SQLite transaction mode (BEGIN IMMEDIATE) that tries to acquire the write lock when the transaction starts, rather than deferring lock acquisition until the first write statement. SQLite's default deferred mode presumes most transactions are read-only and only grabs the lock on first write, which means if the lock can't be acquired the transaction must error immediately to preserve its snapshot. For Rails applications — which wrap every write in a transaction and use a multi-connection pool — deferred mode causes constant SQLite3::BusyException errors under concurrency. Immediate mode lets SQLite's built-in retry mechanism queue the BEGIN itself, dramatically reducing spurious 500s. Configured via `default_transaction_mode: immediate` in database.yml (sqlite3-ruby ≥ 1.6.9).