0.3.1 — ask the index first, which answers in one request

The chain asked OpenLibrary's edition record first and its search index
second. The edition record is the slow one: measured against this
library it takes five to twenty-eight seconds, and it names its authors
by key, so a second request was needed anyway. The index, asked for its
fields by name, returns the title, the authors, the publisher, the
language, a page count and the subjects together, in about a quarter of
a second.

Reordering them turns three requests per book into one, and a pass over
five thousand books from fifty-three hours into well under one. The
edition record is still there for a subtitle or a publisher the index
did not give.

crates/bibfetch/src/source.rs

@@ -55,9 +55,17 @@ impl Source {
         match kind {
             "doi" => &[Source::Crossref],
             "arxiv" => &[Source::Arxiv],
+            // The index first. Asked for the fields by name it answers with
+            // the title, the authors, the publisher, the language, a page
+            // count and the subjects in one request, which is the whole of
+            // what a catalogue wants; the edition record is then needed only
+            // for a subtitle or a publisher the index did not give. Asking it
+            // second cost three requests per book where one usually does, and
+            // at three books in flight that is the difference between two
+            // days and two hours.
             "isbn" => &[
-                Source::OpenLibrary,
                 Source::OpenLibrarySearch,
+                Source::OpenLibrary,
                 Source::GoogleBooks,
             ],
             _ => &[],