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Virtualization Package Your Scripts © Cathy Yeulet, 123RF.com Bundle your custom apps in a Debian package It’s a Wrap Get standard scripts and custom applications into the cloud with the debian packaging system. By Dan Frost You’ve got a cloud. It’s great. You can scale, and you’ve got redundancy. But you have about 20 scripts for a bunch of tasks (e.g., one for when an instance is booted up and another for when its IP changes) and these scripts aren’t getting any shorter, they’re getting better and longer. If you want to manage them in your favorite versioning software (which I hope is Git, but might be something else), how do you get that onto the new instances simply? Enter the not-new-at-all technology of Debian packages. They are straightforward to use across any Debianbased Linux and simple to create, and they provide an ideal way of contain- ing and releasing your scripts. In this article, I’ll show how to create Debian packages and how to install them (which you probably already know). And, I’ll explain how the process will make you feel more comfortable about pushing changes live across your cloud. I talked to cloudy people about how to get code onto new instances, and I tried lots of different things, but the Debian package is such a solid, reliable format that I just had to share it. Debian Packages Debian packages are simply archives that are very easy to install, usually in one line. If you’ve ever worked with packages in Ubuntu or any other Debian-related Linux, you’ve probably needed to download a package from an online source and install it: apt‐get install blarblar.deb On the inside, a Debian package is an archive of binary files, scripts, and any other resources an application needs, plus a handful of control files that the various command-line tools use to install the package. Because this standard package format is so easy to install on any Debianbased Linux, it’s a great way to get standard scripts and custom applications into the cloud. Often you need a few lines to configure a new instance or to connect the instance to the rest of your cloud, and storing all those scripts in a package keeps things very neat. The parts of a Debian package I’m most interested in are the control files, which live in a directory called DEBIAN. Control files tell Debian all about what the package contains, 48 Admin 01 www.admin-magazine.com
Package Your Scripts Virtualization what it’s called, the version, and so on. The second part is the code, which I’ll look at in detail later. Creating the Package To show this process at work, I’ll create a simple package. Suppose I need a simple page that can sit on your server until an application is installed. Good practice dictates that your instances should fail nicely, so if you start 10 instances when your app gets 6 million tweets, you at least want them to deliver a nice page before they’re ready to do business. To begin, n Create a directory called myserver and the directories and files inside it: ./DEBIAN/ ./var/www/index.html n Put the code shown in Listing 1 in ./DEBIAN/control. n Put the file shown in Listing 2 in ./var/www/index.html. If the path to the index.html file looks familiar, it’s because the file structure inside your package mirrors exactly the structure in the target instance. If you want a file in /var/some/where/ here, just create that path inside your package project. Once you’ve created this amazing page, package it up with: $ dpkg‐deb ‐‐build myserver dpkg‐deb: building package U `myserver' in `myserver.deb' When you look in the directory, you’ll see a file called myserver.deb. Now that your project is all packaged up, you can install it. But, before you do, take a look at what’s inside: $ dpkg‐deb ‐‐contents myserver.deb then install: $ sudo dpkg‐deb ‐i myproject.deb After running this command, you’ll find the HTML file sitting there for when Apache starts (Listing 3): The next step is to create a skeleton set of scripts for a cloud provisioning service, such as Scalr or RightScale [1]. Indeed, the scripts I use for hosting and development servers all sit inside one Debian package. A Cloudy Package A tiny HTML file isn’t all that useful in the cloud, so I’ll look at something a bit more useful. Server configuration can be set from the Debian package simply by placing your preferred configuration file in the package: ./etc/apache2/conf.d/ our‐config.conf As long as Apache is configured to include this file (which, in Ubuntu, it often is), it will take effect right away. Although you might want to control this with tools such as Puppet after deployment, starting the instance with a good configuration will help keep the environment sane from the outset. Cloud hosting becomes difficult when you use strange configurations – creating exceptions for some apps or generally working against the grain (e.g., using Tomcat’s configuration style and Apache’s config directories). Avoid customizing the environment too much because it will mean extra maintenance in the future and could limit how you can scale. Another common script for cloud servers will run tasks at certain points in the instance’s life cycle. For instance, a service such as Scalr will run scripts on various events, such as OnHostUp, OnHostInit, and OnIPAddressChanged. You can create some scripts for these events in your Debian package: ./usr/local/myserver/bin/on‐host‐up.sh ./usr/local/myserver/bin/on‐ip‐ U address‐changed.sh The first script should download an HTML file or a PHP file from either S3 or your existing repository and place it in the document root: cd /var/www/ wget ‐O tmp.tgz http://mybucket.s3. U amazonaws.com/website.tgz tar xzf tmp.tgz service apache2 restart These scripts will be like any other script you would write, with one important difference: no more human interaction. Everything has to run automatically when new instances start up, and you really don’t want your script waiting for a human that doesn’t exist. Next, package your project into a .deb file and place it somewhere public from which you can download. This might be where you host, but it is much better to put it somewhere resilient, like S3 [2]. Next, log in to Scalr and add the following lines to a new script that will run on the event OnHostUp (Figure 1). # install the package wget ‐O myserver.deb http://mybucket. U s3.amazonaws.com/myserver.deb deb ‐i myserver.deb # run the script /usr/local/myserver/bin/on‐host‐up.sh Save the Scalr script under the name of the event that you want to trigger it and go to the farm configuration. Now you can add your neatly organized scripts without having to edit Listing 1: Control Files Package: myserver Version: 0.0.1 Section: server Priority: optional Architecture: all Essential: no Installed‐Size: 1024 Maintainer: Dan Frost [dan@3ev.com] Description: My scripts for running stuff in the cloud Listing 2: Message Page 01 02 03 We're getting there... 04 05 06 Give us a moment. 07 We're just getting some more machines plugged in ... 08 09 Listing 3: HTML File 01 $ cat /var/www/index.html 02 03 04 We're getting there ... 05 06 ... www.admin-magazine.com Admin 01 49
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