JAVA-BASED REAL-TIME PROGRAMMING

1. Introduction
control may be delayed due to the dynamics of the other part. Furthermore,
assume it should be simple (no reprogramming, only altering a data value) to
change the sampling period of the temperature control.
A better way is to write two pieces of program, for instance two classes that
handle the two control tasks. How should that be done, and what requirements
does that put on the programming and run-time system?
Example 2: Bank-account transactions
A familiar example of concurrently occurring actions on the same data item,
is bank account transactions. Assume that there are two transactions that
are to be carried out at about the same time. One of them, which we call
A, consists of taking out $1000 from the account via a money machine. The
other transaction, which we call B, is to add a salary of $10000 to the account.
Depending on the actual timing, and depending on how the bank computer
performs the transactions, we obtain different cases. If one transaction is
performed before the other, we have one of the two cases shown in Figure 1.2.
Each sequence of operations expresses the way the computer at the bank
performs the requested transactions.
According to proper software engineering practice, the two sequences are
preferably expressed separately as described in the previous example. Furthermore,
assume that the run-time system of our computer is capable of
interleaving the two sequences in such a way that they appear to be done simultaneously.
Since the interleaving is not specified in the individual programs
as shown in the figure, it could result in any order of operations depending on
the underlying system, and it could possibly be different from time to time.
For instance, we may for the above transactions obtain interleavings according
to Figure 1.3. Even if automatic interleaving of code sequences often is desired
to handle several tasks in parallel, we see that we must be able to specify a
sequence of operations to be performed without being interrupted by other
sequences manipulating the same (shared) data. We shall learn several ways
to solve this type of problem.
// Case 1: Withdraw first
A: Read 5000
A: Amount = 5000 - 1000
A: Write 4000
B: Read 4000
B: Amount = 4000 + 10000
B: Write 14000
// Case 2: Salary first
B: Read 5000
B: Amount = 5000 + 10000
B: Write 15000
A: Read 15000
A: Amount = 15000 - 1000
A: Write 14000
Figure 1.2: Two bank account transactions done in two different orders. In
both cases, the original amount was 5000 and the resulting amount is 14000
as expected.
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