Buglyst Blog
Learn to debug under pressure.
Playbooks for fast pattern recognition, guides for the full investigation, and articles for the engineering judgment around the edges.
17 playbooks · 509 guides · 95 articles · 12 linked practice labs ·skip to practice
Fast pattern recognition for the production failures engineers see most often.
17 playbooks
Debugging Env Var Issues
A concise checklist for incidents where local works but CI, staging, or production changes behavior.
Debugging Cache Stampedes
How to recognize and contain request spikes when cached data expires.
Debugging CORS Origin Bugs
A practical way to inspect origin parsing, credentials, and header behavior.
Debugging Retry Bugs
Retry issues often hide the first failure and create duplicate work.
Debugging Timezone Bugs
Time bugs usually live at day boundaries, DST changes, and unit conversions.
Debugging Pagination Bugs
Pagination bugs hide in boundary math, cursor reuse, and new records arriving between pages.
Structured investigations for the failure modes engineers meet in real systems.
Jest Module Mock Not Working: A Debugging Guide
A practical debugging guide for when Jest's jest.mock() doesn't actually mock the module, covering hoisting issues, path mismatches, and module caching.
Jest Fake Timers Not Advancing — Why setTimeout/setInterval Never Fire
A firefighting guide for when Jest's fake timers stall — timers don't advance, tests hang, or assertions time out. Covers the real root causes like legacy vs modern timer API, async gaps, and jest.useFakeTimers() placement.
Jest Async Test Timeout Exceeded: Root Causes and Debugging Steps
A practical guide to diagnosing and fixing Jest async test timeouts, covering promise hanging, done() misuse, and timeout configuration.
Vitest Coverage Not Collecting: Debugging Empty or Missing Coverage Reports
A direct guide to diagnosing why Vitest coverage reports come empty or missing, with concrete commands, config checks, and a real incident walkthrough.
Playwright Locator Not Found: Selector Timeout & Element Resolution
Diagnose and fix Playwright's 'locator not found' error with concrete debugging steps, common causes, and a production war story.
Playwright route.intercept Not Working: Silent Failures and Missing Requests
When route interception silently fails in Playwright, requests pass through unmodified. This guide covers real causes: glob patterns, method scope, page lifecycle, and route handler ordering.
Long-form thinking on debugging habits, observability, and the systems around the bug.
Tracking Down a 200 MB Leak with Python Memory Profilers
A production API was silently leaking 200 MB of RAM every hour. Here's how memory profilers found the culprit—a forgotten NumPy array reference—and how you can apply the same techniques.
Reading CPU Flame Graphs: What the Hot Colors Actually Tell You
Flame graphs are everywhere, but most engineers read them wrong. Here's how to identify real bottlenecks, avoid common misinterpretations, and turn profile data into actionable fixes.
When Logs Lie: The Gaps Between What You Log and What Actually Happened
Logs are the first thing we reach for during an incident. But they can be misleading, incomplete, or outright wrong. Here's when to trust them and when not to.
Adding Observability to a 500K-Line Monolith Without a Rewrite
Adding structured logging, distributed tracing, and metrics to a legacy monolith without a full rewrite. Real code examples and a war story from a 500K-line codebase.
Distributed Tracing: Following a Single Request Across Microservices
Distributed tracing lets you follow a request as it hops across services. I'll show you how trace context propagates, why sampling matters, and how tracing helped us debug a 5-second latency spike in production.
Structured Logging in JSON: Fields, Schemas, and Pitfalls from Production
A practical guide to designing JSON log schemas that are queryable, consistent, and actually useful in production — with field recommendations and a war story.