initial responders? 	yes	
Tooling 	Were the engineering tools that were to be used during the incident, available and in service? 	yes	
Tooling 	Were the steps taken to mitigate guided by an existing runbook? 	yes	Relevant runbook
Total score (count of all “yes” answers above)	Total score (count of all “yes” answers above)