On Oct 11, 2010, at 2:44 PM, Mircea Markus wrote:
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/j...
>
> 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.
If your class extends Externalizable, things are not that bad tbh. Our Externalizer is the
very best, but Externalizable should work pretty well. Remember that part of the java
serialization problem is the OOS/OIS which we don't use any more since we have JBoss
Marshalling.
>
> * 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(a)jboss.org
> Lead, Infinispan
> Lead, JBoss Cache
>
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>
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>
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