Ways of Setting Up a Shiny App

Shiny Apps can be set up in two different ways.

1. Single-file App: In a single-file app, ui and server are stored in one script. In this case, we create a file named app.R that contains both the server and UI components. This technique is used when developing very simple Shiny Apps and lacks some advantages of the alternative two-files App method.

2. Two-files App: Here, ui and server are stored in two separate scripts, which implies a clear separation between front-end and back-end. This method is preferable when developing more advanced Shiny Apps. It is important that the files are named ui.R and server.R and always stored in a separate folder. In this tutorial, we are going to develop Shiny Apps using the two-files method.

Building a Shiny App from Scratch

To set up, make sure that all required packages are installed and subsequently load the shiny,tidyverse, and plotly packages along with R’s example data set titanic.

## Packages
pkgs <- c("shiny",
          "tidyverse",
          "titanic",
          "plotly")

## Install uninstalled packages
lapply(pkgs[!(pkgs %in% installed.packages())], install.packages)

## Load all packages to library
lapply(pkgs, library, character.only = TRUE)

## Warning: package 'shiny' was built under R version 3.6.3

## Warning: package 'ggplot2' was built under R version 3.6.3

## Warning: package 'tidyr' was built under R version 3.6.3

## Warning: package 'plotly' was built under R version 3.6.3

Next, create a new folder with two R scripts, ui.R and server.R:

ui <- fluidPage()
server <- function(input, output) {
}

Subsequently, launch the Shiny App by pressing the “Run App” button in the top right corner of the Source pane. Also, have a look at the the code chunk below, which provides an example of a Shiny App.

runExample("01_hello")
# To show other Apps, please type runExample(NA) and choose another example

Building the UI

When building a Shiny App, one should have in mind what the app should look like. Hence, we build the UI first. In simple Shiny Apps, the whole UI fits in the fluidPage: every new object is passed comma-separated, and text can be passed to the UI simply by entering strings. In order to format text, Shiny uses HTML wrappers, i.e., functions that take text as arguments (plus further style options).

• h1(): top-level header
• h2(): secondary header
• strong(): make text bold
• em(): make text italicized
• br(): add line break
• titlePanel(): adds an official header

Until now, we only have a plain white page. We need a proper layout to make our app look nicer. sidebarlayout is the simplest layout format. It splits your page in two parts: a sidebarPaneland a mainPanel. Inside sidebarLayout you may use sidebarPanel to specify the appearance of the side panel of your page. The mainPanel contains the results of the server.

ui <- fluidPage(titlePanel("Title of my Shiny App"),
                sidebarLayout(
                  sidebarPanel("My input goes here"),
                  mainPanel("The results go here")
                ))

In order to interact with Shiny Apps, we need control widgets, such as buttons, select boxes, or sliders. These allow us to navigate the app. The type of the widget you choose depends on the design of your app. You can either specify inputs, enter text, or select specific dates to create results. All input functions have two arguments: inputId and label.

inputId tells Shiny to refer to a particular input when retrieving values for the back-end. It is important that the ID names you use are unique. If you have two IDs with the same name, there will not be a warning or error message. Consequently, the app may use the wrong input in the wrong place without notification. An overview of all types of widgets you can work with is provided here. The label option specifies the text displaying the label of the control widget. The control labels go in the sidebarPanel. Here, we specify the possible values, range, and the appearance of the control widget.

Having added the input and the control widgets, we finally need to specify the output elements of our app. These outputs might be plots, tables, text, images, or maps. These will be built in the server file of our two-files app. In the UI, we only build the placeholder, which means that every output function has an outputId argument to identify the output created in server.R. Here, we use plotlyOutput to refer to a plot that will be generated later in server.R. Output elements should always be added to the mainPanel() function. Other types of output elements are:

• dataTableOutput(): data table
• imageOutput(): image
• plotOutput(): plot
• tableOutput(): table
• verbatimTextOutput(): text
• textOutput(): text
• uiOutput(): raw HTML
• htmlOutput(): raw HTML

The code chunk below demonstrates how widget and output elements can be combined.

# The most simple structure of the UI:
ui <- fluidPage(
  titlePanel("Visualizing the Titanic data set"),
  sidebarLayout(
    sidebarPanel(
      checkboxInput("checkbox",
                    "Activate all filters",
                    FALSE),
      radioButtons(
        "buttons",
        "Did the passenger survive?",
        choices = c("did not survive" = 0, "did survive" = 1),
        selected = 0
      ),
      selectInput("selector",
                  "Select the class of the passenger",
                  choices = c(1, 2, 3)),
      sliderInput(
        "slider",
        "Pick a range of fare (in $)",
        min = 0,
        max = 550,
        value = c(10, 50),
        pre = "$"
      ),
      selectInput(
        "selectVars",
        "Select the variable to be displayed",
        choices = c(
          "Age" = "Age",
          "# of Siblings/Spouses on board" = "SibSp",
          "# of Parents/Children on board" = "Parch"
        )
      )
    ),
    mainPanel(plotlyOutput("greatPlot"),
              textOutput("goodText"))
  )
)

Implementing the Server Logic

Having finished the front-end, we now turn to building the server or the back-end. The server logic in Shiny Apps uses an input argument, which contains the values of the input given by the users, as well as an output argument, which contains the plots and tables created as a function of the input arguments. Output objects can be created without any input specification, but always need to be connected with the UI by an outputId. To do so, save the output object into the output list. We then build the object using one of the available render functions. For every object type, there is a unique render function, e.g. renderPlot(). The table below provides a brief overview of which render functions work with which output formats.

