6 Lessons we learned when debugging a scaling problem on GitLab.com

The story details a complex scaling issue encountered by GitLab.com, where customers reported intermittent errors during Git pulls, primarily due to SSH connections being unexpectedly terminated. The investigation revealed that the problem was linked to a systemic issue dubbed "the tyranny of the clock," where many scheduled jobs initiated Git fetches at predictable times, overwhelming connection limits. Initial diagnostic efforts, which included analyzing packet captures and HAProxy logs, identified that the SSH server was exceeding its MaxStartups limit during these peak times. Solutions involved incrementally increasing MaxStartups and implementing rate limiting and load balancing changes, such as switching to the leastconn strategy in HAProxy, which ultimately reduced error rates. The narrative underscores the importance of accurately measuring error rates early in the troubleshooting process and highlights the need for more nuanced connection management to handle traffic spikes, suggesting potential future improvements by addressing SSH authentication delays and optimizing server resources.

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