Re: Distributed Computing confusion

• To: visad-list@xxxxxxxxxxxxx
• Subject: Re: Distributed Computing confusion
• From: "From: Bill Hibbard <hibbard@xxxxxxxxxxxxxxxxx>" <billh@xxxxxxxxxxxxx>
• Date: Fri, 30 Jul 1999 13:40:04 -0500 (CDT)

[I am re-sending this because I don't think it worked
the first time I sent it.  Doug, please let me know if
you get this.  Thanks.]

Hi Doug,
 
> I have a data server running with VisAD data objects only (no display).
> I have a client that gets data references from the server (based on
> Test14,15) and does the display. I am having some confusion about where
> I need to use Remote objects.
>
> - Does my client need a RemoteDisplayImpl or can I add
> RemoteDataReference's to a local DisplayImpl?
 
Yes, the client needs to construct a RemoteDisplayImpl (constructor
argument is the local DisplayImpl) to link (via addReference or
addReferences) with the RemoteDataReference.
 
> I read that one can add a
> RemoteDataReferenceImpl to a local DisplayImpl, but I can't seem to cast
> my RemoteDataReference to a RemoteDataReferenceImpl.
 
That must be error.  If you can recall where you read it, I'll
correct it.
 
> I'll settle for a
> simple "Yes" here. I have it working, but I don't want to add extra
> overhead if it is not needed.
 
In general, VisAD requires that local objects only be linked
with local objects, and remote objects only linked with remote
objects.  When you need a remote object to link, you can construct
it from the local object.  For example,
new RemoteDisplayImpl(DisplayImpl display) and
new RemoteDataReferenceImpl(DataReferenceImpl ref).
 
> - The client application needs to get some data values (doubles) from
> the server. When I run the server as part of the same JVM (using none of
> the Remote stuff) I can use the DataReference getData method and cast
> that to a FieldImpl and call its getValues method. Running remotely, I
> can't seem to cast the results of the (remote)DataReference getData
> method to something like RemoteFieldImpl so I can call a getValues
> method. Do I need a RemoteFieldImpl on the server?
 
If the argument to DataReference.setData(Data d) on the server
is a FieldImpl, then you should be able to cast the return value
of DataReference.getData() to FieldImpl.  That is, moving data
from the server to the client does not change its class to
Remote*.
 
If the server does setData(RemoteFieldImpl field) then the client
can cast (RemoteField) getData().  The client sees the remote
data through the RemoteField interface, and the server implements
this interface using the RemoteFieldImpl class.  The client does
not see RemoteFieldImpl directly, but only through the RemoteField
interface.
 
You would only use RemoteFieldImpl and RemoteField in this way
to avoid the expense of copying the entire Field data object
from server to client.  If you want the getData() call on the
client to immediately download the data from the server, then
use setData(FieldImpl field) on the server, and cast as
(FieldImpl) getData() on the client.
 
Even if the server serves a RemoteFieldImpl, the client can
force it to be local using:
 
  FieldImpl field = (FieldImpl) (ref.getData().local());
 
I know this is a bit complex, so please let me know if it is
not clear.
 
Cheers,
Bill
----------------------------------------------------------
Bill Hibbard, SSEC, 1225 W. Dayton St., Madison, WI  53706
hibbard@xxxxxxxxxxxxxxxxx  608-263-4427  fax: 608-263-6738
http://www.ssec.wisc.edu/~billh/vis.html

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