ORB Technical Details
What's the difference between Location
Transparency and Distribution?
We were careful to say that the ORB took care of routing details, not
distribution details. That's because CORBA is location
transparent: that is, clients make their invocations the
same way regardless of where the target object might be.
It might be remote, out on the network somewhere, but it also
might be local, in the same process as the calling client.
component-based applications, which are composed of several
component types that call each other to perform the total work
process, there are many in-process calls. It's much easier to
program this type of application if you don't have to fix the
local/remote boundary at coding time. Of course, you do have to
fix object location when you assemble the application for
deployment, and when you deploy it on your server.
Where can a reply go?
We were careful to say that the response was routed to its destination,
and not back to the client. Using the new CORBA Messaging
Service, asynchronous replies return to a callback object.
Although this object is usually in the client's process, it can
be anywhere on the network.
What are Session and Stringified
An object reference in session form is an electronic
token that, when associated with an invocation in the way
prescribed by the language mapping, allows the ORB to invoke the
intended object instance. In this form, there is no
information in the object reference that is meaningful to the
client. Many ORBs store the information about the object in
vendor-specific internal structures; for these ORBs, the session
object reference may be a pointer to some part of these
There is a standard form, termed the Interoperable Object
Reference or IOR, that ORBs use to pass object references from
one to another across vendor boundaries. To enable IORs to be
stored, emailed, printed, and retrieved, the ORB will stringify
the reference into a sequence of hexadecimal numbers. Because a
stringified IOR contains only alphabetic characters plus the
digits 0 through 9, it is guaranteed to pass over the network
without being translated by any of the various filters that one
finds in routers and such. Although the format is standard, it
is a very bad idea for a client to interpret a stringified IOR
and and use the information about the object. This breaks
encapsulation, and makes the application dependent on aspects
the object implementation considers its own, and which may
change at any time without notice to the client.
Architecturally, this makes the application subject to
breakdown. Although this may seem harmless enough for a small
application, it can be disastrous for a large application
containing years of effort by hundreds or thousands of
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