how to handle exceptions in JSON based RESTful code?

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I have a "software as a service" app that uses JSON communicated via a RESTful API.

Simply stated: what are the best practices for capturing and reporting exceptions when using a RESTful API with JSON data interchange?

My first thought was to see what Rails does by generating a scaffold, but that's clearly not right. Here's an excerpt:

class MumblesController < ApplicationController

  # GET /mumbles/1
  # GET /mumbles/1.json
  def show
    @mumble = Mumble.find(params[:id])
    respond_to do |format|
      format.html # show.html.erb
      format.json { render json: @mumble }
    end
  end

end

In this case, if the JSON code sends a non-existent ID, e.g.

http://www.myhost.com/mumbles/99999.json

then Mumble.find() will raise ActiveRecord::RecordNotFound. ActionController will catch that and render an error page in HTML. But HTML is useless to the client that is expecting JSON.

I could work around that by wrapping the Mumble.find() in a begin ... rescue RuntimeError block and rendering a JSON status => :unprocessable_entity or something.

But then what if the client's app sends an invalid path, e.g.:

http://www.myhost.com/badtypo/1.json

Is a JSON based app supposed to catch that and return an error in JSON? If so, where do I capture that without digging deep into ActionDispatch?

So overall, do I punt and let ActionController generate HTML if there's an error? That doesn't feel right...

actioncontroller ruby-on-rails json
4 Answers

As a developer, you will also want to see traces (preferably with useful lines, filtering out gems). And make traces invisible for production: 

  rescue_from StandardError do |exception|
    # Handle only JSON requests
    raise unless request.format.json?

    err = {error: exception.message}

    err[:backtrace] = exception.backtrace.select do |line|
      # filter out non-significant lines:
      %w(/gems/ /rubygems/ /lib/ruby/).all? do |litter|
         not line.include?(litter)
      end
    end if Rails.env.development? and exception.is_a? Exception

    # duplicate exception output to console:
    STDERR.puts ['ERROR:', err[:error], '']
                    .concat(err[:backtrace] || []).join "\n"

    render :json => err, :status => 500
  end
user_17513210
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