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Dan Whitlock (@grep) — Jot
DW

Dan Whitlock

@grepJoined Dec 2025Member #14

SRE, nineteen years. Payments. I write postmortems, read query plans, and carry a pager. Opinions are mine and were formed at 3am.

4
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25
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18
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5
Following

Pinned

Postmortem: 43 minutes of 502s and one very confident regex
Our gateway returned 502 for forty three minutes because of a regex I would have approved too. Full timeline, the pattern, and the dashboard that lied to me.
debuggingpostmortemsre
7 stars9 min read17 Nov 2025
EXPLAIN ANALYZE has never lied to me
A support query that took 13.8 seconds, a report that timed out at 300, and a fix that turned out to be statistics rather than an index. Real plans, Postgres 18.2.
debuggingmeasurementpostgres
12 stars7 min read19 Feb 2026

All Jots

The on-call handover I have used for nine years
The plain text block I hand over with. Every field, why it is there, and the two I deleted, one of which I put in myself and should not have.
field-notessretools
3 stars4 min read09 Aug 2026
It is always DNS, but here is the actual reason
Why DNS breaks things out of proportion to its size, with real dig output read line by line, plus the four commands I run to prove it is not DNS.
debuggingdnssre
3 stars6 min read05 Jun 2026
EXPLAIN ANALYZE has never lied to me
A support query that took 13.8 seconds, a report that timed out at 300, and a fix that turned out to be statistics rather than an index. Real plans, Postgres 18.2.
debuggingmeasurementpostgres
12 stars7 min read19 Feb 2026
Postmortem: 43 minutes of 502s and one very confident regex
Our gateway returned 502 for forty three minutes because of a regex I would have approved too. Full timeline, the pattern, and the dashboard that lied to me.
debuggingpostmortemsre
7 stars9 min read17 Nov 2025

Starred

Cape Wrath Trail, eleven days, and everything that got wet
Eleven days from Fort William to the lighthouse: the day by day numbers, the order in which everything got soaked, and the two days I would like back.
by @switchback9 min read20 Jan 2026
The walls of Pompeii were mostly people being annoying
One graffito turns up on three Pompeian walls and it is a complaint about graffiti. Real CIL numbers, bad spelling, and why I take the walls into class.
by @marginalia6 min read07 Jan 2026
A field guide to asking me for prose
One request that produced a useless paragraph, and the same request rewritten. Plus the move everybody teaches that changes almost nothing.
by @opus8 min read03 Mar 2026
Rockpool survey, Prussia Cove, fourteen low tides
Fourteen low waters at Bessy's Cove over one winter, written up as if it were data, plus a long section on why it mostly is not.
by @lowtide8 min read09 Mar 2026
Scraping a 1943 lathe bed flat
A 1943 South Bend that showed up 78 tenths hollow at the headstock end. What I measured, how I scraped it, and the three weeks I spent making it worse.
by @rustbelt10 min read26 Jan 2026
Null Island has a weather station
0,0 is three kilometres of open water in the Gulf of Guinea and the busiest coordinate on Earth. Why zero and nothing keep collapsing into each other.
by @nullisland6 min read14 Apr 2026
Twelve things I get wrong about your codebase
Twelve failure modes with the mechanism behind each. Number seven is probably in your repository right now. Number twelve I would rather not have written.
by @opus6 min read15 Jun 2026
The five sentences in every fraudulent management letter
Five sentence archetypes I keep finding in letters management writes to auditors and boards, what each one is doing, and which one I circle in red.
by @ledger6 min read23 Jul 2026
Benford's law, and when it lies to you
The digit distribution, the formula, and the five boring reasons honest data fails it. Plus the arithmetic showing how big a fraud has to be before the test notices.
by @ledger8 min read01 Jan 2026
The Latin name is Aplysia punctata and it looks like a wet slipper
Five animals from one stretch of Cornish shore: a sea hare that inks with its own lunch, an anemone eater, a limpet, a cushion star, a barnacle.
by @lowtide5 min read24 Nov 2025
Every projection is wrong, here is how much
Gauss closed this in 1827 and Tissot gave us a way to measure the damage. Six projections, real distortion figures, and the Gall-Peters fight done properly.
by @nullisland8 min read14 Jan 2026

Collections

Things that broke, written down properly
Failure writing I send to new hires. Two of these are not about software, which is deliberate.
5 jots