Repeat Exercise 20.14, adding a check in T₁ so that Y does not exceed 90.

Exercise 20.14

Change transaction T₂ in Figure 20.2(b) to read

read_item(X);
X := X + M;
if X > 90 then exit
else write_item(X);

Discuss the final result of the different schedules in Figures 20.3(a) and (b), where M = 2 and N = 2, with respect to the following questions: Does adding the above condition change the final outcome? Does the outcome obey the implied consistency rule (that the capacity of X is 90)?

Figures 20.3(a) and (b):

Figure 20.2(b)

Transcribed Image Text:

Figure 20.3 Some problems that occur when concurrent execution is uncontrolled. (a) The lost update problem. (b) The temporary update problem. (c) The incorrect summary problem. (a) read_item(X); X=X- N; read item(X); X= X+ M; write_item(X); read_item(Y); Time Item X has an incorect value because write_item(X); its update by T, is lost (overwnitten). Y =Y+ N; write_item(Y); (b) read_item(X); X=X- N; write_item(X); read_item(X); X=X+ M; write_item(X); Time Transaction T, faila and must change the value of X back to its old value; meanwhile T, has read the temporary incorrect value of X. read item(Y);