Hi Guys,
I want to discuss a problem I have
found when using the combination of
knowledge agent + classpathResources.
I will try to describe what am I doing
first to give you some context.
I'm deploying drools-camel-server in a
Tomcat 7 container. Inside the
WEB-INF/classes directory I have some DRL
files that I want to use.
My knowledge-services.xml file declares
the following kagent:
<drools:kagent id="kagent1"
kbase="kbase1" new-instance="false">
<drools:resources>
<drools:resource
type="DRL" source="classpath:simple.drl"/>
...
</drools:resources>
</drools:kagent>
When spring parses this configuration
file it creates a KnowledgeAgent instance
with a ChangeSet containing all the listed
resources.
The next step is to start
ResourceChangeNotifierService
and ResourceChangeScannerService.
So far so good.
The problem:
The problem I'm having is not directly
related to drools, but I think it is quite
easy to provide a solution for the people
that is in my same situation.
ClassPathResource is the class that
represents a resource defined as "classpath:"
This class has 2 important methods:
public long getLastModified(){
return
this.classLoader.getResource( this.path
).openConnection().getLastModified();
}
public InputStream getInputStream(){
return this.classLoader.getResourceAsStream(
this.path );
}
The first method is used
by ResourceChangeScannerService to check
whether the resource has changed or not.
It works fine. When the resource in the
filesystem changes, the scanner detects
the change without any problem.
The scanner ends up notifying the
kagent about the change, and the kagent
passes the Resource to an instance of
KnowledgeBuilder.
An here is when things fail.
The kbuilder uses the second method
of ClassPathResource (getInputStream()) to
get the content of the resource. In the
case of Tomcat (and probably some other
environments), it seems that the
classloader (Tomcat's classloader) is
using a cache. So the InputStream returned
doesn't reflect the current state of the
resource.
Long story short: the agent is notified
about a change in the resource, but the
change is never applied to the kbase
because the kbuilder is unable to get it
:P
Solutions:
The first solution is not to use
classpath resources :). You can use just
url resources like
http:// or
file:/. But
honestly, when you have your rules inside
your webapp, it is much
more comfortable and even manageable to
avoid the use of real paths.
What I was thinking about (I already
have a working prototype) is to create a
new Resource type for these cases. This
resource type will let you define your
resources present in your classpath as
usually but it will translate them to URL
Resource internally.
So, in the example above:
<drools:resource type="DRL"
source="URLClasspath:simple.drl"/>
Opinions?