Custom Exceptions in Ruby

A little while ago we talked about exceptions in Ruby. This time we explore ways of creating custom exceptions specific to your app’s needs.

Let's say we have a method that handles the uploading of images while only allowing JPEG images that are between 100 Kilobytes and 10 Megabytes. To enforce these rules we raise an exception every time an image violates them.

class ImageHandler
  def self.handle_upload(image)
    raise "Image is too big" if image.size > 10.megabytes
    raise "Image is too small" if image.size < 100.kilobytes
    raise "Image is not a JPEG" unless %w[JPG JPEG].include?(image.extension)

    #… do stuff
  end
end

Every time a user uploads an image that doesn't meet the rules, our (Rails) web app displays the default Rails 502 error page for the uncaught error.

class ImageUploadController < ApplicationController
  def upload
    @image = params[:image]
    ImageHandler.handle_upload(@image)

    redirect_to :index, :notice => "Image upload success!"
  end
end

The Rails generic error page doesn't offer the user much help, so let's see if we can improve on these errors. We have two goals: inform the user when the file size is outside the set bounds and prevent hackers from uploading potentially malicious (non-JPEG) files, by returning a 403 forbidden status code.

Custom error types

Almost everything in Ruby is an object, and errors are no exception. This means that we can subclass from any error class and create our own. We can use these custom error types in our handle_upload method for different validations.

class ImageHandler
  # Domain specific errors
  class ImageExtensionError < StandardError; end
  class ImageTooBigError < StandardError
    def message
      "Image is too big"
    end
  end
  class ImageTooSmallError < StandardError
    def message
      "Image is too small"
    end
  end

  def self.handle_upload(image)
    raise ImageTooBigError if image.size > 10.megabytes
    raise ImageTooSmallError if image.size < 100.kilobytes
    raise ImageExtensionError unless %w[JPG JPEG].include?(image.extension)

    #… do stuff
  end
end

First, we've added three new classes to the handler that extend from StandardError. For the image size errors, we've overridden the message method of StandardError with an error message we can show to users. The way raise was called in the handle_upload method has also changed, by replacing the custom StandardError message with a different error type we can raise a different, more specific, error.

Now, we can use these custom error types in our controller to return different responses to errors. For instance, we can return the specific error message or a specific response code.

class ImageUploadController < ApplicationController
  def upload
    @image = params[:image]
    ImageHandler.handle_upload(@image)

    redirect_to :index, :notice => "Image upload success!"

  rescue ImageHandler::ImageTooBigError, ImageHandler::ImageTooSmallError => e
    render "edit", :alert => "Error: #{e.message}"

  rescue ImageHandler::ImageExtensionError
    head :forbidden
  end
end

This is already a lot better than using the standard raise calls. With a little bit more subclassing we can make it make it easier to use, by rescuing entire error groups rather than every error type separately.

class ImageHandler
  class ImageExtensionError < StandardError; end
  class ImageDimensionError < StandardError; end
  class ImageTooBigError < ImageDimensionError
    def message
      "Image is too big"
    end
  end
  class ImageTooSmallError < ImageDimensionError
    def message
      "Image is too small"
    end
  end

  def self.handle_upload(image)
    raise ImageTooBigError if image.size > 10.megabytes
    raise ImageTooSmallError if image.size < 100.kilobytes
    raise ImageExtensionError unless %w(JPG JPEG).include?(image.extension)

    #… do stuff
  end
end

Instead of rescuing every separate image dimension exception, we can now rescue the parent class ImageDimensionError. This will rescue both our ImageTooBigError and ImageTooSmallError.

class ImageUploadController < ApplicationController
  def upload
    @image = params[:image]
    ImageHandler.handle_upload(@image)

    redirect_to :index, :notice => "Image upload success!"

  rescue ImageHandler::ImageDimensionError => e
    render "edit", :alert => "Error: #{e.message}"

  rescue ImageHandler::ImageExtensionError
    head :forbidden
  end
end

The most common case for using your own error classes is when you write a gem. The mongo-ruby-driver gem is a good example of the use of custom errors. Each operation that could result in an exception has its own exception class, making it easier to handle specific use cases and generate clear exception messages and classes.

Another advantage of using custom exception classes is that when using exception monitoring tools like AppSignal. These tools give you a better idea as to where exceptions occurred, as well as grouping similar errors in the user interface.

If you liked this article, check out more of what we wrote on AppSignal Academy. AppSignal is all about building better apps. In our Academy series, we'll explore application stability and performance, and explain core programming concepts.

We'd love to know what you thought of this article, or if you have any questions. We're always on the lookout for topics to investigate and explain, so if there's anything magical in Ruby you'd like to read about, don't hesitate to leave a comment.

Comments

[Michael Glass]

Writing literate exceptions is great advice! I'm certainly guilty of wrapping everything in a simple RunTimeError with raise "here's what happened, bub!".

However, I'm wary of the recommendation in this article about using literate errors as an alternate to existing control flow.

Exceptions have the special property of throwing until caught. Unlike other control flow, exceptions need to be caught. If you're planning on catching something in the caller, it's probably good to use control flow that doesn't re-raise.

If you're planning on catching something in the caller of the caller, it's probably a good idea to refactor your code so you're not zipping back and forth between multiple layers. E.g. if you're always going to catch something two levels down, using an exception as a fancy "GOTO" makes reasoning about what's going on quite difficult.

There's a lot written about this "sometimes anti-pattern". Maybe most famously wiki.c2.com/?DontUseExceptionsForF...

Here's an attempt to handle the same situation with explicit control flow, e.g. maybe a case statement.

def upload
  @image = params[:image]

  case ImageHandler.handle_upload(@image)
  when :success
    redirect_to :index, :notice => "Image upload success!"
  when :wrong_dimensions
    render "edit", :alert => "Error: #{wrong_dimension_message(@image)}"
  when :wrong_extension
    head :forbidden
  end
end

In this case, we know that handle_upload is local, expected edges are handled locally, and unexpected edges are thrown (as they should).

[Robert Beekman]

Hi Michael, thanks for your excellent suggestion. I've discussed this while writing the article with my colleagues and we came to the same conclusion.

I still went ahead with the current example, because it's compact, easily understandable and covers all the scenarios I wanted to explain. I'm planning to write another article that covers the "anti-pattern" you rightfully raised.

[Robert Beekman]

Excellent suggestion, thanks!