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> In an asynchronous/event driven server slow SQL queries won't limit your concurrency

Since this is on the JVM, SQL almost always means JDBC and JDBC is blocking so yes, SQL will in fact limit your concurrency.

yes, SQL is blocking, JVM also have threads. The http-kit is asynchronous(epoll on linux), the API for programmers is sync, which is more familiar.

Alternatively, you can use a message queue or a REST service, and whatever lightweight, async database frontend.

That would just move the blocking code elsewhere, and now you have one extra framework / message queue on your stack (free complexity)

The point is to use a non-blocking DB frontend. Not necessarily JVM-based.