Sunday, November 21, 2010

Server-Side Scripting in RHQ

The RHQ platform can be extended in several ways, most notably through plugins that run on agents. There also exists the capability to extend the platform's functionality with scripting via the CLI. The CLI is a remote client, and even scripts that are run on the same machine on which the RHQ server resides are still a form of client-side scripting because they run in a separate process and operate on a set of remote APIs exposed by the server.

In this post I am going to introduce a way to do server-side scripting. That is, the scripts are run in the same JVM in which the RHQ server is running. This form of scripting is in no way mutually exclusive to writing CLI scripts; rather, it is complementary. While a large number of remote APIs are exposed through the CLI, they do not encompass all of the functionality internal to the RHQ server. Server-side scripts however, have full and complete access to the internal APIs of the RHQ server.

Server Plugins
RHQ 3.0.0 introduced server plugins which are distinct from agent plugins. Server plugins run directly in the RHQ server inside a server plugin container. Unlike agent plugins, they do not perform any resource discovery. The article, RHQ Server Plugins - Innovation Made Easy, provides a great introduction to server plugins. Similar to agent plugins, server plugins can expose operations which can be invoked from the UI. They can also be configured to run as scheduled jobs. Server plugins have full access to the internal APIs of the RHQ server. Reference documentation for server plugins can be found here. The server-side scripting capability we are going to look at is provided by a server plugin.

Groovy Script Server
Groovy Script Server is a plugin that allows you to dynamically execute Groovy scripts directly on the RHQ server. Documentation for the plugin can be found here. The plugin currently provides a handful of features including,
  • Customizable classpath per script
  • Easy access to RHQ EJBs through dynamic properties
  • An expressive DSL for generating criteria queries

An Example
Now that we have introduced server plugins and the Groovy Script Server, it is time for an example. A while back, I wrote a post on a way to auto-import resources into inventory using the CLI. We will revisit that script, written as a server-side script.

resourceIds = []

criteria(Resource) { 
  filters = [inventoryStatus: InventoryStatus.NEW] 
}.exec(SubjectManager.overlord) { resourceIds << }

DiscoveryBoss.importResources(SubjectManager.overlord, (resourceIds as int[]))

On line three we call the criteria method which is available to all scripts. This method provides our criteria query DSL. Notice that the method takes a single parameter - the class for which the criteria query is being generated. Filters are specified as a map of property names to property values.
Properties names are derived from the various addFilterXXX methods exposed by the criteria object being built. In this instance, the filter corresponds to the method ResourceCriteria.addFilterInventoryStatus.

The criteria method returns a criteria object that corresponds to the class argument. In this example, a ResourceCriteria object is returned. Notice that exec is called on the generated ResourceCriteria object. This method is dynamically added to each generated criteria object. It takes care of calling the appropriate manager which in this case is ResourceManager. exec takes two arguments - a Subject and a closure. Most stateless session bean methods in RHQ go through a security layer to ensure that the user specified by the Subject has the necessary permissions to perform the requested operation. In the CLI, you may have noticed that you do not have to pass a Subject to the various manager methods. This is because the CLI implicitly passes the Subject corresponding to the logged in user. The second argument, a closure, is called once for each entity in the results returned from exec.

Let's look at a second example that builds off of the previous one. Instead of auto-importing everything in the discovery queue, suppose we only want to import JBoss AS 5 or or AS 6 instances.

resourceIds = []

criteria(Resource) { 
  filters = [
    inventoryStatus:  InventoryStatus.NEW,
    resourceTypeName: 'JBossAS Server',
    pluginName:       'JBossAS5'
}.exec(SubjectManager.overlord) { resourceIds << }

DiscoveryBoss.importResources(SubjectManager.overlord, (resourceIds as int[]))

Here we add two additional filters for the resource type name and the plugin. If we did not filter on the plugin name in addition to the resource type name, then our results could include JBoss AS 4 instances which we do not want.

Future Work
The Groovy Script Server, as well as server plugins in general, are relatively new to RHQ. There are some enhancements that I already have planned. First, is adding support for running scripts as scheduled jobs. This is one of the big features of server plugins. With support for scheduled jobs, we could configure the auto-inventory script to run periodically freeing us from manually having to log into the server to execute the script. The CLI version of the script could be wrapped in a cron job. If we did that with the CLI script though, we might want to include some error handling logic in case the server is down or otherwise unavailable. With the server-side scheduled job, we do not need that kind of error handling logic.

The second thing I have planned is to put together additional documentation and examples. With the work that has already been done, the server-side scripting capability opens up a lot of interesting possibilities. I would love to hear feedback on how you might utilize the script server as well as any enhancements that you might like to see.

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