Class: Rack::Builder

Inherits:
Object
  • Object
show all
Defined in:
lib/rack/builder.rb

Overview

Rack::Builder provides a domain-specific language (DSL) to construct Rack applications. It is primarily used to parse config.ru files which instantiate several middleware and a final application which are hosted by a Rack-compatible web server.

Example:

require 'rack/lobster'
app = Rack::Builder.new do
  use Rack::CommonLogger
  use Rack::ShowExceptions
  map "/lobster" do
    use Rack::Lint
    run Rack::Lobster.new
  end
end

run app

Or

app = Rack::Builder.app do
  use Rack::CommonLogger
  run lambda { |env| [200, {'Content-Type' => 'text/plain'}, ['OK']] }
end

run app

use adds middleware to the stack, run dispatches to an application. You can use map to construct a Rack::URLMap in a convenient way.

Constant Summary collapse

UTF_8_BOM =
'\xef\xbb\xbf'

Class Method Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(default_app = nil, &block) ⇒ Builder

Initialize a new Rack::Builder instance. default_app specifies the default application if run is not called later. If a block is given, it is evaluated in the context of the instance.


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# File 'lib/rack/builder.rb', line 114

def initialize(default_app = nil, &block)
  @use = []
  @map = nil
  @run = default_app
  @warmup = nil
  @freeze_app = false

  instance_eval(&block) if block_given?
end

Class Method Details

.app(default_app = nil, &block) ⇒ Object

Create a new Rack::Builder instance and return the Rack application generated from it.


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# File 'lib/rack/builder.rb', line 126

def self.app(default_app = nil, &block)
  self.new(default_app, &block).to_app
end

.load_file(path) ⇒ Object

Load the given file as a rackup file, treating the contents as if specified inside a Rack::Builder block.

Ignores content in the file after __END__, so that use of __END__ will not result in a syntax error.

Example config.ru file:

$ cat config.ru

use Rack::ContentLength
require './app.rb'
run App

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# File 'lib/rack/builder.rb', line 87

def self.load_file(path)
  config = ::File.read(path)
  config.slice!(/\A#{UTF_8_BOM}/) if config.encoding == Encoding::UTF_8

  if config[/^#\\(.*)/]
    fail "Parsing options from the first comment line is no longer supported: #{path}"
  end

  config.sub!(/^__END__\n.*\Z/m, '')

  return new_from_string(config, path)
end

.new_from_string(builder_script, file = "(rackup)") ⇒ Object

Evaluate the given builder_script string in the context of a Rack::Builder block, returning a Rack application.


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# File 'lib/rack/builder.rb', line 102

def self.new_from_string(builder_script, file = "(rackup)")
  # We want to build a variant of TOPLEVEL_BINDING with self as a Rack::Builder instance.
  # We cannot use instance_eval(String) as that would resolve constants differently.
  binding, builder = TOPLEVEL_BINDING.eval('Rack::Builder.new.instance_eval { [binding, self] }')
  eval builder_script, binding, file

  return builder.to_app
end

.parse_file(path) ⇒ Object

Parse the given config file to get a Rack application.

If the config file ends in .ru, it is treated as a rackup file and the contents will be treated as if specified inside a Rack::Builder block.

If the config file does not end in .ru, it is required and Rack will use the basename of the file to guess which constant will be the Rack application to run.

Examples:

Rack::Builder.parse_file('config.ru')
# Rack application built using Rack::Builder.new

Rack::Builder.parse_file('app.rb')
# requires app.rb, which can be anywhere in Ruby's
# load path. After requiring, assumes App constant
# contains Rack application

Rack::Builder.parse_file('./my_app.rb')
# requires ./my_app.rb, which should be in the
# process's current directory.  After requiring,
# assumes MyApp constant contains Rack application

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# File 'lib/rack/builder.rb', line 65

def self.parse_file(path)
  if path.end_with?('.ru')
    return self.load_file(path)
  else
    require path
    return Object.const_get(::File.basename(path, '.rb').split('_').map(&:capitalize).join(''))
  end
end

Instance Method Details

#call(env) ⇒ Object

Call the Rack application generated by this builder instance. Note that this rebuilds the Rack application and runs the warmup code (if any) every time it is called, so it should not be used if performance is important.


