Sat, 16 Jan 2016

Automating Deployments: Installing Packages

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After the long build-up of building and distributing and authenticating packages, actually installing them is easy. On the target system, run

$ apt-get update $ apt-get install package-info

(replace package-info with the package you want to install, if that deviates from the example used previously).

If the package is of high quality, it takes care of restarting services where necessary, so no additional actions are necessary afterwards.

Coordination with Ansible

If several hosts are needed to provide a service, it can be beneficial to coordinate the update, for example only updating one or two hosts at a time, or doing a small integration test on each after moving on to the next.

A nice tool for doing that is Ansible, an open source IT automation system.

Ansibles starting point is an inventory file, which lists that hosts that Ansible works with, optionally in groups, and how to access them.

It is best practice to have one inventory file for each environment (production, staging, development, load testing etc.) with the same group names, so that you can deploy to a different environment simply by using a different inventory file.

Here is an example for an inventory file with two web servers and a database server:

# production



Maybe the staging environment needs only a single web server:

# staging



Ansible is organized in modules for separate tasks. Managing Debian packages is done with the apt module:

$ ansible -i staging web -m apt -a 'name=package-info update_cache=yes state=latest'

The -i option specifies the path to the inventory file, here staging. The next argument is the group of hosts (or a single host, if desired), and -m apt tells Ansible to use the apt module.

What comes after the -a is a module-specific command. name specifies a Debian package, update_cache=yes forces Ansible to run apt-get update before installing the latest version, and state=latest says that that's what we want to do.

If instead of the latest version we want a specific version, -a 'name=package-info=0.1 update_cache=yes state=present force=yes' is the way to go. Without force=yes, apt wouldn't downgrade the module to actually get the desired version.

This uses the ad-hoc mode of Ansible. More sophisticated deployments use playbooks, of which I hope to write more later. Those also allow you to do configuration tasks such as adding repository URLs and GPG keys for package authentication.

I'm writing a book on automating deployments. If this topic interests you, please sign up for the Automating Deployments newsletter. It will keep you informed about automating and continuous deployments. It also helps me to gauge interest in this project, and your feedback can shape the course it takes.

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