We have to make sure that we accurately assign our IDs to their corresponding render and output functions. Since we named our plotOutput in the UI greatPlot, we have to assign it to this ID. As we use the plotOutput function in the UI, we need the renderPlot function to match it. Furthermore, our plots need a connection to the input and the control widgets. The logic behind the input list is the same as for the output list: As we used the sliderInput function in the UI and named it slider, we now access the respective object. Given the structure of the input, we specify it to fit the output object.

output$greatPlot <- renderPlot({
  plot(rnorm(input$slider[1]))
})

Rendering Plots

There are several ways of increasing the interactivity of plots. For example, one can simply add code to the renderplot() function. Another very simple way of increasing interactivity is filtering. However, when filtering, the whole code will get re-executed every time the user re-specifies anything, which can become very memory-intensive and repetitive when building several plots! We therefore use reactive variables to extract the filtering from the render functions. Reactive variables can now be used for different objects, which change simultaneously with different input. Additionally, we use plotly plots to make it even more interactive. You simply need to replace the plotOutput() with plotlyOutput() in the UI.

# Back-end
# The server function takes input and output as arguments
# input: Input provided by the user to specify a certain appearance, data 
#        or result
# output: The output created by the functions specified in the back end

server <- function(input, output) {
  # Create a reactive variable to use it for different objects interactively
  filtered <- reactive({
    titanic_train %>%
      filter(
        Fare >= input$slider[1],
        Fare <= input$slider[2],
        Pclass == input$selector,
        Survived == input$buttons
      )
  })
  
  # Plot the variables of the titanic_train data set
  # Using if-conditions to specify the variables being plotted
  output$greatPlot <- renderPlotly({
    if (input$checkbox == TRUE) {
      x <- filtered()[, input$selectVars]
      plotHist <- ggplot(filtered(), aes(x)) +
        geom_histogram()
    } else {
      x <- titanic_train[, input$selectVars]
      plotHist <- ggplot(titanic_train, aes(x)) +
        geom_histogram()
    }
    
    ggplotly(plotHist)
  })
  
  output$goodText <- renderText({
    if (input$checkbox == TRUE) {
      x <- nrow(filtered()[filtered()$Survived == 1, ])
      text <-
        paste0("Using your filter ", x, " passengers have survived")
      return(text)
    } else {
      x <- nrow(titanic_train[titanic_train$Survived == 1, ])
      text <- paste0("In total ", x, " passengers have survived")
      return(text)
    }
  })
}

To finally visiualize our plot locally, we simply run shinyApp(ui = ui, server = server). This allows us to see the final Shiny App, including some additional components, which increases the functionality of the app even further.

Deployment using shinyapps.io

Until now, we only ran our apps locally on our machine. In order to present an app to a broader audience, we need to deploy it in the world wide web. We have to take care that our app is protected by a firewall and that we have a stable URL. For simple apps, deploy the app using a free account on shinyapps.io. If done so, go back to RStudio, press the deploy button in the top right corner of the Source pane, re-enter your credentials, select the correct files, and let the magic happen!

Deployment using Shiny Server

For certain apps you might not want to use shinyapps.io, for instance if:

• the size of the app would require a (very) costly plan, or
• you want full control over the app and host it yourself, or
• you love playing around with Unix code.

A free alternative is Shiny Server, which allows you to host your own app in a controlled environment (e.g. inside your organization). The professional version of Shiny Server (Shiny Server Pro) allows you to deploy password-protected apps and use an administrative dashboard that provides you with usage statistics. However, you need an own server to host the app (e.g. Digital Ocean or Amazon Web Services), which may still be costly.

Another alternative is hosting the app on university servers. For instance, members of the University of Mannheim can use the web services provided by the universities of Baden-Württemberg, the bwCloud, for free.

When using Shiny Server, the required steps are a bit more complicated:

1. Register for the bwCloud
2. Log into the bwCloud Dashboard
3. Create an SSH-key pair to connect to your server (find a short intro here)
4. Install PuTTY (Windows) or start a remote connection in the Shell (MAC) as well as Filezilla
5. Set up the SSH-client to access remote connections
6. Build a virtual machine (the server) using your bwCloud dashboard and connect to your virtual machine using SSH with PuTTY

After setting up the server, we need to enter our Unix system and install R, all relevant packages, and Shiny Server using Unix code

1. Install R on your machine

sudo apt-get install r-base

If you are lucky, this will install the correct version. Otherwise, this tutorial might be of help.

2. Install dependencies to install R packages

sudo apt-get -y install libcurl4-gnutls-dev 
sudo apt-get -y install libxml2-dev libssl-dev

3. Install R packages within R, including shiny (easy!)

install.packages("shiny")

4. Install Shiny Server (check for latest version!)

wget https://download3.rstudio.org/ubuntu-12.04/x86_64/shiny-server-1.5.6.875-amd64.deb
sudo gdebi shiny-server-1.5.6.875-amd64.deb

5. Open port 3838 and check whether your firewall is set up

sudo ufw allow 3838

6. Test whether Shiny Server runs correctly. Open this in your browser: http://134.155.108.111:3838/ (replace with your IP-address)
7. Use Filezilla to access your server and upload your app files to the /srv/shiny-server/ folder. It should run when typing in the correct URL associated with your app: http://134.155.108.111:3838/foldername/projectname/
8. The above will likely not work immediately. In this case, you need to go back and troubleshoot… (click here for a tutorial)