Invoke Mono.block() through "nioEventloopGroup-*" threads would end up leading all the threads hang

Rahome 0
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The project I am working for is using Spring WebFlux. I came across a very odd issue. The detail is that some of pieces of code are purely wrote in Reactor style (couples of Flux/Mono pipelines), however, in a inner publishers, I have to call a method where there is "Mono.block()" inside.

The weird thing I aware is that the whole service would become totally stuck, and when I captured a thread dump, I saw all those "nioEventLoopGroup-*" threads were hung.

A fun fact is that if I leverage a "simple" thread (new Thread(..)) to call the method (there is .block inside), everything works fine.

So my question is that, are those "nioEventLoopGroup-*" threads not allowed to call any blocking code.

Sorry for asking a dumb question, but it's blocking issue for now, so I am looking forward your insight.

project-reactor spring-webflux multithreading spring
1 Answers

Reactor, by default, uses a fixed size thread pool. When you use block(), the actual work needs to be done in some thread or another, which depends on the nature of the subscription and the Mono/Flux. Most likely a set of new tasks will be scheduled on the same scheduler, but block() will suspend its thread, waiting for those tasks to complete, so there is one fewer thread for those other tasks to be scheduled on. Evidently you have enough of these calls to exhauast the entire thread pool. All your block() calls are waiting for other tasks to complete, but there are no threads available for them to be run on.

There's no reason to call block() inside a mapping in a reactive stream. There are always other ways of achieving the same goal without blocking - flatMap(), zip() etc etc.

Jelaby0
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