Extend Backend Nodes

Stacki allows the admin to customize the software footprint of a backend node to enable additional functionality.

In a default setup, Stacki installs backend nodes with a very small software footprint. In Stacki parlance, the backend node is brought up to

a ping and a prompt

The backend node will have its network configured, and the SSH daemon is started to allow login access from the frontend.

To make the backend node more useful, other application software and services will need to be installed and configured.

There are several “levels” of installing applications in Stacki, we’re going to look at the simplest case. Assumptions are:

1. The application to be installed is available as an RPM
2. The application can be configured using simple shell command or a script.
3. The developer is has a basic knowledge of editing HTML-like syntax.

Stacki uses a collection of XML files that provide the definition of a system, and the instructions for installing a backend node. To extend the software footprint of a node, we will need to extend the XML framework to accommodate the extra functionality required.

The XML structure is not complicated - think of it as HTML with extra tags. The tags Stacki incorporates, map to kickstart elements you should already be familiar with: pre, post, main, package. From there it's mostly adding shell commands and scripts to install and configure applications.

For more information about the available XML tags, refer to the Wire Reference Guide

Example

In this example, we will extend the backend appliance, by installing the Apache web server and starting the service on the backend node. Further explanation is available in the Concepts section of this page.

The backend appliance definition is a collection of XML files, the principal file being backend.xml.

1. Copy and modify a skeleton.xml template file. The skeleton.xml file resides in

   /export/stack/site-profiles/default/1.0/nodes/
   

   This file contains explanation of the different sections that are allowed in the XML file. Copy the file to extend-backend.xml in the same directory. The new filename instructs Stacki that this file will extend the backend appliance.

2. Edit the file to have the following contents

   <?xml version="1.0" standalone="no"?>
   <kickstart>
   
      <package>httpd</package>
      <package>httpd-tools</package>
      <package>mod_ssl</package>
   
      <post>
      /sbin/chkconfig --add httpd
      /sbin/chkconfig httpd on
      </post>
   
   </kickstart>

   This simple XML file has the following instructions.

   1. It tells the installer to install httpd, httpd-tools, and mod_ssl packages from the repository.
   2. It also tells the installer to run the chkconfig commands after installation.

3. Save the file, and recreate the default distribution. This step picks up extend-backend.xml, and merges it with the default distribution, thereby extending the functionality of the backend node.

   # stack create distribution
   

4. After the distribution is created, verify the kickstart file will generate correctly. List the profile for one of the backend nodes to be installed. This is good hygiene and saves troubleshooting steps later.

# stack list host profile backend-0-0

You should see no errors, and the last bit of output should look like this:

rm -f /tmp/ks-script*

__EOF__

]]>
</section>
</profile>

5. The stack list host profile gives you reasonable assurance backend nodes will install. Set all the backend nodes to install. For more information about re-installing backend refer to the Re-installation section.

   # stack set host boot backend action=install
   

6. Reboot the backend nodes. This will boot the backend nodes into the Stacki installer. Once the installation completes, the node will boot back up into a running state with the Apache web server running.

7. Verify that the Apache web server is running, using the command

   # stack run host backend command='service httpd status'