Distributed Systems Seminar 3
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What is the didifference between a vertical distribution and a horizontal dis-tribution?
Vertical distribution refers to the distribution of the different layers in a multitiered architectures across multiple machines. In principle, each layer is implemented on a different machine. Horizontal distribution deals with the distribution of a single layer across multiple machines, such as distributing a single database.
Consider a chain of processes P1, P2, ..., Pn implementing a multitiered client-server architecture. Process Pi is client of process Pi+1, and Pi will return a reply to Pi1 only after receiving a reply from Pi+1. What are the main problems with this organisation when taking a look at the request-reply performance at process P1?
Performance can be expected to be bad for large n. The problem is that each communication between two successive layers is, in principle, between two different machines. Consequently, the performance between P1 and P2 may also be determined by n - 2 request-reply interactions between the other layers. Another problem is that if one machine in the chain performs badly or is even temporarily unreachable, then this will immediately degrade the performance at the highest level.
In a structured overlay network, messages are routed according to the topology of the overlay. What is an important disadvantage of this approach?
The problem is that we are dealing only with logical paths. It may very well be the case that two nodes A and B which are neighbors in the overlay network are physically placed far apart. As a consequence, the logically short path between A and B may require routing a message along a very long path in the underlying physical network.
Give a compelling (technical) argument why the tit-for-tat policy (i.e., a file can be downloaded only when the downloading client is providing content to someone else) as used in BitTorrent is far from optimal for file sharing in the Internet.
The reasoning is relatively simple. Most BitTorrent clients are operated behind asymmetric links such as provided by ADSL or cable modems. Ingeneral, clients are offered a high incoming bandwidth capacity, but no one really expects that clients have services to offer. BitTorrent does not make this assumption, and turns clients into collaborative servers. Having sym-metric connections is then a much better match for the tit-for-tat policy.
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