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# File 'lib/rack/builder.rb', line 261

def call(env)
  to_app.call(env)
end

#freeze_appObject

Freeze the app (set using run) and all middleware instances when building the application in to_app.


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# File 'lib/rack/builder.rb', line 244

def freeze_app
  @freeze_app = true
end

#map(path, &block) ⇒ Object

Creates a route within the application. Routes under the mapped path will be sent to the Rack application specified by run inside the block. Other requests will be sent to the default application specified by run outside the block.

Rack::Builder.app do
  map '/heartbeat' do
    run Heartbeat
  end
  run App
end

The use method can also be used inside the block to specify middleware to run under a specific path:

Rack::Builder.app do
  map '/heartbeat' do
    use Middleware
    run Heartbeat
  end
  run App
end

This example includes a piece of middleware which will run before /heartbeat requests hit Heartbeat.

Note that providing a path of / will ignore any default application given in a run statement outside the block.


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# File 'lib/rack/builder.rb', line 237

def map(path, &block)
  @map ||= {}
  @map[path] = block
end

#run(app = nil, &block) ⇒ Object

Takes a block or argument that is an object that responds to #call and returns a Rack response.

You can use a block:

run do |env|
  [200, { "content-type" => "text/plain" }, ["Hello World!"]]
end

You can also provide a lambda:

run lambda { |env| [200, { "content-type" => "text/plain" }, ["OK"]] }

You can also provide a class instance:

class Heartbeat
  def call(env)
   [200, { "content-type" => "text/plain" }, ["OK"]]
  end
end

run Heartbeat.new

It could also be a module:

module HelloWorld
  def call(env)
   [200, { "content-type" => "text/plain" }, ["Hello World"]]
  end
end

run HelloWorld

Raises:

  • (ArgumentError)

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# File 'lib/rack/builder.rb', line 192

def run(app = nil, &block)
  raise ArgumentError, "Both app and block given!" if app && block_given?

  @run = app || block
end

#to_appObject

Return the Rack application generated by this instance.


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# File 'lib/rack/builder.rb', line 249

def to_app
  app = @map ? generate_map(@run, @map) : @run
  fail "missing run or map statement" unless app
  app.freeze if @freeze_app
  app = @use.reverse.inject(app) { |a, e| e[a].tap { |x| x.freeze if @freeze_app } }
  @warmup.call(app) if @warmup
  app
end

#use(middleware, *args, &block) ⇒ Object

Specifies middleware to use in a stack.

class Middleware
  def initialize(app)
    @app = app
  end

  def call(env)
    env["rack.some_header"] = "setting an example"
    @app.call(env)
  end
end

use Middleware
run lambda { |env| [200, { "content-type" => "text/plain" }, ["OK"]] }

All requests through to this application will first be processed by the middleware class. The call method in this example sets an additional environment key which then can be referenced in the application if required.


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# File 'lib/rack/builder.rb', line 149

def use(middleware, *args, &block)
  if @map
    mapping, @map = @map, nil
    @use << proc { |app| generate_map(app, mapping) }
  end
  @use << proc { |app| middleware.new(app, *args, &block) }
end

#warmup(prc = nil, &block) ⇒ Object

Takes a lambda or block that is used to warm-up the application. This block is called before the Rack application is returned by to_app.

warmup do |app|
  client = Rack::MockRequest.new(app)
  client.get('/')
end

use SomeMiddleware
run MyApp

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# File 'lib/rack/builder.rb', line 208

def warmup(prc = nil, &block)
  @warmup = prc || block
end