Re: Roles and Components in Merlin : another newbie

Fri, 16 Jan 2004 20:11:01 -0800

Stephen,

That was a great explanation. Thanks.

I found from the Fulcrum website that you are involved in that project also. And I read one of ur emails in the list in which u mentioned that all fulcrum components now run in Merlin.

As I mentioned, In the Fulcrum code that I checkout I found the interfaces using roles and there were no avalon tags. I just looked into one component: the "pool" component. So does that mean the component won't run in Merlin ?

I basically came to Avalon via Fulcrum and to Fulcrum via Turbine. I understand Merlin in the latest and the greatest.

So is the process of convertion of Fulcrum components and services to Merlin under way right now. Fulcrum site still mentions ECM but I can assume that its outdated.

But Fulcrum components in their present form ( the new ones ) must be compatible to some Avalon standard/pattern. What standard is that ? ECM, Fortress or Merlin ?

I want to follow the Avalon container standard ( or whatever u call it ), where Fulcrum has moved or is moving.

On http://jakarta.apache.org/turbine/fulcrum/changes-report.html, what is meant by the first change in the list, which involves you.

Thanks a lot for ur help

M






From: Stephen McConnell <[EMAIL PROTECTED]>
Reply-To: "Avalon framework users" <[EMAIL PROTECTED]>
To: Avalon framework users <[EMAIL PROTECTED]>
Subject: Re: Roles and Components in Merlin : another newbie
Date: Sat, 17 Jan 2004 02:32:22 +0100

Mathews Julien wrote:

So does having
"String ROLE = InterfaceName.class.getName();" doesn't matter.


The notion of ROLE has never been used within Merlin.


And how are roles resolved now in Merlin ?


Forget about the term "role" - think instead in terms "component", "services" and "dependencies. Any component compliant with the Avalon Meta package declares itself as a component by simply including @avalon.component at the class level (in fact that's not even a requirement - but that's another subject).

 /**
  * @avalon.component name="fred"
  */
  public class Fred
  {
  }

If the component has a dependency then the component needs to declare this using @avalon tags. Dependencies that are managed my Merlin include all deployment and runtime features that the Avalon Meta Model declares. These include classic service dependencies (accessible via a ServiceManager), context dependencies (accessible via a Context interface, derived context interface, or custom contextualization strategy), not to mention logging, parameterization and configuration dependencies, together with lifecycle stage dependencies.

Here is a simple addition to the above component to include a service dependency:

 /**
  * @avalon.component name="fred"
  */
  public class Fred
  {
      private Gizmo m_gizmo;

     /**
      * @avalon.dependency type="Gizmo" key="my-gizmo"
      */
      public void service( ServiceManager manager )
        throws ServiceException
      {
          m_gizmo = (Gizmo) manager.get( "my-gizmo" );
      }
  }

The key things in the above example - the component is declaring explicitly that it needs a service that conforms to the Gizmo interface contract. Secondly, it is saying that it will lookup that service in the service manager using the key "my-gizmo". From here it is the containers responsibility to establish and associate the component relative to the requested key.

A complete description of the @avalon tags is available here:

http://avalon.apache.org/meta/tools/tags/index.html

The specification of the component type descriptors build from tags which provides binary component portability is defined here:

http://avalon.apache.org/meta/meta/info/type/index.html

An API supporting dynamic component type definition construction is available here:

http://avalon.apache.org/meta/api/index.html

The bottom line is ... if you can describe the contract using the @avalon tags you have source portability across any container that is compliant with the Avalon Meta standard. If you use the tools bundled with the package you can automatically escalate to binary component compatibility.

Finally, Merlin ... actually there isn't much to say. Merlin is based on the Avalon Meta Model, it defines a parallel architecture with respect to meta data management, a comprehensive model-driven architecture, and ultimately - an efficient and reliable runtime platform.

Cheers, Stephen.

--

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