Chapter 29

Chapter 29 Middleware

Objectives

To present and contrast the major approaches to supporting distributed software, by synchronous object invocation and by asynchronous message passing.
To show how heterogeneity can be supported.

Points to emphasise

The differences between the OOM and MOM paradigms. OOM, with synchronous invocation, requires all components to be running at the same time. MOM is inherently asynchronous and senders and receivers are decoupled. The providers of OOM have seen this difficulty and have provided (apparently) asynchronous extensions.
Contrast with the web. Are the approaches complementary?

Possible difficulties

Middleware standards are large, widely available on the web and described in very general terms. If the students don’t get hands-on experience they find it difficult to see what’s really going on. Referring back to Chapter 15 on RPC and Java RMI should help with stubs and skeletons.

This area is evolving rapidly.

Teaching hints

Practical work on Java using JVM and web browsers or using Java RMI is easy to set up. Students may already have used it for concurrent but not distributed programming. It is suitable for individual project work too, e.g. a distributed diary and room booking system using RMI could be a final year undergraduate project.
Discuss whether Java is a closed-world, single language approach to distributed software development. CORBA is predicated on the requirement for general interoperability. Discuss how other middleware is supporting heterogeneity.
How has the web affected other forms of middleware?