Scripting News in RHQ 4.5.0

RHQ 4.5.0 (which we released today) contains a great deal of scripting enhancements that I think are worth talking about in more detail. In my eyes, the changes make the scripting in RHQ ready for serious use.

CommonJS support

This, I think, is huge. In the previous versions of RHQ, the only way of reusing functionality from another script was to use the exec -f command in the interactive CLI shell (in another words, this was NOT available in the batch mode, which is how majority of people are using the CLI). So if you needed to implement something bigger and needed to split your code in several files (as any sane person would do), you only had 1 option – before executing the “script”, you needed to concatenate all the scripts together.

This sucked big time and we knew it 😉 But we didn’t want to just add functionality to “include files” – that would be too easy 😉 At the same time it wouldn’t solve the problem, really. The problem with just “including” the files into the current “scope” of the script is that this would mean that each and every variable or function in those files would have to be uniquely named because javascript lacks any sort of namespace/package resolution. Fortunately, the CommonJS spec solves this problem.

Here’s how you use a module. Notice that you assign the module to a variable and that’s how you prevent the “pollution” of your scope. The loaded module can have methods and variables with the same name as your script and they won’t influence each other:

var myfuncs = require("modules:/myfuncs");
myfuncs.helloworld();

You may wonder what that "modules:/myfuncs" identifier means. It is an URI that the underlying library uses to locate the script to load. This “sourcing” mechanism is pluggable and I will talk about it more in the following chapter. To see some examples of the modules, you can look at the samples/modules directory of your CLI deployment and you can also read some documentation about this on our wiki.

Locating the scripts

With the ability to load the scripts there comes the problem of locating them. For the standalone CLI, the obvious location for them is the filesystem, but what about alert notification scripts on the RHQ server? These scripts are stored in RHQ repositories which don’t have a filesystem location. The solution is not to tie the scripts to the filesystem but have a generic way of locating them using URIs and a pluggable way of resolving those URIs and loading the scripts from various locations. This means that you can for example load the script from an RHQ repository in your standalone CLI installation, or to define 1 central location for your local CLI scripts and use the “modules” URIs to refer to them. Or you can easily implement your own “source provider” and for example load the scripts from your internal git repo or ftp or whatnot. RHQ comes with a small set of predefined source providers, documented here.

With this ability at hand, you can make an RHQ repository a central place for your scripts that you will then be able to use universally – both in the standalone CLI installations and also in the alert notification scripts.

Python

In previous versions, our scripting was tied to Javascript. Thanks to quite a bit of refactoring, the RHQ scripting integration is now language independent and language support is pluggable (see my previous post where I detail how this was done in case of Python).

What this all means is that you can now write your CLI scripts in Python and still use the same API as you were able to use before from Javascript only. I.e. you will find the ResourceManager, AlertManager and all the other predefined variables that define the RHQ API available in Python, too. The only thing that this initial implementation doesn’t support is code-completion in the interactive CLI shell.

Last but not least, the ability load the scripts from various locations is available in Python, too, using an automatically installed path_hook. You can read about how to use it on our wiki. This also means that you can now write your alert notification scripts in Python, too.

When running an alert notification script (i.e. an alert notification of the type “CLI Script”), the language of the script is determined from the script file extension – “.py” for python and “.js” for javascript. When you start the CLI shell, you pass your language of choice using the --language commandline parameter – “javascript” or “python” are the obvious possible values for it.

Conclusion

In my opinion, these changes are great and will allow our users to really start building useful tools using our scripting support. If you feel like you’ve come up with a script module you would like to share with the RHQ community, why don’t you just send a pull request to our github repo with sample scripts?

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