How to Send All Errors to the Front-end in a Shiny App

• The content is a blog article that explains how to use a custom error handler and a custom message handler to send all errors to the front-end in a Shiny app, a web framework for R that allows you to create interactive web applications.

If you are developing a Shiny app, you might want to display any errors that occur on the server-side to the front-end, so that the user can see what went wrong and how to fix it. However, by default, Shiny does not provide a built-in way to do this. In this article, I will show you how to use a custom error handler and a custom message handler to send all errors to the front-end in a Shiny app.

What is Shiny?

Shiny is a web framework for R that allows you to create interactive web applications with minimal coding. You can use Shiny to build dashboards, data visualizations, reports, and more. Shiny apps consist of two main components: a user interface (UI) and a server function. The UI defines the layout and appearance of the app, while the server function defines the logic and functionality of the app.

Problem

The problem is that when an error occurs on the server-side of a Shiny app, the default behavior is to display a generic error message on the front-end, such as “An error has occurred. Check your logs or contact the app author for clarification.” This message is not very helpful for the user, as it does not tell them what caused the error or how to fix it. Moreover, it does not give any feedback to the app developer, as it does not log the error message or stack trace.

Solution

The solution is to use a custom error handler and a custom message handler to send all errors to the front-end in a Shiny app. A custom error handler is a function that overrides the default error handling behavior of Shiny. A custom message handler is a function that handles custom messages sent from the server-side to the front-end using session$sendCustomMessage.

Here are the steps to implement the solution:

1. Define a custom error handler function that takes an error object as an argument and returns a list containing the error message and stack trace. For example:

customErrorHandler <- function(error) {
  # Get the error message and stack trace
  errorMessage <- conditionMessage(error)
  errorTrace <- paste0(capture.output(traceback(3)), collapse = "\n")
  
  # Return a list with the error message and stack trace
  return(list(message = errorMessage, trace = errorTrace))
}

2. Set the custom error handler as an option in your server function using options(shiny.error = customErrorHandler). For example:

server <- function(input, output, session) {
  # Set the custom error handler
  options(shiny.error = customErrorHandler)
  
  # Rest of your server code ...
}

3. Define a custom message handler function on the front-end that takes a message object as an argument and displays it using an alert or any other UI element. For example:

Shiny.addCustomMessageHandler("display-error", function(message) {
  // Display the error message and stack trace using an alert
  alert("Error: " + message.message + "\n\nStack trace:\n" + message.trace);
});

4. Send any errors that occur on the server-side to the front-end using session$sendCustomMessage(type = “display-error”, message = customErrorHandler(error)). For example:

server <- function(input, output, session) {
  # Set the custom error handler
  options(shiny.error = customErrorHandler)
  
  # Try to read a non-existent file and catch any errors
  tryCatch({
    data <- read.csv("non-existent-file.csv")
  }, error = function(e) {
    # Send the error to the front-end using a custom message
    session$sendCustomMessage(type = "display-error", message = customErrorHandler(e))
  })
  
  # Rest of your server code ...
}

Result

The result is that when an error occurs on the server-side of a Shiny app, instead of displaying a generic error message on the front-end, it displays a detailed error message and stack trace that tells the user what caused the error and how to fix it.

Frequently Asked Questions (FAQs)

Here are some frequently asked questions related to this topic:

Question: How can I customize the appearance of the error message on the front-end?

Answer: You can use any UI element or library that suits your needs to display the error message on the front-end. For example, you can use shinyalert, which provides customizable modal dialogs for Shiny apps.

Question: How can I prevent errors from crashing my app or stopping other functionalities?

Answer: You can use tryCatch blocks around any code that might cause errors and handle them gracefully. For example, you can use a finally clause to execute some cleanup code or resume the normal execution of the app.

Question: How can I log errors to a file or a database for debugging purposes?

Answer: You can use any logging library or package that suits your needs to log errors to a file or a database. For example, you can use futile.logger, which provides a flexible and powerful logging system for R.

Disclaimer: The solution may not work for all scenarios and may require further customization. Please use it at your own risk and test it thoroughly before deploying it to production. Shiny is a product of RStudio, Inc. and is not affiliated with this blog or its author.