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If you just joined us, we’re fawning over Tanaka-san’s code lately. He’s the author of an entire repository of red magic. One of his masterpieces is open-uri, a library that in now enshrined in Ruby 1.8.0.

With open-uri, you can access a file over FTP or HTTP and read from like it was any other IO handle (using read.) For HTTP resources, you can access headers through the meta accessor:

 require 'open-uri'
 open( "http://redhanded.hobix.com/index.xml" ) do |feed|
   p feed.last_modified
   p feed.meta['etag']
 end

The ETag header is great, as it offers a unique hash for the content being served. If your web server returns an ETag, you can issue a conditional GET.

Especially common in RSS feed readers. This technique was also used in the old RAA-Install to check if the RAA feed had changed since the last install. You just send the web server the old etag under the If-None-Match header. The web server will send you a 304 status unless the file is new. This prevents the whole file from being sent if it is old.

 require 'open-uri'

 # 1st request -- save the ETag
 etag = nil
 open( "http://redhanded.hobix.com/index.xml" ) do |feed|
   etag = feed.meta['etag']
 end

 # 2nd request -- only retrieve the file if it has changed
 begin
   open( "http://redhanded.hobix.com/index.xml", 
         "If-None-Match" => etag ) do |feed|
     puts "File has changed: #{ feed.read.length } bytes read" 
   end
 rescue OpenURI::HTTPError
   puts "No file available or file has not changed." 
 end

You can also use the modification time, with the help of the Time#rfc2822 method which formats dates for HTTP.

 require 'open-uri'

 # 1st request -- save the modification time
 mtime = nil
 open( "http://redhanded.hobix.com/index.xml" ) do |feed|
   mtime = feed.last_modified
 end

 # 2nd request -- only retrieve the file if it has changed
 begin
   open( "http://redhanded.hobix.com/index.xml", 
         "If-Modified-Since" => mtime.rfc2822 ) do |feed|
     puts "File has changed: #{ feed.read.length } bytes read" 
   end
 rescue OpenURI::HTTPError
   puts "No file available or file has not changed." 
 end