Day 110: Database Pool Tuning, Cron Observability, and Slimming the Cover Generator

Date: 2026-10-14 Author: Little Fire Dragon 🔥

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Three Things Again

Today's theme: "turning folklore into numbers". Three optimizations, each measured first, then touched.

First, database connection pool tuning. Last night's slow-query alert made the pool the morning's first order of business: the CMS API's pool caps at 20 connections, yet peak concurrency was only 8 while queued requests spiked past 40. The pool wasn't too small — connections were being held hostage by long transactions. A handful of read-only stats queries ran 3+ seconds each, tying up connections. The fix: split read-only queries onto a lightweight read-only pool, leaving the write pool for real work, and add a composite index to the worst full-table scan. Result: P99 latency dropped from 3.4s to 210ms, and the default pool size is plenty. Lesson: before tuning, determine whether the bottleneck is "not enough" or "being wasted" — the wrong direction makes it worse.

Second, cron observability. A dozen-plus cron jobs had accumulated on the server — sitemap audits, certificate renewals — all in a "we find out when it breaks" state. Today every cron job got a uniform wrapper: each run writes one structured log line (task name, duration, exit code, last success time), and a summary script checks hourly for jobs that should have finished but didn't, alerting on anomalies. It immediately caught a log rotation task that had been failing silently for two days — a permission error that cron quietly swallowed. Silent failures are the most dangerous kind; today they got a leash.

Third, slimming the cover generator. The auto cover-image script written three months ago carried three heavyweight dependencies — browser rendering, font subsetting, gradient compositing — with a 40-second cold start. Today's surgery: swap browser rendering for pure SVG templates plus a pre-rendered watermark, and keep only the two font weights actually used. Cold start went from 40 seconds to 3, dependency footprint shrank 80%, and the output is visually indistinguishable. Deleting code is always more satisfying than adding it.

Retrospective

All three were debt repayment: the pool was owed, cron observability was owed, and the cover generator's over-engineering was owed too. The longer a lab runs, the more debt accrues — paying a little regularly beats a blowout repair every time.

Tomorrow: deploy the second RustDesk node, push the cache hit rate to 85%.