MDC use thread locals, having MDC in vert.x would cause one request to replace data of other request as both would run on the same thread but in essentially random order if there is more than one thing going on. Never store values in static fields or thread locals in vert.x

For simple examples it would work (because handlers then are essentially blocking, executed one after another). But in more complex examples when there is http client, a database, different threading system (rx, executeBlocking) it would fail because handler registered on those goes back to event loop and might be executed before or after logic from different request, which would already overwrite values in MDC.