Monthly Archives: October 2012

Simple Configuration Service

The Basics

So I’ve seen a few implementations of ‘config services’ being built in all sorts of {ruby,python,php,java} applications. It really gets on my nerves to see people implementing things that could be much simpler. As Engineers we should really strive to get the most simple product built in a flexible way. So here is my go at implementing a ‘config service’ (basically a key value store optimised for reads). This was originally tweeted in a set of four tweets (1, 2, 3, 4).

Tweet 1

A 'Config Service' that fits in a tweet:
<Location /config>
  DAV svn
  SVNPath /var/svn/config
  SVNAutoversioning on

Simply place this in your apache server configuration. You’ll need to create the SVN repo, which can be done with ‘svnadmin create /var/svn/config’ then chmod it to be writiable and readable by apache. What we are doing here is turning on WebDAV (which lets us upload and make directories on the apache server) and turning on SVN backed autoversioning which means that every change we make will be recorded in the SVN repo.

Tweet 2

To get a variable:
curl -s http://config/config/namespace/key

This simply performs a GET request for a key. You can’t do this until you add some keys to the server.

Tweet 3

To change a variable:
echo "value" | curl -T- http://config/config/namespace/key

This creates/modifies a key. You will need to have the namespace created first, which is in the next tweet.

Tweet 4

Make a new namespace:
curl -XMKCOL http://config/config/namespace

This creates a namespace to put keys in.



The service above doesn’t support users, so anyone can modify the keys, and the svn logs look like this:

r5 | (no author) | 2012-10-15 12:36:33 +1100 (Mon, 15 Oct 2012) | 2 lines

Autoversioning commit:  a non-deltaV client made a change to

All we need to do is require the user to authenticate when doing PUT requests to the server. You can use this method to restrict certain namespaces too. Generate a htpasswd file for the users and place it in /var/svn/users and change the code to read as follows.

<Location /config>
  DAV svn
  SVNPath /var/svn/config
  SVNAutoversioning on
  AuthName "Config Service"
  AuthType Basic
  AuthUserFile /var/svn/users
  <LimitExcept GET>
    require valid-user

Viewing the logs

Simply do this:

svn checkout http://config/config
cd config
svn log

You can also use this to make bulk/atomic changes in the same way you would make changes to a subversion repository.

Ansible Talk @ Infra Coders

Here are the notes that I used in my talk at Infrastructure Coders. Each section was also put on the screen as a ‘slide’. The configuration that I used in the demo is available at GitHub. A full video of the meetup is available on the Infrastructure Coders Youtube Channel, my talk starts at 25:05.


0. There is nothing in the hat
  - Start a RHEL install
    - Cmd line: console=ttyS0 ks=http://admin01/ns3.cfg
    - If you want to follow the demos grab the ansible config from my github
    - You will need to substitute hostnames in the ansible hosts file
    - You should copy the firewall config from ns2 (remove port 647 if paranoid)

1. The problem
  - Ansible is the combination of several functions
  - There was a plan to build config management on func
  - However func is a pain to setup
  - Puppet and Chef have a steep learning curve
  - Ansible was also built to simplfy complexrelease procedures
  - You need to know ruby to extend Puppet/Chef

2. Ansible
  - Designed so you can figure out how to use it in 15 minutes
  - Designed to be easy to setup
  - Doesn't require much to be installed on the managed host
  - Designed to do config management/deployment/ad hoc
  - Other people do security better, just use SSH
  - You can extend ansible in any language that can output JSON

3. Simple Ansible Demo
  - Ansible hosts file
  - Ansible can be run directly on the command line
    - Run cat /etc/redhat-release
    - Get info using the setup module
  - It can prompt for auth, or use key based auth
    - On the new machine show it prompting
    - Run the rhelsetup script on the new machine
    - Install vim-enhanced

4. Playbooks
  - This is the method of scripting Ansible
  - Done in YAML
  - Executed in order *gee thanks puppet*
  - Designed to be as easy as possible

5. Example playbook
  - Playbook for the name servers
    - Can have multiple plays in a book
    - Can serialise if you dont want all to be down at once
  - Template config for the name servers
  - Firewall install script

6. My thoughts
  - Config management has been around a while, its going from art to science
  - Ansible covers more ground than puppet and chef do
  - Ansible doesn't compromise on simplicity to do that
  - I don't have to focus on the nodes, I can focus on services
  - There is something missing
    - Disk config is done in kickstarts
    - Network config can't be done by Ansible
    - Need to find a way to cover both with one

The playlist of all the videos is available at Youtube.