Logging in Rack middleware with Clogger

ruby rack

Feb 2013

Thanks to Rack middleware and Rails engines, it is possible to easily combine web application modules in Ruby. Good examples of this pattern are the Spree engine or some chapters of Josè Valim’s book “Crafting Rails Applications”. In the context of the design of modular web applications, I found logging specifics of HTTP requests with Clogger very helpful.

What is Clogger?

The main function of Clogger is to print and format parameters in a Rack::Request. It’s a bit like ‘sprintf’ for HTTP requests, and can be mounted as middleware in a Rack application. So far, I mainly used the logger with Sinatra or in Rails application. Here are 2 examples:

Logging HTTP headers in a Rails application

Authentication over the web can be tricky. For development purposes, it’s often helpful to see some more details of a HTTP header. For this, let’s write a simple client sending some header tokens to a Rails app.

The client:

require 'faraday'
client = Faraday::Connection.new(:url => "")
client.get '/example' do |req|
  req.headers['x_token'] = 'some meta data'

The Clogger setup in a Rails server config/application.rb:

class Application < Rails::Application
  config.middleware.use Clogger, :format => '"$request" ? $query_string $status $body_bytes_sent "$http_referer" "$http_user_agent" $request_time -- $http_x_token', :logger => $stdout 

Results in:

  "GET HTTP/1.1" ?  200 1083 "-" "Ruby" 0.502 -- some meta data

Monitoring cookies in Sinatra

A similar logging setup can easily be done in Sinatra. Simply include Clogger in config.ru

require 'clogger'

use Clogger, :format => '"$request" ?$query_string $status "$http_cookie" $request_time $http_host', :logger => $stdout

Now the cookies are dumped for every request. This can be helpful for debugging once in a while.


The take away from this post is that the Rack middleware can be a nice tool for debugging HTTP header content. Rack can bring modular code back to a web application, and this is great to create ‘layers’ of functionality upon which other applications can build on. A similar approach was derived in the Faraday stack to write modular client-side middleware, and will may be discussed in a later post.

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