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Using Rack::Parser to encapsulate parsing logic in web service applications

Sinatra is often used for web service applications. One common need these applications have is to parse incoming messages, typically JSON or XML.

Often this is a two step process – first parsing the message string into predictable data structures, then loading that into models. So a typical route might look like

post '/messages' do
  message = Message.from_hash( ::MultiJson.decode(request.body) )
  halt 201, {'Location' => "/messages/#{}"}, ''

And your Message model would have an appropriate #from_hash method that grabs what it needs from the parsed message and throws it into a new instance.

If your application has several different endpoints, all using the same content-type, you could save some repetition by moving it to a helper:

helpers do
  def parsed_body

post '/orders' do
  order = Order.from_hash( parsed_body )
  # ....

This works fine for simple scenarios. But if you have multiple content-types, need to validate the message format, or do other pre-processing on it, moving the parsing logic out of your application itself starts to become attractive.

One option is to move it into a module that your app extends.

But another option is to do the parsing in a middleware. That way, your app is not responsible for doing the basic parsing at all – it's done and exposed to your app by the time the request arrives.

The idea is to mimic what Rack does to process form data, which is to populate the env[rack.request.form_hash] with a hash, and then expose that through the params hash. But instead of parsing url parameters or multipart form data, it will parse the XML or JSON body.

Rack::Parser is a Rack middleware that does just that, and lets you configure custom parsing routines for any given content-type.

So using Rack::Parser, the example above would be changed to

# in
use Rack::Parser, :content_types => {
  'application/json'  => { |body| ::MultiJson.decode body }

# in application
post '/orders' do
  order = Order.from_hash( params['order'] )
  # ....

This is quite convenient if your app accepts either form data or JSON for a given endpoint – since the code is then identical whether a user submits a form directly or some web-service client submits a JSON request.

Custom parsing

Note that the content-type handlers are just procs (or anything that responds to #call) that take a single parameter – the request body.

This makes it quite easy to set up custom parsing, validation, or other pre- processing.

Rack::Parser also has recently added support for error handling, so that errors in parsing can be mapped into a suitable http response, and logged. This is quite useful for handling validation errors.

For instance, say you have set up vendor-specific json message formats, which you want to validate using a validate_input class method on your models.

class MyJsonParser

  def initialize(validator_class)
    @klass = validator_class

  def call(body)
    json = ::MultiJson.decode(body)
    if @klass.respond_to?(:validate_input)


use Rack::Parser, :parsers => {
  'application/vnd-my-message+json' =>,
  'application/vnd-my-order+json'   =>

Then any parsing error raised in MulitJson.decode or validation error in validate_input will result in a 400 (bad request) response in the same content-type as the request – like

{"errors": "Validation error in input - cannot set 'private_info'"}

This is the default; you can also completely customize how errors are converted into responses, per content-type. Both the error and the content type is passed down to the handler.

use Rack::Parser, :handlers => {
  'application/xml' => proc { |err, type| [400, '<?xml version="1.0"?><response><error>Something went wrong</error></response>'] }

For more info