Distributed Systems: Concepts and Design by George Coulouris. Question 2.5

2.5) Suggest some applications for the peer process model, distinguishing between cases when the state of all peers needs to be identical and cases that demand less consistency.

Answer:

Cooperative work (groupware) applications that provide a peer process near to each user. 

Applications that need to present all users with identical state - shared whiteboard, shared view of a textual discussion 

Less consistency: where a group of users are working on a shared document, but different users access different parts or perhaps one user locks part of the document and the others are shown the new version when it is ready. 

Some services are effectively groups of peer processes to provide availability or fault tolerance. If they partition data then they don’t need to keep consistent at all. If they replicate then they do.

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Distributed Systems: Concepts and Design. by George Coulouris. Exercise Solutions 3. Question 2.4

2.4) A search engine is a web server that responds to client requests to search in its stored indexes and (concurrently) runs several web crawler tasks to build and update the indexes. What are the requirements for synchronization between these concurrent activities?

Answer:

The crawler tasks could build partial indexes to new pages incrementally, then merge them with the active index (including deleting invalid references). This merging operation could be done on an off line copy. Finally, the environment for processing client requests is changed to access the new index. The latter might need some concurrency control, but in principle it is just a change to one reference to the index which should be atomic.

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Distributed Systems: Concepts and Design. by George Coulouris. Exercise Solutions 3. Question 2.2

2.2) For the applications discussed in Exercise 2.1 state how the servers cooperate in providing a service.

Answer:

Web: Web servers cooperate with Proxy servers to minimize network traffic and latency. Responsibility for consistency is taken by the proxy servers - they check the modification dates of pages frequently with the originating web server.

Mail: SMTP servers do not necessarily hold mail delivery routing tables to all destinations. Instead, they

simply route messages addressed to unknown destinations to another server that is likely to have the relevant tables.

Netnews: All NNTP servers cooperate in the manner described above to provide the newsfeed mechanism.

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