Exercises: Using Shiny Apps

Task: Explore the Connections pane

The app 03_databse/app.R is a Shiny application that reads data from a PostGres database.

The RStudio IDE has a pane for “Connections” that allows you to establish database connections and browse information contained in the database tables.

See these support articles:

As well as the article at https://db.rstudio.com/

And you can watch this video walkthrough:

Task: Deploy a Shiny app with database connection

Run the app 03_database/app.R in the IDE to make sure everything works.

Notice that the connection gets established by this line of R code:

con <- DBI::dbConnect(odbc::odbc(), "Postgres Dev")

Deploy 03_database/app.R to Connect (using the publish button in the Source pane).

While you wait, can you figure out how “PostGres Dev” is defined?

Hint:

cat /etc/odbc.ini 
cat /etc/odbcinst.ini

Task: Change runtime settings

While you’re viewing the deployed app in Connect, find the “runtime settings”:

The Connect admin guide explains these settings at:

Read this part of the admin guide, then change the settings to allow parallel execution of your app:

Setting New value
Max processes 6
Min processes 1
Max connections per process 2
Load Factor 0.1

The setting pane looks like this:

Task: Load the app

  • Open the metrics dashboard in a new browser tab

    • As an administrator, you can find the “metrics” as a pane in the admin dashboard:

  • Return to your app, and progressively open the app in 6 new tabs

    • Do this by pasting the app URL in 6 new browser tabs
    • Each new tab will establish a new connetion to your app.

Notice in this recording how the CPU indicator goes into the red very quickly, as each shiny connection puts a new demand on the database. However, even a 2CPU machine can handle the 6 simultaneous connections, albeit with some delay to the user:

Simulation: shimmer

Open the Shimmer and shiny simulation app at https://colorado.rstudio.com/rsc/shimmer_and_shiny/

  • Read the instructions
  • Perform the simulation
  • Adjust setting until all the measurements are in the “blue”

Try to develop an intuitive understanding of the runtime settings.

Ideas to try:

  • Increase the number of users
  • Increase the number of requests per user
  • Increase the app response time

Then modify the server size and runtime settings until all the indicators are in the “blue”