`bibfetch::cover(&agent, isbn, cover_id, size)` asks Open Library's covers
store by the ISBN's thirteen-digit form, then its ten-digit form, then by a
cover id a lookup record named, and returns the image or `Ok(None)`. The
answer is checked before it is trusted: JPEG or PNG by content type and by
magic bytes, between 1 KB and 10 MB, at most three redirects, none of them
off HTTPS. One request at a time per process; a 429 or 503 is waited out
for its Retry-After, up to a minute.
`idnorm::isbn` converts between the two forms, and only for an ISBN whose
check digit agrees. Lookup records carry `cover_id` from the edition record
and the search index. The CLI gains `bibfetch cover <isbn> -o <file>`.
An ISBN is not a DOI. The body that assigns a DOI is the only one that has
it, so one register is the whole answer. A book's identifier is assigned by
its publisher and recorded by whoever gets round to it, and the record that
comes back is as often as not missing the thing the caller wanted: asked
about 9780199205592, OpenLibrary's edition record names *The First World
War* and the press that printed it, and nobody at all as its author, because
an edition record names its authors by key.
So an ISBN now has a chain rather than a register. The edition record first,
then OpenLibrary's own index — which answers with `author_name` filled in
and costs nothing — and Google Books last, because it answers anonymous
callers only until a daily quota runs out, after which it replies 429 to
everyone behind the same address, as it is doing here today.
The first complete answer wins, an incomplete one is kept in case nothing
better comes, and a register being down is a failure only when every
register is. `source` on the record says which one answered.
colophon comes with it: the imprint reader was taking `Oxford is a
registered trade mark of Oxford University Press` for a publisher and
`Oxford N`, off `Oxford New York`, for a place. A legal notice mentioning a
press is not the press, and a city is the city and not the first letter of
the next one.
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