The Praxis Request and Response classes encapsulate the gory details of the HTTP protocol; specifically, they parse requests and generate responses so your business logic can focus on the content of messages, not the representation of that content. Handlers are the “glue” that Praxis uses to parse request payloads and generate response bodies so that your application sees requests as structured data (hashes and arrays), and responds with the same.

Each handler understands a particular encoding mechanism: JSON, WWW-form, and so forth. Praxis uses a heuristic to decide which handler is appropriate for a given HTTP body.


Register new handlers at application startup time by invoking the handler DSL method inside your app’s config block. Each handler is identified by a short string name and implemented using a Class that responds to a few handler-interface methods.

Praxis::Application.configure do |app|
  app.handler 'xml', Praxis::Handlers::XML

Built-in Handlers

The Praxis core contains handlers for plain text, JSON, form-encoding, and XML, but only plain text, JSON, and form-encoding are registered automatically because XML has external dependencies.

To enable the XML handler, register it as shown in the example above, then add two gems to your application’s Gemfile.

gem 'builder', '~> 3.2'
gem 'nokogiri', '~> 1.6'

XML Data Representation

Praxis’ XML handler parses and generates documents that are compatible with Ruby On Rails’ #to_xml serialization mechanism. In brief:

  • attributes and their values are represented as named tags with inner CDATA
  • the type attribute indicates the data type of each value
  • a special type value indicates an array of objects

This representation scheme does not have an XML DTD or schema because its tag names are open-ended, but its predictable naming scheme allows you to define a schema that covers your application’s media types.

For more information, please refer to ActiveSupport documentation.

Handler Selection

Praxis looks at the content_type of a request or response in order to determine the appropriate handler. Specifically, it asks for the handler_name of the content type; this is a method of MediaTypeIdentifier that applies a simple heuristic:

  • If the content type’s suffix (e.g. +json, +xml) matches a handler name, use that handler
  • If the content’s subtype (e.g. json in application/json) matches a handler name, use that handler
  • Otherwise, assume www-form-urlencoded handler for requests and json for responses

This heuristic works because all of the structured-syntax suffixes defined in RFC6839 happen to coincide exactly with the subtype of the corresponding Internet media type: +json, application/json and text/json all imply the same thing about the encoding of data, although they have different implications about the meaning of the data.

Implementing a Custom Handler

Write your own handler by creating a Class that responds to three methods:

Check that your handler’s dependencies are all satisfied and raise a helpful exception if not.
Given a String, decode into structured data and return structured data (Hash or Array).
Given structured data, encode to String and return that string.

Use the XML handler as an implementation guide. When you’re finished implementing, register your handler at app startup and handle with impunity!