James Crisp

When offering higher levels of uptime on a web site backed by MySQL, a good approach is to set up a MySQL master-slave configuration for failover between servers. This generally works quite well, but once in a while, there is a problem or error that causes the replication to cease. The slave then ceases to process updates and gets out of sync with the master.

The script below is a quick and easy approach to monitoring the status of replication on the slave. If the slave thread or IO ceases, the slave gets more than 120 seconds behind the master, or there is an error, the script will email all the slave status information to an email address you specify to alert you that you need to log in and sort things out. I run the script from cron so that I get notified fairly soon if a problem arises.

RAILS_ENV = 'production'
ALERT_EMAIL_ADDRESS = '[email protected]'

require 'open3'
require 'socket'
require "#{File.dirname(__FILE__)}/../../config/environment.rb"

r = ActiveRecord::Base.connection.execute("show slave status").fetch_hash
unless  r["Slave_IO_Running"] == "Yes" && r["Slave_SQL_Running"] == "Yes" &&
  r["Last_Errno"] == "0" && r["Seconds_Behind_Master"].to_i < 120

    status = "*** STATUS ***\\n" + r.to_a.collect { |i| "#{i[0]}: #{i[1]}\\n" }.join
    subject = "MySQL Slave Replication Down on #{Socket.gethostname}"

    Open3.popen3("mail -s \"#{subject}\" #{ALERT_EMAIL_ADDRESS}") do |stdin, stdout, stderr|
       stdin.write(status)
    end
end

Note: This script relies on being part of a rails app to get a database connection. It would be fairly easy to modify it to include db credentials and open the connection.

Once a business web application reaches a certain size, the need often arises to generate PDFs from HTML/CSS.

Up until recently, the story around this for a MRI Rails application was not good. You could either use tools like Prawn, which require a description of the layout in a specific DSL, or pay for a tool like Prince XML which can convert from HTML, but which costs quite a bit. Those using JRuby were in a stronger position as they could use the Java PDF library called Flying Saucer.

The good news is that PDF generation for MRI Ruby is now easy and free, thanks to webkit, the open source webkit wrapper called wkhtmltopdf and mileszs’s wickedpdf plugin. I was really excited to come across this plugin and started to use it right away. However, it had a couple of issues:

• Temp file handling caused errors when two PDFs were being generated within the same second (eg, 2 requests at almost the same time)
• Problems generating PDF were not reported

Galdomedia forked the code and updated it to use standard Ruby temp files. This was great for ruby 1.7, but not good for Ruby 1.6 which does not allow you to set the extension on temp files (wkhtmltopdf relies on having a .html extension).

As my production servers run Ruby 1.6, I needed a different approach. My fork uses streams rather than temporary files, and adds some basic error handling and basic integration tests.

To install in a rails app:

script/plugin install git://github.com/jcrisp/wicked_pdf.git

Or clone the code from GitHub.