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chapter17 实践习题1
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chapter17 实践习题1
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C H A P T E R
17
Database System
Architectures
Practice Exercises
17.1 Instead of storing shared structures in shared memory, an alternative
architecture would be to store them in the local memory of a special
process, and access the shared data by interprocess communication
with the process. What would be the drawback of such an architecture?
Answer: The drawbacks would be that two interprocess messages
would be required to acquire locks, one for the request and one to
confirm grant. Interprocess communication is much more expensive
than memory access, so the cost of locking would increase. The process
storing the shared structures could also become a bottleneck.
The benefit of this alternative is that the lock table is protected better
from erroneous updates since only one process can access it.
17.2 In typical client–server systems the server machine is much more pow-
erful than the clients; that is, its processor is faster, it may have multiple
processors, and it has more memory and disk capacity. Consider in-
stead a scenario where client and server machines have exactly the
same power. Would it make sense to build a client–server system in
such a scenario? Why? Which scenario would be better suited to a
data-server architecture?
Answer: With powerful clients, it still makes sense to have a client-
server system, rather than a fully centralized system. If the data-server
architecture is used, the powerful clients can off-load all the long and
compute intensive transaction processing work from the server, freeing
it to perform only the work of satisfying read-write requests. even if
the transaction-server model is used, the clients still take care of the
user-interface work, which is typically very compute-intensive.
A fully distributed system might seem attractive in the presence of
powerful clients, but client-server systems still have the advantage of
simpler concurrency control and recovery schemes to be implemented
17
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