bookshelf/docs/METADATA-SOURCES.md

Book metadata lookup — why scans miss, and what else we could ask

Research note, 2026-09-09. Written in response to "out of the 3 barcodes I've scanned, only 1 has been discovered properly."

Nothing in here has been implemented. SPEC's two-source design (Open Library primary, Google Books fallback) is unchanged. This is the evidence for deciding whether to change it.

Update, same day — the coverage hypothesis is dead. The user supplied the two ISBNs that failed: 9781883937386 (The Hittite Warrior) and 9781883937676 (Shadow Hawk), both Bethlehem Books. Both are fully present in Open Library — title, author, publisher, page count and cover art — and the app's own parser handles both real responses correctly (regression test: OpenLibraryClientTest.parses the real responses for the two books the app failed to identify). Whatever went wrong on the phone was upstream of the metadata sources. Everything below still holds as background, but do not act on "add a third source" until we know why a book Open Library has did not reach the lookup. See What actually failed.

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Short version

Three separate defects were making lookups look far worse than the underlying data actually is, and all three are now fixed (commit 356f639). They are not the same problem as "this book isn't in the database":

1. A cover that loaded fine still rendered as nothing, so a successful lookup looked like a failed one. That alone could account for the book you did find appearing broken.
2. Open Library's cover URL was synthesized for every book whether or not art existed, and a missing cover comes back as a 200 with a 43-byte 1×1 transparent GIF — a "successful" load that paints nothing.
3. Because that synthesized URL was never blank, the merge rule could never fall through to Google Books' thumbnail. The documented fallback was dead code for covers.

What is left is a real coverage question, and there the measurements point at one thing above all others: the Google Books fallback is probably not answering at all. Every keyless request from this machine returned HTTP 429, and the app turns any non-200 into null, which the UI presents as "No match found — enter the details by hand." A rate-limited lookup and a book that genuinely exists nowhere are, right now, indistinguishable to both you and me.

My recommendation is to fix the diagnosis before buying more data. Details in Recommendation.

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What I measured

Sample. 60 ISBNs drawn from the Harvard Library catalog — deliberately a third party, so the sample doesn't presuppose the answer by coming from one of the two sources under test. Ten publishers, weighted toward the small Catholic and homeschool presses that a MODG family's shelf actually carries (Ignatius, TAN, Sophia Institute, Bethlehem Books, Baronius) alongside mainstream trade (Penguin, Random House, Scholastic, Crossway, Loyola).

Method. Direct HTTP against each API, one ISBN at a time, 1.2 s apart. A source "hits" only if it returns a usable title.

Results

Source	Hit	Miss	Error	Hit rate
Open Library Books API (what the app calls today)	53	4	3	88%
Google Books, keyless (the app's fallback)	0	0	60 × HTTP 429	0%
Harvard LibraryCloud	56	4	0	93% *
Open Library cover art exists for the ISBN	47	13	0	78%

* Harvard is where the sample came from, so its number is inflated by construction. It's here to show the API works and answers by ISBN-13, not as a fair comparison.

Two further observations from the same runs:

• Concurrency is punished. The same 60 ISBNs run six-at-a-time dropped Open Library from 88% to 70%, entirely through transport errors. The app makes one request per scan, so this doesn't bite in normal use — but it does mean "Open Library missed" in a log is not proof the book is absent. By the end of this research my own IP was refused outright for a while.
• 17% of successful Open Library lookups have no cover art at all (9 of 53). Even with everything working, roughly one book in six will legitimately show the placeholder. That is a data fact, not a bug, and it's worth knowing before you read a placeholder as a failure.

What this does not tell us

Worth saying plainly, because it bounds how much weight the numbers carry:

• n = 60, and the sample comes from a research library. It under-represents recent mass-market paperbacks, reprints and print-on-demand editions — which is exactly where Open Library is thinnest. Real shelf coverage is probably below 88%.
• Every request came from a datacenter IP. The Google Books 429 may partly be this host sharing a quota pool with other tenants; your phone, on a residential or mobile IP, may well get answers. That's precisely why the app needs to be able to tell us which it got.
• I don't know which three ISBNs you scanned. If you still have the books to hand, those three numbers are worth more than another 60 sampled ones.

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The options

A. Give Google Books an API key

Free, 1,000 requests/day, no billing account required. Turns the fallback from "silently 429" into a working source. Roughly a dozen lines: a key in local.properties → BuildConfig → &key= on the query.

The key ships inside the APK and can be extracted, so restrict it to the Books API in the Google Cloud console. At our volume, someone stealing it costs us nothing but the quota.

