Parallel reader/writer failures

[brodycj]

A client reported failures when testing with multiple readers running in parallel with a writer due to database locking. The solution is to fix the CoffeeScript/JavaScript to manage queue locking between the main database handle (with write access) and the parallel readers with a similar state-based locking style as described in https://www.sqlite.org/lockingv3.html.

Alternative may be to use WAL. Pitfalls of WAL discussed in storesafe/cordova-sqlite-storage-help#10. I wonder if WAL may be less vulnerable to database corruption due to its more sequential nature, for further investigation.

[brodycj]

Alternative solution may be to use WAL. From https://www.sqlite.org/wal.html#overview:

WAL provides more concurrency as readers do not block writers and a writer does not block readers. Reading and writing can proceed concurrently.

Another advantage of WAL is that it may be more robust against possible corruption. From storesafe/cordova-sqlite-storage-help#10 (comment):

I was thinking that WAL may be more robust against possible database corruption and found a couple threads that confirm WAL may be more reliable, at least in certain cases:

• http://sqlite.1065341.n5.nabble.com/WAL-mode-is-reliable-td41264.html
• https://marc.info/?l=sqlite-users&m=132052237302135&w=2