The process of bootstrapping a Praxis application consists of executing a well known set of ordered stages described in the Bootloader. An application can easily hook into the boot process by either executing Ruby code as the files required by stages are loaded or by installing before and after hooks. As part of this bootstrapping process the application will typically install its configuration into the Praxis application singleton, initialize or finalize its objects, and load application-specific source code.

A Praxis application is represented by the Praxis::Application singleton class. As a singleton, you can have only one application instance and it is always referencable using Praxis::Application.instance. This allows the bootstrapping process to easily locate, configure and change any aspect of the application code.

To kick-off the bootstrapping process, call the setup method of the Praxis::Application singleton and run it through your favorite rack server. Here’s a simple example of a file to achieve that:

# Example of a minimalistic
require 'bundler/setup'
require 'praxis'

application = Praxis::Application.instance

run application


The bootloader is the class that determines the sequence of stages that will be executed as the application starts. This process is automatically triggered when the setup method of the application is invoked.

The Bootloader works by setting up a set of ‘stages’, and executing them in order. Each stage has a unique name. An application can dynamically register before and after hooks for each existing stage in order to execute code at the right moment of the bootstrapping process. It is possible to register a before/after callback for a stage that hasn’t been registered yet (as late as the before hooks for that stage are run).

Stages can have sub-stages, and sub-stages can have their own sub-stages and so on. This allows you to create logical groups of stages. Before hooks of a parent stage get executed before any of the before/after hooks of its sub-stages. After hooks of a parent stage get executed after all the before and after hooks of its sub-stages.

Note: Currently, stages can only be removed. In the future (if needed) we might want to add support for adding stages within a given position relative to others, so that they can be arranged and/or attached in correct order.

Default boot stages

The Bootloader will automatically create the following stages by default:

  • Requires the config/environment.rb file
  • Instantiates each plugin specified by the use directive and calls its setup! method
  • Sets the default file layout if the app hasn’t defined one yet. The layout facility lets applications define the structure and order of files to be loaded. See the layout section for details.
Loads the files specified in the initializers key of the file layout which typically globs all files within the ‘initializers’ directory from the application root.
Loads the files specified in the libs key of the file layout which typically globs all files within the ‘lib’ directory from the application root.
Loads application design
  • loads each group of named files under the :design key of the layout
  • the load order follows how the entries are defined in file_layout. This can be changed by overriding AppLoader#layout_order.
Loads the application code (implementation):
  • loads each group of named files under the :app key of the layout
  • the load order follows how the entries are defined in file_layout. This can be changed by overriding AppLoader#layout_order.
  • for each loaded “Controller” sets ts app_config and root accessors.
traverses all declared routes in of your resource definitions and creates appropriate entries in the main Praxis router. Requests matching the correct path, version, and conditions cause the router to invoke the matching method name (with the typed params as method arguments) in the controller that implements the action.
determines if there are any files that exist within the application path, but haven’t been loaded by any of the file_layout path patterns. If there are unloaded files in the paths, it will print a warning indicating which files were not loaded. Turn this off by deleting the stage delete_stage :warn_unloaded_files in the configure block.


The layout is a way for you to tell Praxis where your source code lives. It is essentially a nested set of name/pattern pairs. The layout tells Praxis which files it should require at different stages of the bootloader. Praxis preserves the load order of the stages as listed in the layout. Naming each pattern allows your application to install before and after hooks when the stage is executed. See Request Life Cycle Callbacks for more information on before and after hooks.

If you don’t specify a custom layout, Praxis will install the following default layout:

Praxis::Application.instance.layout do
  layout do
    map :initializers, 'config/initializers/**/*'
    map :lib, 'lib/**/*'
    map :design, 'design/' do
      map :api, 'api.rb'
      map :media_types, '**/media_types/**/*'
      map :resources, '**/resources/**/*'
    map :app, 'app/' do
      map :models, 'models/**/*'
      map :controllers, '**/controllers/**/*'
      map :responses, '**/responses/**/*'


Praxis’ configuration facility allows you to define, set and use your application’s configuration. More specifically, Praxis lets you:

  • Define the structure and types of the available configuration parameters which gives your application a clean, validated and coerced set of values that match your specifications. This allows the application to minimize boilerplate code and not worry about consistent error reporting during application startup.
  • Set the actual values of the application configuration in a plugabble way so that the application deployer can decide how to best provide the correct parameters to each of your apps. While reading the values from one or multiple configuration files might be appropriate in some cases, reading values from environment variables or accessing a config service might be more appropriate in others.
  • Access the values using a fully typed configuration object which allows the application code to be clean and readable, and ignorant of where the values came from.

Defining the configuration structure must be done at boot time, typically within the environment stage. Setting the configuration values is also typically done at boot time. While this might vary from app to app, it is common to set all parameters early on in the boot process as well. Technically speaking, it is possible to re-set the configuration at runtime, although this should only be done if the application code is able to re-read those values when they change. Accessing configuration can be done at any time.


Given an instance of your application, you can define the structure of its configuration by passing a block to #config. Within the block, you can define attributes in the same way you define attributes in a MediaType or params/payload in an action. The DSL within such a #config block follow the syntax of an Attributor::Struct. You can call define with a block multiple times if you need to define configuration in multiple places.

Praxis::Application.instance.config do
  attribute :db do
    attribute :hostname, String, regexp: /^host-/
    attribute :port, Integer, default: 80, values: [80,8080,443]
    attribute :username, String
    attribute :password, String

  attribute :log_level, String, required: true


After you’ve defined your application’s configuration, you can set the actual values to use when the application starts. You can call config= with any object that satisfies your configuration definition. Since Praxis doesn’t mandate any particular configuration store it is up to us to go fetch the values from the right place. In this example, we’ll use a simple YAML file:

values = YAML.load('./config/application.yml'))
# {
#   'db' => {
#     'hostname' => 'host-1234',
#     'username' => 'root',
#     'password' => 'mydbpass'
#   },
#   'log_level' => 'info'
# }
Praxis::Application.instance.config = YAML.load(values)

If there is any problem in loading the configuration, Praxis will halt and will describe the exact problem it encountered. Typical loading errors include invalid or uncoercible types received, values outside the specified range, or missing attributes marked as required.


Accessing the configuration data is done through the config method in the Application singleton. The object retrieved will have the characteristics and types as you defined it. Structs like the one above will expose parameters as method accessors. Other types as Hash or Attributor::Collection will expose values as underlying hashes or arrays.

For example, retrieving the config data for the example above can be done by:

app_config = Praxis::Application.instance.config
=> 'host-1234'
=> 80

Note: While we didn’t explicitly set a db port, the system picked a suitable default as specified in its attribute.


Praxis provides a Logger for your application to use, accessible through the logger method in the Application singleton:

Praxis::Application.instance.logger.error "failed to catch tuna"

If you don’t specifically set the logger, Praxis will create one for you and connect it to STDOUT. If you want to set your own, just use the setter method and pass an object that implements the logger interface:

file ='log.log', File::WRONLY | File::APPEND)
Praxis::Application.instance.logger =