1_faraday.rb

require 'logger'
$logger = Logger.new(STDOUT)

require 'active_support/cache'
$cache = ActiveSupport::Cache.lookup_store(:memory_store)
$cache.logger = $logger
$cache.clear

class EtagMiddleware

  def initialize(app, options = {}, &block)
    @app = app
    @cache = options.fetch(:cache, &block)
    @cache_key_prefix = options.fetch(:cache_key_prefix, :faraday_etags)
  end

  def call(env)
    # Only cache "safe" requests
    return @app.call(env) unless [:get, :head].include?(env[:method])

    cache_key = [ @cache_key_prefix, env[:url].to_s ]
    cached = @cache.fetch(cache_key)

    if cached
      env[:request_headers]["If-None-Match"] ||= cached[:response_headers]["Etag"]
    end

    @app.call(env).on_complete do

      if cached && env[:status] == 304
        env[:body] = cached[:body]
      end

      if !cached && env[:response_headers]["Etag"]
        @cache.write(cache_key, env)
      end

    end

  end

end


require 'faraday'

Faraday.register_middleware :request, :etag => EtagMiddleware


connection = Faraday.new(url: "http://localhost:4567") do |builder|
  builder.response :logger, $logger
  builder.request :etag, :cache => $cache
  builder.adapter :net_http
end

2.times do

  response = connection.get "/"

  response.on_complete do
    puts "---"
    p response.body
    puts "---"
  end

end

2_sinatra.rb

require 'sinatra'

get "/" do
  puts "Got this If-None-Match header: #{env["HTTP_IF_NONE_MATCH"].inspect}"
  etag "abcdef"
  "Hello from Sinatra"
end

3_sinatra_output.txt

Got this If-None-Match header: nil
localhost - - [22/Apr/2012:14:19:14 CEST] "GET / HTTP/1.1" 200 18
- -> /
Got this If-None-Match header: "\"abcdef\""
localhost - - [22/Apr/2012:14:19:14 CEST] "GET / HTTP/1.1" 304 0
- -> /

4_faraday_output.txt

I, [2012-04-22T14:19:14.360599 #92297]  INFO -- : get http://localhost:4567/
D, [2012-04-22T14:19:14.360696 #92297] DEBUG -- request: 
D, [2012-04-22T14:19:14.360781 #92297] DEBUG -- : Cache read: [:faraday_etags, "http://localhost:4567/"]
D, [2012-04-22T14:19:14.364850 #92297] DEBUG -- : Cache write: [:faraday_etags, "http://localhost:4567/"]
I, [2012-04-22T14:19:14.365021 #92297]  INFO -- Status: 200
D, [2012-04-22T14:19:14.365072 #92297] DEBUG -- response: x-frame-options: "sameorigin"
x-xss-protection: "1; mode=block"
content-type: "text/html;charset=utf-8"
etag: "\"abcdef\""
content-length: "18"
server: "WEBrick/1.3.1 (Ruby/1.9.3/2012-04-20)"
date: "Sun, 22 Apr 2012 12:19:14 GMT"
connection: "close"
---
"Hello from Sinatra"
---
I, [2012-04-22T14:19:14.365193 #92297]  INFO -- : get http://localhost:4567/
D, [2012-04-22T14:19:14.365221 #92297] DEBUG -- request: 
D, [2012-04-22T14:19:14.365262 #92297] DEBUG -- : Cache read: [:faraday_etags, "http://localhost:4567/"]
I, [2012-04-22T14:19:14.367825 #92297]  INFO -- Status: 304
D, [2012-04-22T14:19:14.367876 #92297] DEBUG -- response: x-frame-options: "sameorigin"
x-xss-protection: "1; mode=block"
etag: "\"abcdef\""
server: "WEBrick/1.3.1 (Ruby/1.9.3/2012-04-20)"
date: "Sun, 22 Apr 2012 12:19:14 GMT"
connection: "close"
---
"Hello from Sinatra"
---