Effort: hours. Cost: free. Likely the single biggest win.

B. Tell the difference between "not found" and "couldn't ask"

Both clients collapse every non-200, timeout and parse failure into null, and MetadataRepository collapses that into "no match", and the UI writes "No match found." A book that's offline, rate-limited, or hit a 500 is reported to you as a book that does not exist.

Distinguishing these gets you a retry button instead of a manual-entry form, and gets me a real answer next time you say "it missed."

Effort: half a day. Cost: free. Do this regardless of what else we choose.

C. Retry with backoff

One retry on 429/5xx, a couple of seconds apart. Standing at a bookshelf, a two-second retry is invisible; a manual-entry form is not.

Effort: an hour. Cost: free.

D. Add a third free source

Only worth doing after A–C, when we can see what's actually still missing. Ranked by what I'd try first:

Source	Key?	Notes
Open Library search.json	No	Searches the whole OL index rather than the edition table the Books API reads. Cheapest possible fallback — same service, one more request, no new failure modes.
Harvard LibraryCloud	No	Demoted. Free, no registration, answers by ISBN-13 (?identifier=<isbn>), verified working — but it returned numFound: 0 for both of the user's real failing books. It is a research library: strong on scholarly and older material, and it simply does not hold small-press children's historical fiction. The 93% in the table above is inflated by construction (the sample came from Harvard) and is misleading in exactly the direction that matters. Not a fit for this shelf.
Library of Congress	No	The SRU endpoint (port 210) is blocked from here; the loc.gov JSON API responds. Excellent for US imprints. Needs more probing before I'd commit.
K10plus SRU	No	Free German-led union catalogue, large and international. Cataloguing conventions differ enough that merging would need care.

Dead ends, so nobody re-investigates them: OCLC Classify (retired 2021), Goodreads API (retired 2020), Amazon Product Advertising API (requires an affiliate account with qualifying sales), WorldCat Search (requires OCLC membership — institutional pricing).

E. Pay for ISBNdb

~$15–50/month depending on tier. Genuinely better coverage than anything free, including cover art, and a single clean API. It is also a subscription for a two-person home library, and I'd want proof that A–D leave a real gap before recommending it.

Effort: hours. Cost: $180–600/year.

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What actually failed

Both failing ISBNs resolve cleanly against the source the app already uses:

Check	9781883937386	9781883937676
Open Library Books API	Hittite warrior, Joanne S. Williamson	Shadow hawk, Andre Norton
Cover art	yes	yes
App's own parser (unit test)	parses	parses
ISBN-13 checksum	valid	valid
Harvard	not held	not held

So the request either never went out, or went out and failed in a way the app reported as "not found". Candidates, in the order I'd chase them:

1. The barcode never decoded into a valid ISBN-13. ScanCodeFilter drops anything that fails the checksum, silently and with no UI feedback at all — no sheet, no message, nothing. A book whose barcode carries a price add-on, or is worn, or is a UPC-A rather than a Bookland EAN, looks to the user exactly like a camera that isn't working. This is my leading theory, and it fits "wasn't able to scan" better than "wasn't found".
2. The HTTP request failed. Both clients turn every non-200, timeout and parse error into null, which reaches the user as "No match found". The metadata OkHttpClient is constructed with no call timeout, so a stalled connection hangs on default socket timeouts.
3. The lookup ran and the sheet was dismissed before it landed. While Loading, the sheet passes an empty onDismissRequest, so it can't be swiped away — but onScanned early-returns whenever a sheet is already showing, so a stuck sheet blocks every subsequent scan.

Nothing here is a data-source problem. Note the sting in (1) and (2): both failure modes are invisible or actively misleading, which is why three scans produced no usable diagnosis.

Recommendation

Revised after the two real ISBNs came in. Adding sources is now the wrong next move: the books that failed are already in the source we query.

Do B first and on its own — make the app say what happened. A scan that decodes nothing should say so on the camera screen; a lookup that fails should offer retry, not a manual-entry form captioned "No match found". Add a call timeout while in there.

Then rescan those two books. The app will tell us which of the three candidates above it is, and that determines everything after it. A (the free Google Books key) and C (retry/backoff) are still worth doing — cheap, and the 429 result is real — but they are no longer the leading theory.

D and E are on hold. Harvard specifically is off the list for this shelf. Paying ISBNdb for coverage we demonstrably already have would be the wrong order.

One thing worth deciding separately: 17% of books legitimately have no cover art anywhere. The placeholder now looks deliberate rather than broken, but if you want covers on everything, that's a different feature — photograph the book, store it as the cover — and not a metadata-source problem at all.