SQLite is a popular and fast database engine. While it can't be used with Vercel, we do offer other storage solutions.
SQLite needs a local file system on the server to store the data permanently when write requests are made. In a serverless environment, this central single permanent storage is not available because storage is ephemeral with serverless functions.
As a function receives more concurrent traffic, the serverless environment will create new instances of the function and each instance will not be able to share the same storage. While Fluid Compute can send multiple requests to one function, it does not offer guarantees around reading from persistent storage like a local file system.