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 l Higher-level abstractions (RPC, RMI, …) l Location transparency l Independent of communication protocols l Independent of hardware/operating systems l 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. l Client/Server l Object References l Interfaces l Remote Method Invocation l 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 l Proxy: the local representative of the remote object. l Dispatcher: relays a request to the appropriate skeleton method. l 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 l Defined by OMG to hide the intricacies of network programming. l An ORB (Object Request Broker) receives invocations from a client and deliver them to a target object. l 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 l Each object has an interface defined in IDL. l An interface defines the operations that can be called by the clients. l An interface can be implemented in one language and called from by another. l The CORBA IDL includes features such as inheritance of interfaces, exceptions, and compound data types.

CORBA Programming with Java l Define the interfaces using IDL and compile them into Java interfaces. l Implement the interfaces with Java classes. l Write a server main function that creates instances of these classes and then inform the underlying CORBA implementation. l Register the server. l 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