[infinispan-dev] Defining new commands in modules

[Mircea Markus]

I've implemented a POC as per your suggestions, my comments bellow.
On 29 Sep 2010, at 20:06, Manik Surtani wrote:

> So this is an extension to the discussion around a GenericCommand that has been going around.  IMO a GenericCommand is a big -1 from me for various reasons - the whole purpose of the command pattern is so we have strongly typed and unit testable commands.  This will help the ongoing work by Mircea, Sanne and Israel on various modules that need to define custom commands.
> 
> I proposed the following solution to Mircea earlier today, I'll repeat here for you guys to discuss.  Note that this is a *half baked* solution and needs more thought!  :-)
> 
> * If a module needs to define custom commands, it should define its own ReplicableCommand implementations in its' own module.
> * It should define a sub-interface to Visitor (MyModuleVisitor) with additional methods to handle the new commands
> * Interceptors defined in this module should extend CommandInterceptor AND implement MyModuleVisitor
> * These new commands can be created directly, or via a new CommandFactory specially for these commands.
directly should not be a problem. 
> 
> Now for the un-finished bits.  :)
> 
> * How does RemoteCommandFactory instantiate these new commands?  The module should have a way of registering additional command IDs with RemoteCommandFactory.fromStream().  See 
> 
> http://fisheye.jboss.org/browse/Infinispan/branches/4.2.x/core/src/main/java/org/infinispan/commands/RemoteCommandsFactory.java?r=2327#l59

> 
> Perhaps RemoteCommandFactory.fromStream() should look up the ID in a map of command creator instances, and each module can register more of these with the RemoteCommandFactory?
that's a more complex issue and has to do with managing the serialization of custom commands through our marshalling framework. Right now we cannot register marshaller at runtime, until https://jira.jboss.org/browse/ISPN-244 is implemented.
My POC is based on java serialization - far from being optimal it can represent an approach till 5.0 is released.
> 
> 
> * How do interceptors defined in the core module handle commands it isn't aware of?  handleDefault()?  
what about just invokeNext? this would make only the interceptors that are extensions to do any work.
> Or should we define a new handleUnknown() method in Visitor for this case, which would default to a no-op in AbstractVisitor?  E.g., in a module-specific command such as MyModuleCommand, I would implement:
works as well. 
> 
> class MyModuleCommand implements ReplicableCommand {
> 
> public Object acceptVisitor(InvocationContext ctx, Visitor visitor) throws Throwable {
>     if (Visitor instanceof MyModuleVisitor) {
>           return ((MyModuleVisitor) visitor).visitMyModuleCommand(ctx, this);
>     } else {
>           return visitor.handleUnknown(ctx, this);
>     }
>  }
> 
> }
> 
> Cheers
> Manik
> 
> PS: There is no JIRA for this.  If we like this approach and it works, I suggest we create a JIRA and implement it for 4.2.  The impl should be simple once we resolve the outstanding bits.
there is one actually: https://jira.jboss.org/browse/ISPN-256
I'll attache the patches to it.
Just to conclude, I've POC both approaches GenericCommand and ExtendingCommand, here are my thoughts on it:
- extending visitor
PROS:  ExtendingCommand is a better OO API, more strongly typed
CONS: it is harder for the user to understand; integrating custom serialization might be difficult(impossible at the time), also it will have to manage command uniques. 

- GenericCommand
CONS: 
- less OO API
- would require a "convention" for not "overusing" it. Overuse would be in client code though, not in ours
PROS:
- easier for the user to understand and use
	- no need for extra classes needed
	- no understanding of how the interceptor chain internals work needed (for writing acceptVisitor). 
	- no understanding of how serialization works  is needed. Nor command uniques.
- easy streight-forward solutions

I would go for GenericCommand, but Extending approach works for me as well.

Cheers,
Mircea
> --
> Manik Surtani
> manik at jboss.org
> Lead, Infinispan
> Lead, JBoss Cache
> http://www.infinispan.org
> http://www.jbosscache.org
> 
> 
> 
> 
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