In the previous post, I mentioned a use case for the Virgo buildpack of running Virgo as an Ubuntu Juju charm. With some advice from the guys at Canonical, for which I'm grateful, a first prototype is now working.
The purpose of a charm is to instantiate a service in a cloud. I'm testing on Amazon EC2, although the charm should run without change on other clouds. The crucial contents of a charm are a simple metadata description and a series of hooks to control the lifecycle of the service associated with the charm and its relationships with other entities in the cloud.
So far, I've implemented two simple hooks. An install hook downloads and installs a JRE and Virgo as well as a test application. It does this by installing the test application and then running the Virgo buildpack compile operation against the application, which in turn downloads and installs a suitable JRE and Virgo (actually, Virgo Server for Apache Tomcat). A start hook starts Virgo using a command similar to that output by running the Virgo buildpack release operation against the test application.
The following pieces of work are necessary to tidy up the prototype:
- Make the start hook reuse the output of the release operation which is a piece YAML containing the basic shell command necessary to start Virgo. It will probably be simplest to rewrite the start script in Ruby so that parsing the YAML file will be trivial.
- Somehow synchronise the termination of the start script with the completion of starting Virgo. Currently, the start hook "fires and forgets" using an asynchronous invocation of the Virgo start script. The result is that the start hook returns before Virgo has started, which gives the wrong impression of the state of the charm, especially if Virgo startup fails for any reason.
- Implement any other hooks which are required as a bare minimum, including, presumably, a stop hook.
- Provide a mechanism to deploy a specified application to Virgo in place of the test application.
I've licensed the code under the Apache v2 license and would be delighted to accept contributions if anyone would like to raise issues, make some of the changes mentioned above, or otherwise improve the charm.