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Software Fault Tolerance Techniques and Implementation phần 10

Đầu vào biến thể Thiết lập trạng thái = NIL Nhận kết quả biến thể, R, và lòng khoan dung, một ngẫu nhiên chọn một đầu ra biến thể, x Xây dựng tính khả thi thiết lập FS = {x, ri ∈ R x - ri ≤ A, i = 1, ., n} là tính khả thi thiết lập có sản phẩm nào? Không Có Thiết lập trạng thái = NO KẾT QUẢ ĐÚNG (ngoại lệ) chính thức đa số cử tri | Adjudicating the Results 301 Figure 7.14 Formal majority voter operation. FormalMajorityVoter input_vector e r This Decision Mechanism determines the correct or adjudicated result r given the input vector input_vector of variant results and the maximum allowed distance e via the Formal Majority adjudication algorithm. Set Status NIL r NIL FS NIL Receive Variant Results input_vector e Was a Result Received from each Variant No Set Status NO CORRECT RESULT Exception Go To Out 302 Software Fault Tolerance Techniques and Implementation Yes Continue Randomly Select a Variant Output x. Construct the Feasibility Set FS where FS x y in input_vector x - y z Is FS empty No Set r Randomly Selected Value in FS Set Status SUCCESS Yes Set Status NO CORRECT RESULT Exception Out Return r Status FormalMajorityVoter 7.1.5.2 Example An example of the formal majority voter operation is shown in Figure 7.15. Suppose we have a fault-tolerant component with three variants n 3. If the results of the variants are r 17.632 r2 17.674 r1 18.795 then the input vector to the voter is 17.632 17.674 18.795 . 17.632 17.674 18.795 17.632 17.674 18.795 Input vector e 0.05 Comparison Ị17.632 - 17.674Ị 0.042 e 117.632 - 18.7951 1.121 e K FS 17.632 17.674 Randomly select from FS 17.632 as the result r Output r 17.632 and status 17.632 SUCCESS Figure 7.15 Example of formal majority voter. Adjudicating the Results 303 Suppose the variant result selected as the focal point X is r2. The other variant results are checked to see if they are within the tolerance E 0.05 of X. IX - r J I 17.674 - 17.632 I 0.042 0.05 IX - r31 117.674 - 18.795 I 1.121 0.05 X Since q matches X r2 within s the FS fj r2 17.632 17.674 . One of these values say 17.632 is randomly selected from FS as the adjudicated result. 7.1.5.3 Discussion The formal majority voter expects a result from each variant and when all variant results are not present the voter can fail. A way to avoid this type of failure is to make the formal majority .