I wrote a library[0] to let you concurrently update a sqlite db in s3 safely. It uses the little known sqlite sessions extension plus s3 compare-and-swap on a small metadata file to make this work reasonably efficiently and safely. I have been enjoying it for a bunch of small projects where I want a lambda function to have a db for state but I don't want to pay for a full database instance.
[0]: https://github.com/psanford/s3db