Resposta sd

Distributed Systems: Concepts and Design
Edition 4
By George Coulouris, Jean Dollimore and Tim Kindberg
Addison-Wesley ©Pearson Education 2005

Chapter 10
10.1

Exercise Solutions

Early file-sharing applications such as Napster were restricted in their scalability by the need to maintain a central index of resources and the hosts that hold them. What other solutions to the indexing problem can you identify?

10.1 Ans.
The choice is application-dependent:
i) The web can be used, but this requires manual intervention to announce new resources and users must invoke a search engine to find the resources they want (maybe via a web service interface). ii) If the aim is to provide access to all resources for all users, some form of reliable replication is needed.
Chapter 15 details several with differing costs and consistency models. Gossip and Bayou are potentially scalable to internet-wide. They offer availability guarantees, but only after ‘convergence’ which may be slow. iii)If guaranteed availability isn’t required (as in media file sharing) then some form of localized information sharing may be more suitable. For example, each node contacts other nodes with GUIDs in the same range as its own to discover the resources they have available or can access. This would offer quick response with a subset of the available resources and possibly more resources with a slower response. 10.2

The problem of maintaining indexes of available resources is application-dependent. Consider the suitability of each of your answers to Exercise 10.1 for (a) music and media file sharing, (b) longterm storage of archived material such as journal or newspaper content, (c) network storage of general-purpose read-write files.

10.2 Ans.
a) Solution (i) is suitable for slowly - changing collections of resources such as media files. It is used by
BitTorrent, supplemented by a tracker service for each resource that holds up-to-date information about
the