[bob1029]

I think SQLite is maybe the best option if you can get a bit clever around the scalability implications.

For instance, you could maintain an in-memory copy of the log DB schema for each http/logical request context and then conditionally back it up to disk if an exception occurs. The request trace SQLite db path could then be recorded in a metadata SQLite db that tracks exceptions. This gets you away from all clients serializing through the same WAL on the happy path and also minimizes disk IO.