Hacking Thy Fearful Symmetry

Varys' Little Birds

July 21st, 2012

One of the great joys of Perl and CPAN is how it allows you to stand on the shoulders of giants. By picking the right tools, applications that are not that trivial can be built in a matter of days, if not hours, and the goal of my little project of today is to demonstrate that very thing.

So. Grab a helmet and put your mouth-piece on, for this time I aim at nothing else than to blow your mind to awestruck smithereens.

The Specs

In a vague, semi-related follow-up to Dumuzi, I was wondering last week if I could have system checks that I could install on different machines and query via a web server. Those checks would come with two modes: passive checking, where we only collect information, and testing, where we check if everything's peachy.

On top of that, why not have the results of the checks stored in a local history database.

And it would also really be good if I could also run the same checks, with a minimum of code change. Or, since we're in full dream mode, maybe no code change at all.

Sounds like a fair application. In those three little paragraphs, we managed to squeeze needs for http, cli and database stacks, which need to be all put together in a seamless way. Okay. So, how many lines of code will be required to build that app (that, for giggles, I'll call Varys). Well, let's see...

A Check for Disk Usage

First thing, we need checks. As a sample, we'll write one that reports on the disk partitions and, possibly, check that none is getting too full.


Checks are going to be what we write over and over again, so we want to make it as easy to use as possible. All we are expecting from it are attributes that can either be parameters passed to the check (labeled via the Input trait), or collected data (labeled via the Info trait). Plus a test() function that will return a hashref with a success result, and whatever other information we want to provide (in this case, the list of bloated partitions).

In truth, the only piece of boilerplate that we need is the class_has '+store_model' stanza, which is required as we are going to use the DBIx::NoSQL::Store::Manager system I put together a few weeks ago. But more details on that later on.

The Checks Inner Mechanisms

Of course, checks don't run on pure pixie magic. It's close to it, but not quite. The role of the sparkling dust, in this case, is played by the parent class Varys::Check, which takes care of setting all the common stuff and hook points for the overall systems that will be using those checks:


As you can see, we're using MooseX::App::Cmd for our cli invocation of the checks. In consequence, we have to tweak things such that the same class will not complain when used outside of that harness (lines 20-23), and we have to provide an execute method (line 92).

We're also using that DBIx::NoSQL::Store::Manager add-on I wrote on top of DBIx::NoSQL, so we also need to have a store key (lines 25-29).

The rest are things all checks will share: attributes for the name of the check, the timestamp of when it is run, if the test has to be executed and a last attribute to store the results of the said potential test, and the info method, which serialize the information of the check in a format we'll be able to bandy around.

The Web Service

For the web service, we are using dear lithe and nimble Dancer:


The brievity of the code talks for itself. For each check, we are creating a GET action (for simple information retrieval) and a POST action (for running the test). A pre-serializing hook takes care that all results are kept in our store, and the serializing itself is taken care of by Dancer and our checks' info() method. Oh yes, and we've thrown in some basic auth, because letting anybody run stuff on your machines? Not smart.

The result:


The CLI Application

With what we already have, adding the cli application is only a question of throwing a script called varys.pl:


Yup, just that. And with that, we can now do:



Aaaand that's it for now. The code, as usual, is available on GitHub. The DBIx::NoSQL::Store::Manager-related classes are still hidden in my Galuga project, but should be CPANized in a not-so-distant future.

Seen a typo or an error? Submit an edit on GitHub!