Distributed Objects and Middleware

Sockets and Ports

Source: G. Coulouris et al., Distributed Systems: Concepts and Design

Crossing Hosts/Platforms

Source: G. Coulouris et al., Distributed Systems: Concepts and Design

Provisions of Middleware Higher-level abstractions (RPC, RMI, …) Location transparency Independent of communication protocols Independent of hardware/operating systems Use of several programming languages

The Middleware layer

Source: G. Coulouris et al., Distributed Systems: Concepts and Design

* The operating system includes basic network protocols.

Distributed Objects

The object-based programming model is extended

to allow objects in different processes to interact

with one another. Client/Server Object References Interfaces Remote Method Invocation Exceptions

A CORBA IDL Example

Source: G. Coulouris et al., Distributed Systems: Concepts and Design

Local and Remote Method Invocations

Source: G. Coulouris et al., Distributed Systems: Concepts and Design

A Remote Object and Its Interface

Source: G. Coulouris et al., Distributed Systems: Concepts and Design

Client Proxy (Stub) and Server Skeleton (Stub)

Source: G. Coulouris et al., Distributed Systems: Concepts and Design

The RMI Software

Proxy: the local representative of the remote object.

Dispatcher: relays a request to the appropriate skeleton method.

Skeleton: unmarshals the request and invokes the corresponding method in the remote object.

Java Remote Interface Examples

Source: G. Coulouris et al., Distributed Systems: Concepts and Design

Java Server Example

Source: G. Coulouris et al., Distributed Systems: Concepts and Design

Java Server Example (cont’d)

Source: G. Coulouris et al., Distributed Systems: Concepts and Design

Java Client Example

Source: G. Coulouris et al., Distributed Systems: Concepts and Design

CORBA

Defined by OMG to hide the intricacies of network programming.

An ORB (Object Request Broker) receives invocations from a client and deliver them to a target object.

The main communication protocol is GIOP (General Inter-ORB Protocol), known as IIOP when implemented over the Internet.

The CORBA Architecture

Source: G. Coulouris et al., Distributed Systems: Concepts and Design

CORBA Object Interfaces

Each object has an interface defined in IDL. An interface defines the operations that can be

called by the clients. An interface can be implemented in one language

and called from by another. The CORBA IDL includes features such as

inheritance of interfaces, exceptions, and compound data types.

CORBA Programming with Java

Define the interfaces using IDL and compile them into Java interfaces.

Implement the interfaces with Java classes. Write a server main function that creates instances

of these classes and then inform the underlying CORBA implementation.

Register the server. Write a client main function to connect to the

server and to use server’s objects.

CORBA IDL Interfaces: Shape and ShapeList

Source: G. Coulouris et al., Distributed Systems: Concepts and Design

Java Interface Generated from the CORBA Interface ShapeList

Source: G. Coulouris et al., Distributed Systems: Concepts and Design

Java Implementation of Shapelist

Source: G. Coulouris et al., Distributed Systems: Concepts and Design

Java Implementation of ShapeList (cont’d)

Source: G. Coulouris et al., Distributed Systems: Concepts and Design

Java Implementation of a ShapeList Client

Source: G. Coulouris et al., Distributed Systems: Concepts and Design