docs: explain SQLite concurrency caveats + DSN tuning (RUK-137) - #22
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The original ticket asked for runtime WAL verification, but auditing the code showed nothing actually depends on WAL being on — the "In WAL mode the transaction provides row-level locking" comment in sqlite/get_for_processing.go is a vestigial note, not a real dependency. Adding code to enable+check WAL would have been ceremonial. What is real and worth telling users: - SQLite serializes writes at the file level. Multiple goque workers calling CureTasks / GetTasksForProcessing in parallel hit SQLITE_BUSY immediately because dbtx.WithinTx uses BEGIN DEFERRED: the SELECT takes a SHARED lock, the UPDATE can't upgrade to RESERVED, and _busy_timeout does NOT apply to lock upgrades. The fix is to start tx as BEGIN IMMEDIATE via the _txlock=immediate DSN parameter — surfaced in README as a copy-pasteable snippet. - Single-binary constraint: even with the DSN tuning, two processes pointing at the same .sqlite.db file will deadlock. If you need real concurrency, the message stays the same: PG or MySQL. Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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The original ticket asked for runtime WAL verification, but auditing
the code showed nothing actually depends on WAL being on — the
"In WAL mode the transaction provides row-level locking" comment in
sqlite/get_for_processing.go is a vestigial note, not a real
dependency. Adding code to enable+check WAL would have been
ceremonial.
What is real and worth telling users:
SQLite serializes writes at the file level. Multiple goque workers
calling CureTasks / GetTasksForProcessing in parallel hit
SQLITE_BUSY immediately because dbtx.WithinTx uses BEGIN DEFERRED:
the SELECT takes a SHARED lock, the UPDATE can't upgrade to
RESERVED, and _busy_timeout does NOT apply to lock upgrades. The
fix is to start tx as BEGIN IMMEDIATE via the _txlock=immediate
DSN parameter — surfaced in README as a copy-pasteable snippet.
Single-binary constraint: even with the DSN tuning, two processes
pointing at the same .sqlite.db file will deadlock.
If you need real concurrency, the message stays the same: PG or
MySQL.
Co-Authored-By: Claude Opus 4.7 noreply@anthropic.com