Distributed Systems - Guide to Cloud Computing for Business and Technology Managers - page 81

sends the request to the remote ORB, which locates the appropriate object
residing in a server and passes back an object reference to the requester. An
object residing in a client can then make the remote method invocation of
a remote object. When this happens the ORB marshals the arguments and
sends the invocation over the network to the remote objects ORB that invokes
the method locally and sends the results back to the client.
DCOM, developed by Microsoft, is a protocol that enables communication
between two applications running on distributed computers in a reliable,
secure, and efficient manner. DCOM is an extension of the Component Object
Model (COM). COM is an object-based programming model and defines
how components and their clients interact. COM allows the development of
software components using a variety of languages and platforms to be eas-
ily deployed and integrated. The distributed COM protocol extends the pro-
gramming model introduced by COM to work across the network by using
proxies and stubs. Proxies and stubs allow remote objects to appear to be in
the same address space as the requesting object. When a client instantiates a
component that resides outside its address space, DCOM creates a proxy to
marshal method calls and route them across the network. On the server side,
DCOM creates a stub, which unmarshals method calls and routes them to an
instance of the component.
Java RMI is a package for writing and executing distributed Java programs
by facilitating object method calls between different Java Virtual Machines
(JVM) across a network. Java RMI hides most of the aspects of the distri-
bution and provides a conceptually uniform way by which local and dis-
tributed objects can be accessed. An RMI application consists of a server
interface, a server implementation, a server skeleton, a client stub, and a cli-
ent implementation. The server implementation creates remote objects that
conform to the server interface. These objects are available for method invo-
cation to clients. When a client wishes to make a remote method invocation
it invokes a method on the local stub, which is responsible for carrying out
the method call on the remote object. The stub acts as a local proxy. A server
skeleton exists for each remote object and is responsible for handling incom-
ing invocations from clients.
CORBA, DCOM, and Java RMI enjoyed considerable success, but they were
beset by shortcomings and limitations when used in Web environments. For
example,
• They tend to create tightly coupled distributed systems
• Some are vendor and platform specific (e.g., COM/DCOM only runs
on Wi ndows)
• The distributed systems developed run on closely administered
environments
• Some use complex and proprietary protocols, and specific message
formats and data representation