R Shiny: async downloadHandler - r

Question

I have a shiny app which takes a large amount of time downloading zip files. I am trying to use the futures and promises packages to manage the downloads so that other users can access the app while downloads are in progress.
The app looks as below:
library(shiny)
ui <- fluidPage(
downloadButton("Download", "Download")
)
server <- function(input, output){
output$Download <- downloadHandler(
filename = "Downloads.zip",
content = function(file){
withProgress(message = "Writing Files to Disk. Please wait...", {
temp <- setwd(tempdir())
on.exit(setwd(temp))
files <- c("mtcars.csv", "iris.csv")
write.csv(mtcars, "mtcars.csv")
write.csv(iris, "iris.csv")
zip(zipfile = file, files = files)
})
}
)
}
shinyApp(ui, server)
I've tried wrapping the write.csv inside a future function and setting `and while this does not throw an error, the app is not available for other users during the download.
library(shiny)
library(promises)
library(future)
plan(multiprocess)
ui <- fluidPage(
downloadButton("Download", "Download")
)
server <- function(input, output){
output$Download <- downloadHandler(
filename = "Downloads.zip",
content = function(file){
withProgress(message = "Writing Files to Disk. Please wait...", {
temp <- setwd(tempdir())
on.exit(setwd(temp))
files <- c("mtcars.csv", "iris.csv")
future(write.csv(mtcars, "mtcars.csv"))
future(write.csv(iris, "iris.csv"))
zip(zipfile = file, files = files)
})
}
)
}
shinyApp(ui, server)
I've also tried wrapping the entire downloadHandler function inside the future function, but I get the error:
Error in .subset2(x, "impl")$defineOutput(name, value, label) :
Unexpected MulticoreFuture output for DownloadUnexpected
MultiprocessFuture output for DownloadUnexpected Future output for
DownloadUnexpected environment output for Download
How can I handle the entire downloadHandler asyncronously? I am using the open source version of shiny server.

Answer 1

Don't know if you still need an answer for this, but I think you were very close. I have wrapped both the write.csv and zip in future as below and it works for multiple users on my testing.
library(shiny)
library(promises)
library(future)
plan(multiprocess)
ui <- fluidPage(
downloadButton("Download", "Download")
)
server <- function(input, output){
output$Download <- downloadHandler(
filename = "Downloads.zip",
content = function(file){
withProgress(message = "Writing Files to Disk. Please wait...", {
temp <- setwd(tempdir())
on.exit(setwd(temp))
files <- c("mtcars.csv", "iris.csv")
future({
Sys.sleep(15)
write.csv(mtcars, "mtcars.csv")
write.csv(iris, "iris.csv")
zip(zipfile = file, files = files)})
})
}
)
}
shinyApp(ui, server)