COMP 9243 — Week 8 c ( 17 s 1 )

Middleware consists of a layer of services added between those of a regular network OS and the actual applications. These services facilitate the implementation of distributed applications and attempt to hide the heterogeneity of the underlying system architectures (both hardware and software). Middleware is usually based on a particular paradigm, or model, for describing distribution and communication. Providing such a paradigm automatically provides an abstraction for programmers to follow, and provides direction for how to design and set up the distributed applications. Recall from the first lecture that there are several advantages and disadvantages to the middleware approach. The greatest advantage is that it runs in user space and can provide independence from OS, network protocol, programming language, etc. Unfortunately, this independence means that a lot of standard OS services must be reimplemented by the middleware, leading to an overabundance of features and bloat. One direction that current middleware is taking is to increase flexibility (and reduce bloat) by focusing on adaptation and reflection. The most popular (and widely used) middleware abstraction is that of distributed objects. Besides distributed objects, other abstractions include RPC, transaction processing, message passing, events, shared data spaces, and web services. In this lecture we focus on distributed object-based middleware, message-oriented middleware, and (event-based) publish/subscribe middleware.