SQLite concurrency and why you should care about it

In SQLite, transactions by default start in “deferred” mode. This means they do not take a write lock until they attempt to perform a write.

You get SQLITE_BUSY when transaction #1 starts in read mode, transaction #2 starts in write mode, and then transaction #1 attempts to upgrade from read to write mode while transaction #2 still holds the write lock.

The fix is to set a busy_timeout and to begin any transaction that does a write (any write, even if it is not the first operation in the transaction) in “immediate” mode rather than “deferred” mode.

https://zeroclarkthirty.com/2024-10-19-sqlite-database-is-lo...

I've always tried to avoid situations that could lead to SQLITE_BUSY. SQLITE_BUSY is an architecture smell. For standard SQLite in WAL, I usually structure an app with a read "connection" pool, and a single-entry write connection pool. Making the application aware of who _actually_ holds the write lock gives you the ability to proactively design access patterns, not try to react in the moment, and to get observability into lock contention, etc.

I mean, you're not wrong, and that is one way to solve it, but the whole point of a sensibly-designed WAL -- never mind database engine -- is that you do not need to commit to some sort of actor model to get your db to serialise writes.

These are performance optimizations. SQLite does serialize writes. Avoiding concurrent writes to begin with just avoids some overhead on locking.

"performance optimisation" --- yeees, well, if you don't care about data integrity between your reads and writes. Who knows when those writes you scheduled really get written. And what of rollbacks due to constraint violations? There's we co-locate transactions with code: they are intertwined. But yes, a queue-writer is fine for a wide range of tasks, but not everything.

It's that we need to contort our software to make sqlite not suck at writes that is the problem.

This is just FUD. The reason SQLite does locking to begin with is to avoid data corruption. Almost every statement this blog post makes about concurrency in SQLite is wrong, so it's little surprise that their application doesn't do what they expect.

>Who knows when those writes you scheduled really get written

When a commit completes for a transaction, that transaction has been durably written. No mystery. That's true whether you decide to restrict writes to a single thread in your application or not.

> When a commit completes for a transaction, that transaction has been durably written. No mystery. That's true whether you decide to restrict writes to a single thread in your application or not.

Usually this is true but there are edge cases for certain journaled file systems. IIRC sqlite.org has a discussion on this.

> there are edge cases for certain journaled file systems. IIRC sqlite.org has a discussion on this.

Can't currently find it but I guess it comes under the "if the OS or hardware lies to SQLite, what can it do?" banner?