Rack Cheatsheet
July 2022
Rack interface
Middleware
Middleware is the atom of Rack Land. It’ has a very simple interface! It’s
#initialize must accept a single argument, this is the next middleware in
the stack. The current middleware should send #call to this middleware in
it’s own #call method.
Middleware must also respond to #call, which must accept a single
argument and must return the “magic triple”. The argument passed is the Rack
“environment hash”, it contains information about the HTTP request and
Rack-specific variables1. The magic triple is broken down below.
[
200, # HTTP Status Code, must be an `Integer`
{"Content-Type" => "text/plain"}, # Headers, must be a `Hash`, but can be empty
["Hello world."] # Body, must respond to `#each`
]
Applications
Applications are just a special case middleware. Because they’re at the bottom
of the stack they do not need to implement #initalize, they must respond
to #call.
Rack-up DSL
The entry point to Rack! Normally added to a config.ru file and booted
with a sever with a Rack handler build in (like Puma).
Running an application
Using a proc:
run proc {
[200, {"Content-Type" => "text/plain"}, ["Hello world."]]
}
Using a class:
class HelloWorld
def call(_env)
[200, {"Content-Type" => "text/plain"}, ["Hello world."]]
end
end
run HelloWorld.new
Adding middleware
Without configuration
use Rack::ShowExceptions
With configuration
use Rack::Auth::Basic, "Rack Cheatsheet" do |username, password|
OpenSSL.secure_compare('secret', password)
end
Two applications, one config.ru
APP_1 = proc {
[301, {"Location" => "/hello-world"}, []]
}
APP_2 = proc {
[200, {"Content-Type" => "text/plain"}, ["Hello world."]]
}
map "/hello-world" do
run APP_2
end
run APP_1
Warming up applications on boot
warmup do |app|
client = Rack::MockRequest.new(app)
client.get('/warm-cache')
end
Request handing
Using the environment hash directly can be a little unwieldy, Rack::Request
is a helpful wrapper to make it more ergonomic.
run lambda { |env|
request = Rack::Request.new(env)
unless request.scheme == 'https'
return [301, {"Location" => "https://#{request.host}#{request.fullpath}"}, []]
end
if request.get?
[200, {"Content-Type" => "text/plain"}, ["Secure hello world."]]
else
[405, {}, []]
end
}
The response object
A convenience class for generating responses, calling #to_a or #finish will
return a magic triple.
run lambda { |_env|
response = Rack::Response.new
response.write "<p>"
case Time.now.hour
when 0..11
response.write "Good morning!"
when 12..17
response.write "Good afternoon!"
when 18..23
response.write "Good evening!"
end
response.write " <time>It's #{Time.now.strftime("%l:%M %P")}</time>.</p>"
response.finish
}