Cases2 cases

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1. IL-0455 Calls from an AWS service through NAT Gateway to an on-premises service returned 5xx errors every busy weekend

   Woowa Brothers2020

   Symptoms

   5xx errors rose on API requests from service A, running in AWS, to service B, running on premises, and user requests failed. At the same time the L4 session count of service B went up.

   High error rateOutbound connection exhaustion

   Trigger

   High weekend traffic from service A, which called service B without a connection pool, through the NAT gateways.

   Root cause

   Capacity exhaustion

   Component

   NAT gatewayOn-premises load balancer

   Contributing factor

   Unknown limit

   Detection

   Rising 5xx errors on service A and a rising L4 session count on service B.

   Remediation

   • Added service A instances, which lowered but did not end the errors
   • Replaced the service A instances, which brought errors and the L4 session count back to normal

   Action items

   • Switch service A to a connection pool
   • Find the services that use NAT Gateway between AWS and the IDC and switch them to connection pools

   Lessons

   The fix is often easy to find once the cause is analysed, but without confirming the root cause a service keeps running with the risk still in it, so Woowa Brothers treats finding the root cause as essential for preventing recurrence, alongside resolving the failure first.

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2. IL-0454 Baemin billing servers could not open new MariaDB connections at the start of a card-discount event

   Woowa Brothers2016

   Symptoms

   At 18:00, when a card-company event giving 3,000 won off to the first 1,000 customers started, the Pinpoint graph of the Baemin billing system turned red shortly afterwards: 140 of 14,000 requests failed in about one minute. Most errors looked like requests from the billing system that the payment gateway (PG) did not receive because of a network problem, but other errors pointed at the billing system itself.

   High error rateIntermittent connection failures

   Trigger

   A card-company discount event for the first 1,000 customers started at 18:00 and sent about 14,000 requests to the billing system in about one minute.

   Root cause

   Software defect

   Component

   DatabaseClient-side DNS resolver and cache

   Contributing factor

   Unpatched known bug

   Detection

   The team was watching NewRelic and Pinpoint during the 18:00 event, and the Pinpoint graph turned red shortly after the event began.

   Remediation

   • Changed the Gradle settings to use mariadb-connector-j 1.5.5 instead of 1.4.6

   Lessons

   A new DB connection failed because the driver did a DNS lookup of the local host name that took about 6 seconds, longer than the server's 5-second connect_timeout. Other ways to avoid it are to use localhost as the host name or to register the IP address and host name in /etc/hosts so the address is returned first. Above all, the author recommends keeping libraries on recent versions.

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