SQLite journal mode enabled via `PRAGMA journal_mode=WAL`. Under the default rollback journal, reads and writes contend for the same lock — only one operation at a time, read or write. WAL mode permits many concurrent readers alongside the single writer (e.g. 15 reads during one stream of writes). Became part of the default SQLite configuration shipped by Rails 7.1, along with five other pragmas that make the default SQLite-on-Rails setup more resilient.