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Defining the maximum number of concurrent users for a Spring Reactive Web application on a Netty Server

Spring Webflux, using project Reactor is very promising in terms of using your resources in a more efficient way. It's far from obvious how to calculate the resources required to support a certain number of users, however.

When building a standard web application (not Reactive) which will run on a Tomcat, you can simply define the number of webthreads required to support your users. This does not seem to count for Spring Reactive Web applications, however. A small number of threads is used, and less memory is used, but where in the past you could just define "X webthreads available per server" this doesn't seem possible to do. Making our sysops and devops friends a bit unhappy.

What's a solution for this issue? Because it's hard to "sell" a Reactive solution, when it's one that's not very deterministic when it could break down.