R Shiny - Continuous Background Task

Question

I would like to know if it is possible to maintain a continuous background job while the Shiny application is running. This means, for example, that a stream can load data from a web page into a database while shinyApp is running.

Moreover, could it be possible to interact with data from an external background “stream”? (for example, during loading, plot creation, or something else).

The video tutorial says that: "Code outside the server function will be run once per R (working) session." So, the code I need, I think, should be placed outside the server function.

Is it possible to reach the scene that I am describing? Or do I need another external instance of R (outside shinyApp scripts)?

Thanks in advance.

Answer 1

I thought about this, and I think it is possible, but the implementation that I mean depends on the platform. In this case, I will consider ubuntu 14.04.

Suppose you have some kind of computationally intensive task:

ui.R:

library(shiny) fluidPage( numericInput('number','Number',10000000), textOutput('CalcOutput') ) 

server.R

 library(shiny) function(input,output,session) { output$CalcOutput <- renderText({ sort(runif(input$number)) }) } 

transfer the operation to the function of the corresponding variables in the subfile:

newfile.R

 saveRDS(sort(runif(commandArgs(TRUE)[1])), file = 'LargeComputationOutput') 

change your server. R

 function(input, output) { observe({ # Starts script as a background process, but completes instantaneously system(paste('Rscript newfile.R',input$number,'&')) }) CalculationOutput <- reactive({ invalidateLater(5000) validate( need(file.exists('LargeComputationOutput'),'Calculation In Progress'), need(file.info('LargeComputationOutput')$mtime > Sys.time()-5,'Calculation In Progress') ) x <- readRDS('LargeComputationOutput') }) output$CalcOutput <- renderText({ CalculationOutput()[300] }) } 

This is still a bit wrong, but it is a proof of concept that you can move intensive operations into subprocesses and detect a reactive listener when doing these calculations.

EDIT: Shiny will also need permissions to write to the appropriate location.

Answer 2

I found a solution to this problem using a future package. Please see my answer in Calling a brilliant JavaScript callback from the future.