Where are the "I/O dispatcher" threads coming from in WebFlux (Netty) or MVC (Tomcat-NIO) Application?

Question

I have 2 Spring Boot Applications, one WebFlux on Netty and one MVC on Tomcat. I know the apps use the following threads: WebFlux (Netty):

reactor-netty-nio-*

MVC (Tomcat-NIO):

http-nio-8091-exec-*

But both Applications also create a number of I/O dispatcher threads, according to the number of logical cores. Can somebody help me understand where they are coming from?

If you are using apache-httpclient, then they are probably used by that. In case of WebFlux you can use reactor-netty as http client as well. In that case the threads (reactor-http-nio) would be shared between the http server and client which could be more efficient. — Martin Tarjányi
Thank you. Yeah I´m using Spring-Data-Elasticsearch and that dependency pulls in apache-httpcomponents. But I´m using the ReactiveElasticsearchClient on top of WebClient with reactor-netty HttpClient. — safarione