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The advantage in loading on demand is that

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The advantage in loading on demand is that transaction processing can start rightaway; however transactions may see long and unpre- dictable delays in disk access until the entire database is loaded into memory. 26.3 Is a high-performance transaction system necessarily a real-time system? Why or why not? Answer: A high-performance system is not necessarily a real-time sys- tem. In a high performance system, the main aim is to execute each trans- action as quickly as possible, by having more resources and better uti- lization. Thus average speed and response time are the main things to be optimized. In a real-time system, speed is not the central issue. Here each transaction has a deadline, and taking care that it Fnishes within the deadline or takes as little extra time as possible, is the critical issue. 26.4 Explain why it may be impractical to require serializability for long- duration transactions. Answer: In the presence of long-duration transactions, trying to ensure serializability has several drawbacks:- a. With a waiting scheme for concurrency control, long-duration trans- actions will force long waiting times. This means that response time will be high, concurrency will be low, so throughput will suffer. The probability of deadlocks is also increased. b. With a time-stamp based scheme, a lot of work done by a long- running transaction will be wasted if it has to abort. c. Long duration transactions are usually interactive in nature, and it is very difFcult to enforce serializability with interactiveness. Thus the serializability requirement is impractical. Some other notion of database consistency has to be used in order to support long duration transactions. 26.5 Consider a multithreaded process that delivers messages from a durable queue of persistent messages. Different threads may run concurrently, attempting to deliver different messages. In case of a delivery failure, the
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Exercises 3 message must be restored in the queue. Model the actions that each thread
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