第七章一些问题

[suzhiyang]

闫老师：

您好！

 

第七章（P319)：有三个问题没看明白...

4 Q: During the discussion of consistency models, we often referred to the 

contract between the software and data store. Why is such a contract needed? 

A: If a program expects a sequentially consistent data store and cannot live 

with anything less, the store must provide sequential consistency. However, 

to improve performance, some systems provide a weaker model. It is then 

essential that the software agrees to abide by the rules imposed by this 

model. Generally, it means that programs obeying the rules will perceive 

what looks like a sequentially consistent data store. 

 

 

13. Q: When using a lease, is it necessary that the clocks of a client and 

the server, respectively, are tightly synchronized? 

A: No. If the client takes a pessimistic view concerning the level at which 

its clock is synchronized with that of the server, it will attempt to obtain 

a new lease far before the current one expires. 

 

 

21. Q: State-based leases are used to offload a server by letting it allow 

to keep track of as few clients as needed. Will this approach necessarily 

lead to better performance? 

A: No, for the simple reason that for some clients it would still be better 

to inform them when updates happened. Not maintaining any state may lead to 

the situation that these clients will often poll the already busy server. 

 

 

谢谢！

2010-06-06

---

suzhiyang

[Hongfei Yan]

4. 是上面应用(process代表)和下面基础设施(data store)之间的合约。如果process不需要严格的一致，data store就可以提供更好性能的服务。如果process要求高，下面也要提供这些服务才成。
page 277 A consistency model is essentially a contract between processes and the data
store. It says that if processes agree to obey certain.rules, the store promises to
work correctly. Normally, a process that performs a read operation on a data item,
expects the operation to return a value that shows the results of the last write operation
on that data.
Each model effectively restricts the values that a read operation on a data item can return.

13. 不需要严格同步，因为可以提前很多续约。

14.
page 305
The last criterion is that of state-space overhead at the server. When the server
realizes that it is gradually becoming overloaded, it lowers the expiration time of
new leases it hands out to clients. The effect of this strategy is that the server
needs to keep track of fewer clients as leases expire more quickly. In other words,
the server dynamically switches to a more stateless mode of operation, thereby
offloading itself so that it can handle requests more efficiently.

本意是降低服务器端负载，但是由于记录客户端状态的减少，导致客户轮训服务器次数增加，从而加重服务器负担。

2010/6/6 suzhiyang <suzh...@gmail.com>

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