Currently the one of the major issue Distributed System is facing is the improvement of faster and efficient method to access the Internet, How to introduce QoS in web services provided to client? Tremendous growth in number of users accessing the Internet has lead to this problem. For such a major problem, the authors of this paper talk more about the replication of webservices and its content delivery policies and how it helps to deal with QoS improvement.
Initially many methods were used to improve the access by users to internet, such as caching, prefetching and pushing. But none of them were good enough for large number of users accessing the internet. So the advanced approach of data replication was introduced to attack the problem and make accessing internet easier.
So what is replication and how it enables us to improve the way user is accessing the internet? Replication simply means creating multiple copies of a resource and distributing it throughout the network on servers. Doing so enables us to reduce the single server load on a network. Since accessing the web is done throughout the globe and is also time zone dependent Geographical replication is used since it facilitates the use of the available resources.
Circumstance that created this problem is the Centralized Server Model, In CSM there is one original web server which processes the data and sends it to the local ISP where a group of client access it. So if we focus on the communication path between client and the Centralized Server we can identify some bottleneck that cause the network infrastructure issues. The authors of the paper state there are 4 bottlenecks in CSM they are as follows : First Mile, The peering points, The backbone capacity, The last mile. First mile problem occurs when bandwidth is less. Higher the number of users higher should be the bandwidth. The peering point occurs when there are multiple uses multiple routers in a network racing with one another. The backbone capacity is the infrastructure unable to carry the load of client requests.
Last mile problem occurs when number of client trying to access network over broadband is less but over dial up connection is high.
Previous solutions were caching, prefetching, pushing came up short because cache misses are very expensive. Prefetching consumed lot of resources. Pushing was an incompetent idea because to send a file throughout the globe pushing the data would be consuming a lot of time and power. Currently there are 2 different approaches in achieving replication of web service example Clustering and Geographical replication.
The main goal of the geographical replication is to hide the communication latency, because the copy of the file will be nearby to the client. It is a promising area from the industrial point of view. Authors have assumed that there wont be any data consistency problems appearing because of the replication. Authors have gone into depth of the geographical replication to talk about the subtypes in it and comparison between them. When there are multiple webservers there is a severe need for binding policy to client requests, this can be on the following geographical basis request close to client and request close to server: – Server Side approach and the client side approach. From the above mentioned things we can get that
Authors concluded that after comparing server side replication approach and client side replication approach there is no winning strategy just a trade-off between two situations. In server side replication approach server resource allocation and usage can be managed. Author suggest that there should be a hybrid model which comprises of both server and client replication approach. This can also be the future work to be done.