When using UndertowClient to call another service from the current service handleRequest, the following error occurs under very light load.

java.io.IOException: UT001033: Invalid connection state
at io.undertow.client.http.HttpClientConnection.sendRequest(HttpClientConnection.java:336)

This looks like a bug in the first place, however, it might be that the connection cannot handle too many concurrent requests given further investigation. As UndertowClient supports both HTTP 1.1 and 2.0, it would be necessary to support connection pool internally. In most of the cases, HTTP 2.0 won't need connection pool unless under heavy load but HTTP 1.1 needs a connection pool in case the target server doesn't support HTTP 2.0 or developers choose to use HTTP 1.1 in the client side code.