How to get a remote-file's mtime before downloading it in Ruby?

Question

I have the below code, which simply downloads a file and saves it. I want to run it every 30 seconds and check if the remote-file\'s mtime has changed and download it if it has.

Answer 1

The official Net::HTTP 2.6.5 docs have a concrete example of If-Modified-Since which was mentioned by https://stackoverflow.com/a/1509202/895245

uri = URI('http://example.com/cached_response')
file = File.stat 'cached_response'

req = Net::HTTP::Get.new(uri)
req['If-Modified-Since'] = file.mtime.rfc2822

res = Net::HTTP.start(uri.hostname, uri.port) {|http|
  http.request(req)
}

open 'cached_response', 'w' do |io|
  io.write res.body
end if res.is_a?(Net::HTTPSuccess)

Here is a full script that actually runs:

#!/usr/bin/env ruby

require 'net/http'
require 'time'

uri = URI('https://upload.wikimedia.org/wikipedia/commons/thumb/9/95/Illumina_iSeq_100_flow_cell_top.jpg/451px-Illumina_iSeq_100_flow_cell_top.jpg')
file_path = 'cached_response'
req = Net::HTTP::Get.new(uri)
if File.file?(file_path)
  req['If-Modified-Since'] = File.stat(file_path).mtime.rfc2822
end
res = Net::HTTP.start(uri.hostname, uri.port, use_ssl: true) {|http|
  http.request(req)
}
if res.is_a? Net::HTTPSuccess
  File.open(file_path, 'w') {|io|
    io.write res.body
  }
end

but TODO it is updating the file every time, even though Wikimedia seems to interpret If-Modified-Since: https://wikitech.wikimedia.org/wiki/MediaWiki_caching