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I was experimenting promises and as I understand, as pointed out in this issue, an async process running for the same user is supposed to block execution of all other intra-session activities. However this doesn't seem to be a universal principle.

In the example app below, I have an artificially drawn out plotting operation and some other elements to see how they interact. One of these elements is a downloadButton/downloadHandler pair. While the async operation is running, it is still possible to press the downloadButton and get an output, which was unexpected for me. My questions are:

  • Can I stop downloadHandler from being active while an async process is running?
  • Why is downloadHandler special?


library(future)
library(promises)
library(shiny)
plan(multisession)


server = function(input, output) {
    # slow async plot
    output$slow_plot <- renderPlot({
        input$slow_plot_button
        future({
            print('I am slow')
            Sys.sleep(10)
            runif(20)
        }) %...>% plot(.)
    })
    # fast normal plot
    output$fast_plot = renderPlot({
        print('I am fast')
        input$fast_plot_button
        plot(runif(20))
    })
    # something that has no UI output
    observeEvent(input$non_ui_update,{
        print('Button press yay!')
    })
    # trigger happy downloadHandler
    output$download = downloadHandler('file',
                    content = function(file){
                        cat('is a file',file = file)
                    })}



ui = fluidPage(
    titlePanel("Async test"),
    sidebarLayout(
        sidebarPanel(
            actionButton('slow_plot_button','slow plot button'),
            actionButton('fast_plot_button','fast plot button'),
            actionButton('non_ui_update','non ui update'),
            downloadButton('download','download')
        ),
        mainPanel(
            plotOutput("slow_plot"),
            plotOutput("fast_plot")
        )
    )
)



shinyApp(ui,server)

Edit: Removed additional information about downloadButton downloading the whole page at first as it is not an integral part of the question

OganM
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  • Any idea how it has access to reactive variables from the main session? Because as far as I know sessions created by future can't do that. – OganM Oct 24 '18 at 18:58
  • I deleted my previous comment because I'm not sure if it's technically correct, I don't know how internally downloads are handled. I've always assumed a different session because it's a new page that opens up, but now I realize that perhaps that's not correct, maybe the same session is being used, which is what allows you to use any variables easily. – DeanAttali Oct 24 '18 at 19:40
  • hmm that kinda breaks the accepted answer as well. Without knowing further only solution I can think of is to manually disable other buttons and re-enable at promise completion via `finally` but this is not scalable – OganM Oct 24 '18 at 19:53
  • I deleted my answer since it didn't answer your question :). Also, I believe the file download is handled in the same session as well, but I bet it being a separate HTTP request has something to do with the different behavior. – greg L Oct 25 '18 at 01:24
  • If your download handler depends on data that's calculated asynchronously, you could at least prevent the handler from writing content until the future is complete. For example, `content = function(file) data() %...>% cat(file = file)`. Then you wouldn't have to disable buttons. The download handler would block until the async operations it depends on are complete. – greg L Oct 25 '18 at 01:31

0 Answers0