Operating Systems and Middleware

Suppose you sit down at your computer to check your email. One of the messages includes an attached document, which you are to edit. You click the attachment, and it opens up in another window. After you start editing the document, you realize you need to leave for a trip. You save the document in its partially edited state and shut down the computer to save energy while you are gone. Upon returning, you boot the computer back up, open the document, and continue editing.

This scenario illustrates that computations interact. In fact, it demonstrates at least three kinds of interactions between computations. In each case, one computation provides data to another. First, your email program retrieves new mail from the server, using the Internet to bridge space. Second, your email program provides the attachment to the word processor, using the operating system's services to couple the two application pro grams. Third, the invocation of the word processor that is running before your trip provides the partially edited document to the invocation running after your return, using disk storage to bridge time.

In this book, you will learn about all three kinds of interaction. In all three cases, interesting software techniques are needed in order to bring the computations into contact, yet keep them sufficiently at arm's length that they don't compromise each other's reliability. The exciting challenge, then, is supporting controlled interaction. This includes support for computations that share a single computer and interact with one another, as your email and word processing programs do. It also includes support for data storage and network communication. This book describes how all these kinds of support are provided both by operating systems and by additional software layered on top of operating systems, which is known as middleware.