The entry said to measure first and put the number in the commit, so: a plain page
renders in 14µs, a twelve-picture gallery in 1.23ms. Of that, ~102µs per picture
was reading, hashing and decoding bytes the previous request had already read.
Remembering that one fact — keyed by path, size and modification time — brings the
same gallery to 63µs. 19.5× faster, 21× fewer bytes allocated, twenty-odd lines.
After which nothing is slow enough to justify caching whole pages, so ADR-0044
declines the page cache and leaves the parked validity model parked, now with a
measurement rather than an intuition behind its trigger.
That parked model has five axes and was written before any code existed. The
problem it would have been built for turned out to be one repeated file read.
Benchmarks live in internal/web so they measure through the real handler, which is
also what conventions.md wants before any cache goes in the render path. The
invalidation risk has its own test: an edited picture is a different key, so the
memo cannot serve yesterday's dimensions. Everything runs clean under -race, since
the map is read by concurrent requests.
A pass over the content at startup writes three widths per picture into a cache
outside the site root, named by the source's content hash and the width (ADR-0042).
Idempotent by construction: a rerun stats and skips, an edited picture takes a new
name, and nothing stale can be served under an old one. Restarting the evidence
site made 0 derivatives the second time, as it should.
Ahead of the request rather than during it, because resampling is felt and there is
no page cache yet to hide it. Outside the site root, because the engine reads that
directory and must not leave generated files in somebody's content git — a lost
cache costs one startup pass and no correctness.
Markup now carries the original as src, the derivatives as srcset closed by the
original at its own width, and width/height from the original — which retires most
of the latent row about the output floor; only a gallery's alt is still empty, and
a filename cannot supply that.
Two things the work itself decided:
`Fragment.Items` became `Fragment.Pictures`, ADR-0037's own revisit trigger. Items
had one consumer, so widening it beat adding a second list beside it.
"A browser can show it" and "we can resample it" are different questions, and
conflating them nearly deleted content: an SVG has no decoder here, so a single
predicate would have dropped SVGs from galleries silently. Undecodable and
unsupported pictures are now rendered as they are, without a size or a srcset.
bdeshi