Why has the cache hit ratio for Fastly's Image Optimizer service dropped by 15% over the past week?

Introduction

The recent 15% drop in cache hit ratio for Fastly's Image Optimizer service over the past week is a critical issue that demands immediate attention. This metric directly impacts service performance, user experience, and operational costs. I'll approach this problem systematically, focusing on identifying the root cause, validating hypotheses, and developing both short-term fixes and long-term solutions.

Step 1

Clarifying Questions (3 minutes)

• Looking at the timing, I'm thinking there might have been a recent change. Has there been any deployment or configuration update in the past week?

Why it matters: Recent changes often correlate with performance shifts. Expected answer: Yes, there was a minor update. Impact on approach: If yes, we'd focus on that change; if no, we'd look at external factors.

• Considering user behavior, have we seen any unusual traffic patterns or spikes recently?

Why it matters: Abnormal traffic could strain the system and affect cache performance. Expected answer: Traffic has been within normal ranges. Impact on approach: If abnormal, we'd investigate traffic sources; if normal, we'd look at internal systems.

• Regarding content, has there been a shift in the types or sizes of images being processed?

Why it matters: Different image characteristics can impact caching efficiency. Expected answer: No significant change in image types or sizes. Impact on approach: If changed, we'd analyze new content patterns; if not, we'd focus on system issues.

• Thinking about infrastructure, have there been any changes in our CDN setup or cache configuration?

Why it matters: CDN changes could directly affect cache hit ratios. Expected answer: No recent CDN changes. Impact on approach: If changed, we'd review CDN setup; if not, we'd look at application-level issues.

• Considering monitoring, are we confident in the accuracy of our cache hit ratio measurement?

Why it matters: Ensures we're solving a real problem, not a measurement issue. Expected answer: Monitoring systems are reliable and unchanged. Impact on approach: If unreliable, we'd audit monitoring systems; if reliable, we'd proceed with deeper analysis.