Hey!

I'm making one issue due to fact, that I was using exactly the same test scenario, but with different configuration. I believe, that this will make it easier to understand what is going on.

I've already spoken about this with @marcingrzejszczak as a part of his free consultations, thank you once again @marcingrzejszczak for this opportunity!

Bug in visual form

Configuration

• http.client.requests:
  • web app
  • server with virtual threads
  • RestClient with ReactorClientHttpRequestFactory
• http.server.requests:
  • WebFlux app
  • Undertow server
  • WebClient with ReactorClientHttpConnector

Version:

• Spring Boot 3.5.4

How to reproduce

Everything is here, please check readme file
https://github.com/mateusz-nalepa/spring-http-client-server-seconds-bug-MCVE

Potential places how to fix it

• http.client.requests
  • DefaultWebClient.DefaultRequestBodyUriSpec#exchange
    • do not start observation for http.client here, instead start http.client.pending
    • make sure http.client.pending metric/observation starts on server thread
  • ExchangeFunctions.DefaultExchangeFunction#exchange
    • end observation for http.client.pending
    • start observation for http.client
    • make sure, both of this happens on client thread
• http.server.requests
  • TomcatVirtualThreadsWebServerFactoryCustomizer
  • instead of new VirtualThreadExecutor("tomcat-handler-")
  • do something like

 new MonitoredExecutor(
 new VirtualThreadExecutor("tomcat-handler-")
 )
 class MonitoredExecutor internal constructor(private val delegate: Executor) : Executor {
 override fun execute(runnable: Runnable) {
 delegate.execute(MonitoredRunnable(runnable))
 }
 }
 class MonitoredRunnable internal constructor(private val delegate: Runnable) : Runnable {
 private val startTime = System.nanoTime()
 override fun run() {
 val duration = Duration.ofNanos(System.nanoTime() - startTime)
 VirtualLogger.log(this, "Http server pending request took: $duration")
 delegate.run()
 }
 }

Very important note

I've provided only two configurations of server + client. But probably the same applies for other servers/clients.

Why is this important?

In classical old approach test scenario takes 19s as expected.
In webflux/virtual in takes 27s and we are blind.