The Shiny-app on open-source edition Shiny-server has only one concurrent, which means it can run only for one user at a time point. But it can host multiple Shiny-apps, which can run synchronously. So, if we create severl Shiny-apps with different names but same function, then we can let more users use our service at same time. But users don’t how to choose the Shiny-app with small user number. This post will show you how to create a Shiny-app to redirect user to the Shiny-app with lower load.

I have no knowledge about server load balancer, the following method is ONLY what I thought it can be.

• First, we need to know the load information about Shiny apps on our server, like which apps are running, how many users for each app, etc.

• Then, create a normal Shiny app to detect which app has little user number than others, and using JavaScript to redirect user to that app.

• CPU information about Shiny-app

  The following is the R code than generates a data frame containing which Shiny-app are running and the user number of each Shiny-app.

“` ruby ## Setup work directory; setwd(“/srv/shiny-system/Data”) I <- 0 for (i in 1:60) { system(“top -n 1 -b -u shiny > top.log”) dat <- readLines(“top.log”) id <- grep(“R $”, dat) Names <- strsplit(gsub(“^ +|%|\+”, “”, dat[7]), “ +”)[[1]] if (length(id) > 0) { # ‘top’ data frame; L <- strsplit(gsub(“^ *”, “”, dat[id]), “ +”) dat <- data.frame(matrix(unlist(L), ncol = 12, byrow = T)) names(dat) <- Names dat <- data.frame(Time = Sys.time(), dat[, -ncol(dat)], usr = NA, app = NA) dat$CPU <-as.numeric(as.character(dat$CPU)) dat$MEM <-as.numeric(as.character(dat$MEM)) # Check if connection number changed; for (i in 1:length(dat$PID)) { PID <- dat$PID[i] system(paste(“sudo netstat -p | grep”, PID, “> netstat.log”)) system(paste(“sudo netstat -p | grep”, PID, “» netstat.log2”)) system(paste(“sudo lsof -p”, PID, “| grep /srv > lsof.log”)) netstat <- readLines(“netstat.log”) lsof <- readLines(“lsof.log”) dat$usr[i] <- length(grep(“ESTABLISHED”, netstat) & grep(“tcp”, netstat)) dat$app[i] <- regmatches(lsof, regexec(“srv/(.)”, lsof))[[1]][2] } dat <- dat[, c(“app”, “usr”)] } else { dat <- data.frame(app = “app”, usr = 0) } write.table(dat, file = “CPU.txt”) }

“`

To make it run automatically, schedule it under /etc/crontab like the following:

“` ruby

• • • • • root Rscript //CPU.R

“`

• Create the Shiny-app for redirecting.

ui.R

ruby shinyUI(bootstrapPage( tags$style("#link {visibility: hidden;}"), # This app doesn't need user interface; textInput(inputId = "link", label = "", value = ""), # Redirecting link; tags$script(type="text/javascript", src = "redirect.js") # JavaScript for redirecting; ))

server.R

“` ruby shinyServer(function(input, output, session) { CPU <- read.table(“Data/CPU.txt”) App <- data.frame(app = c(“app1”, “app2”, “app3”, “app4”)) App <- merge(App, CPU, all.x = TRUE) App$usr[which(is.na(App$usr))] <- 0 Link <- paste(“http://192.168.150.36/”, App$app[which.min(App$usr)], sep = “”) updateTextInput(session, inputId = “link”, value = Link) })

“`

redirect.js

“` ruby setInterval(function() { var link = document.getElementById(‘link’).value; if (link.length >1) { window.open(link, “_top”) } }, 50)

“`