diora-web/docs/database.md

# The database

One SQLite file, `data/db.sqlite3` (on the server `/home/marwin/diora-prod-data`, mounted into
`diora-prod` at `/app/data`). The books live in it too, encrypted, so it is large (2.5 GB in
October 2026) and some writes are big (a scanned book of 30 MB).

## Write-ahead log (since 2026-10-19)

`diora/sqlite.py` puts every connection in WAL mode (`PRAGMA journal_mode=WAL`, set once per
file, it stays) with `synchronous=NORMAL`; `settings.DATABASES` has `transaction_mode:
IMMEDIATE`, so a transaction takes the write lock when it begins.

Why: in the rollback-journal mode before, a long write kept all readers out. The podcast
refresh (cron, every 15 minutes) and big uploads made requests fail with "database is
locked", GET /books/ among them — the apps then saw no new books and no changed titles.
With WAL, readers read on while one writer writes; writers still take turns (20 s timeout).

Next to `db.sqlite3` there are now `db.sqlite3-wal` and `db.sqlite3-shm`. They belong to the
database: a copy of `db.sqlite3` alone may miss the latest writes. To copy it safely:

    docker exec diora-prod python -c "import sqlite3; s=sqlite3.connect('data/db.sqlite3'); d=sqlite3.connect('data/backup.sqlite3'); s.backup(d)"

or stop the container first. Back to the old mode, if ever needed:
`PRAGMA journal_mode=DELETE` (with no other connection open) and remove `diora/sqlite.py`'s hook.

## The book list never reads past the file

A book's row is `user, meta_ct, meta_iv, data_ct (the file), data_iv, uploaded_at, is_read,
cover_ct, cover_iv`. SQLite stores a big value in a chain of overflow pages, and to reach a
column *after* it, it walks the whole chain: listing 427 books read gigabytes and took 37 s
(the desktop app gives up after 10 s, so it never synced). Since then `books.models.book_rows()`
reads `id, meta_ct, meta_iv` from the rows (they stand before the file) and `uploaded_at,
is_read` / whether there is a cover from the covering indexes `ebook_list_idx` and
`ebook_cover_idx` — no list query touches a file. `books/tests.py` checks the query plans.

## The files in a table of their own (since 2026-10-19)

`books.models.EBookFile` (one row per book, `book_id` its key) holds `data_ct`/`data_iv`;
`EBook` has only the small fields. Before, every change of a title, a folder or "gelesen"
rewrote the whole row with its file (megabytes, for a scanned volume tens of them), and every
`EBook.objects.get()` loaded the file. `EBook.objects.create(data_ct=…, data_iv=…)` still
works (the manager makes the EBookFile too); the API is unchanged.

Migration `books/0008_ebook_file` copied 430 files in one statement and dropped the two
columns: about 2 minutes on the server (rehearsed on a copy), the site down meanwhile. The
pages the old columns used are free now and get reused; `VACUUM` (exclusive, minutes, at a
quiet time) would give them back to the disk.