Goals

You will learn:

  • Tools for finding bugs & diagnosing errors in Shiny applications
  • Little-known ways to look under the hood
  • How to debug problems in deployed apps
  • Cheat code for OSX IDE JavaScript debug tools

Non-goals

Outline

We'll cover three main areas:

  • Debugging
  • Tracing
  • Errors

Debugging

"Finding your bug is a process of confirming the many things that you believe are true — until you find one which is not true."

– Norm Matloff

Author of The Art of R Programming and The Art of Debugging

Breakpoints

Breakpoint (armed)

Breakpoints

Stepping after execution

Breakpoints

Pros

  • Can inspect local values/stack
  • Can step through following code
  • Doesn't change code

Cons

  • Only works inside shinyServer
  • Can't tell you why something isn't happening
  • Interrupts program flow; not conditional

browser()

Conditional breakpoint

if (input$bins > 50)
  browser()

browser()

Pros

  • Similar advantages to breakpoints
  • Valid anywhere an R expression is valid
  • Can be made conditional

Cons

  • Changes your code!
  • Interrupts program flow
  • Can't tell you why something isn't happening

Tracing

Showcase Mode

runApp(..., display.mode="showcase")

Showcase Mode

Pros

  • Very easy to use
  • Helps find unnecessary or unexpected execution

Cons

  • Doesn't scale well to larger apps
  • Won't show you why a reactive or observer executed

Reactive Log

options(shiny.reactlog=TRUE)
runApp(...)
showReactLog()

Reactive Log

Pros

  • Can show you why something isn't happening!
  • No more thorough way of visualizing reactivity

Cons

  • Lots of data to sift through
  • Not real-time

"printf" tracing

# generate bins based on input$bins from ui.R
x    <- faithful[, 2] 
bins <- seq(min(x), max(x), length.out = input$bins + 1)
cat(file=stderr(), "drawing histogram with", input$bins, "bins\n")
Listening on http://127.0.0.1:5757
drawing histogram with 30 bins
drawing histogram with 35 bins

"printf" tracing

Pros

  • Actual values at runtime, w/o modifying flow
  • Only method that works after deployment

Cons

  • Limited to textual representation of values
  • Have to guess values to log
  • Watch out for reactive side effects!

"printf" tracing

Tracing on Shinyapps.io

rsconnect::deployApp( ... )
rsconnect::showLogs(streaming = TRUE)
2016-01-29T01:26:12 shinyapps[77594]: 
2016-01-29T01:26:12 shinyapps[77594]: Starting R with process ID: '26'
2016-01-29T01:26:14 shinyapps[77594]: drawing histogram with 30 bins
2016-01-29T01:26:14 shinyapps[77594]: drawing histogram with 35 bins

Tracing in Shiny Server

tail -f /var/log/shiny-server/myapp-20160131-104403-8492.log

Only while R session is alive!

Client/server tracing

options(shiny.trace = TRUE)
SEND {"config":{"workerId":"","sessionId":"04531d50d12554bd981b24b9d3983cc4"}}
RECV {"method":"init","data":{"bins":30,".clientdata_output_distPlot_width":610,
      ".clientdata_output_distPlot_height":400,".clientdata_output_distPlot_hidden":false,
      ".clientdata_pixelratio":1,".clientdata_url_protocol":"http:",
      ".clientdata_url_hostname":"127.0.0.1",".clientdata_url_port":"5569",
      ".clientdata_url_pathname":"/",".clientdata_url_search":"",
      ".clientdata_url_hash_initial":"",".clientdata_singletons":"",
      ".clientdata_allowDataUriScheme":true}}

Error Handling

Tracebacks

bins <- seq(min(x), max(x), length.out = input$bins + 1)
if (input$bins > 40)
  stop("too many bins")
Warning: Error in renderPlot: too many bins
Stack trace (innermost first):
    76: renderPlot [server.R#20]
    68: output$distPlot
     1: shiny::runApp

Full tracebacks

options(shiny.fullstacktrace = TRUE)
Warning: Error in renderPlot: too many bins
Stack trace (innermost first):
    79: h
    78: .handleSimpleError
    77: stop
    76: renderPlot [server.R#20]
    75: ..stacktraceon..
    74: func
    ...

Pausing on errors

options(shiny.error = browser)

JavaScript errors: Browser

Right-click, inspect element, Console.

JavaScript exceptions: first-chance

JavaScript dev mode on OS X

defaults write org.rstudio.RStudio WebKitDeveloperExtras -bool true

Links

Wrapup

Any questions?