parallel::mclapply fails in

I have a Shiny app that uses parallel::mclapply to run a simulation many times in parallel, then gathers all results and summarizes them. The app works fine on my local machine, but after deploying it to, it immediately fails when I hit my actionButton. The log gives a clue:

Warning in parallel::mclapply(1:input$simrep_count, function(i) { : all scheduled cores encountered errors in user code

Is there something that prevents using mclapply within Or am I just missing something?

FYI the app works fine with lapply instead of mclapply so I assume it's a parallelization issue.

The documentation warns the use within shinyapps has only be tested on MacOS so maybe not compatible with Shiny running on Ubuntu servers.


It is strongly discouraged to use these functions in GUI or embedded environments, because it leads to several processes sharing the same GUI which will likely cause chaos (and possibly crashes). Child processes should never use on-screen graphics devices. Some precautions have been taken to make this usable in on macOS, but users of third-party front-ends should consult their documentation. Note that tcltk counts as a GUI for these purposes since Tcl runs an event loop. That event loop is inhibited in a child process but there could still be problems with Tk graphical connections.

Thanks @andresrcs , what documentation are you referring to?

parallel::mclapply() documentation