enters into the component. Each component reads messages from a queue, computes the answers or takes other action, and returns, sometimes, a result.

The way in which to decide whether or not a boundary is required is to ask yourself two simple, and related, questions: • What am I trying to protect with this barrier? • What are the consequences if the barrier fails?

In the case of operating systems, the answers are well understood. We are protecting a common resource with KV access to the physical hardware from malicious or buggy code; and the consequences of failure are systems that KODE VICIOUS, known to mere mortals as George V. crash, lose data, and are impossible to use. Neville-Neil, works on networking and operating system

Boundaries, like any other software construct, are a code for fun and profit. He also teaches courses on various matter of “just enough, but not too much.” The bor- subjects related to programming. His areas of interest are ders of the state of California provide another example code spelunking, operating systems, and rewriting your bad from the real world. In the United States you rarely see code (OK, maybe not that last one). He earned his bachelor’s a border post except where there is an interface to a degree in computer science at Northeastern University in foreign country or at a seaport or airport. In part, this is Boston, Massachusetts, and is a member of ACM, the Usenix for convenience. If every truck carrying goods in the U.S. Association, and IEEE. He is an avid bicyclist and traveler who had to stop at each state, this would add time and cost currently lives in New York City. thCaAtCMtrulcikfientgimceomepmanhie aslfwopuaglde naodt:t Loalyeoruatte.1Ca9li/fo 4/rn08ia 4:04 ©P2 M009 PaAgCM e 11542-7730 /09/0200 $5.00

is different. To protect (note the word) its agriculture, it has erected borders with other states in order to inspect vehicles transiting its borders. The state has decided that the cost of not protecting its borders from fruit flies and other agricultural pests is too high to ignore.

In a complex software system there are probably only a few components that need this extra kind of protection, and it’s the job of a good engineer to find the right places and put the proper protections in place.

 

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