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Unformatted text preview: September 2006 Discussion Writing correct concurrent programs is harder than writing sequential ones. This is because the set of potential risks and failure modes is larger - anything that can go wrong in a sequential program can also go wrong in a concurrent one, and with concurrency comes additional hazards not present in sequential programs such as race conditions, data races, deadlocks, missed signals, and livelock. Testing concurrent programs is also harder than testing sequential ones. This is trivially true: tests for concurrent programs are themselves concurrent programs. But it is also true for another reason: the failure modes of concurrent programs are less predictable and repeatable than for sequential programs. Failures in sequential programs are deterministic; if a sequential program fails with a given set of inputs and initial state, it will fail every time. Failures in concurrent programs, on the other hand, tend to be rare probabilistic events. Because of this, reproducing failures in concurrent programs can be maddeningly difcult. Not only might the failure be rare, and therefore not manifest itself frequently, but it might not occur at all in certain platform congurations, so that bug that happens daily at your customer's site might never happen at all in your test lab. Further, attempts to debug or monitor the program can introduce timing or synchronization artifacts that prevents the bug from appearing at all. As in Heisenberg's uncertainty principle, observing the state of the system may in fact change it. So, given all this depressing news, how are we supposed to ensure that concurrent programs work properly? The same way we manage complexity in any other engineering endeavor - attempt to isolate the complexity. Structuring programs to limit concurrent interactions It is possible to write functioning programs entirely with public, static variables. Mind you, it's not a good idea, but it can be done - it's just harder, and more fragile. The value of encapsulation is that it makes it possible to analyze the behavior of a portion of a program without having to review the code for the entire program....
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