UOW-Caching and new ways to improve response time (Paper)


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UOW-Caching and new ways to improve response time (Paper)

  1. 1. Caching and New Ways to Improve Response Time Guson Kuntarto (2636177)AbstractThis essay will be focused on how a web cache proxy server work that can improve theend-user response time. To justify there is a significant improvement using a web cacheproxy server, an example from recent study about web caching technology will be used.This is done by comparing the mean of response times that using a web cache andwithout using a web cache.1. IntroductionIn the past few years, Internet usage especially World Wide Web (www) has beenincreased exponentially [3]. This forces Internet service providers (ISP) to providebandwidth in order to satisfy the end-users. Because often end-users request large amountof files from servers and send them over the Internet, without enough bandwidth thatavailable on the network the traffic congestion may be happened. Moreover, it may leadto the increasing number of response times from end-users perspective. In other words,end-users tend to wait a bit longer to be able to get information that they needed.Alternatively, implementing a web cache proxy server could be a better solution.Configurationally, it is placed between the Internet and the end-users that act to provideservices between end-users and the Internet by reducing the number of requests sentacross the Internet to the origin servers [1]. So, it has to provide a large amount ofdedicated temporary storage to serve users. In addition, it must contain an up-to-date dataor fresh data from the popular web sites that frequently accessed by end-users [1]. Byimplementing a web cache proxy server Internet bandwidth usage can be saved and theend-user response time can be reduced [1].In the second section of this essay will be explained caching technology terminology. Inthird section, it will explain how the web cache proxy server works. Then, some 1
  2. 2. examples regarding to response time will be presented and compared in the section four.Finally, conclusion will be drawn in the last section.2. Terminology“Cache” means to store of recent retrieved information for further reference [2]. Thus, aweb cache is a collection of locally stored of web objects such as web pages, embeddedand other types of digital objects retrieved over the Internet using the hypertext protocol(HTTP) [1] [5]. It is maintained by a web browser and used for a private or not shared[2]. Whereas, a web cache proxy server is essentially a shared web cache. A cache’sprogram running on a dedicated server that archives and returns documents frequentlyrequested by a group of web clients such as squid [5].Response time can be defined as the amount of latency that the end-user experienceswhen requesting a web objects. This measurement starts when the web browser initiates arequest and completes when the server responds with the data [1].3. How does the web cache proxy server work?3.1. A web cache proxy server topology Computer Computer PROXY ORIGIN SERVER SERVER Computer Internal Network Figure 3.1 – A simple topology of web cache proxy serverThe web cache proxy server usually placed between the internal network and Internet asit seen on figure 3.1. It plays important rules to copy a web objects close to the users andmaintain the freshness of the web object, so that user do not have to get the objects fromthe origin servers [5]. By returning local copies of objects whenever possible, a caching 2
  3. 3. proxy can improve response times; reduce network traffic and the effective bandwidthavailable to end-users [5]. Caching proxies typically implement a cache replacementpolicy including Least Recently Used (LRU), Least Frequently Used (LFU), and Hyper-G to determine when web objects have to be removed from their archive [3][5].Basically, LRU policy works to remove the documents that have not been accessed forthe longest time whereas LFU policy acts to delete the documents that are least frequentlyasked by end-users [4]. Finally, Hyper-G performs by removing the documents, whichare LFU, and if it is found there are two items that have identical LFU, then it will deletethe one that is LRU. And if it is found two documents that have identical LFU and LRU,Hyper-G policy will perform task to remove one that is larger [4].3.2. Scenario of web cache proxy server WEB CACHE PROXY SERVER Request Cache Miss CLIENT WEB SERVER Cache Hit Store/ access History CACHE VERSIONING Figure 3.2 – scenario of web cache proxy server [6]There are three scenarios that involve in web caching technology, including: cache-hit,and cache-miss. Cache hit happen when the request from the users found the fresh copyof data on the cache and sends it directly to the users [2]. Whereas, cache-miss happenwhen the request from the users could not find the fresh copy of data that is requested onthe cache. The next task is proxy server will get the copy from the origin server, save iton the cache for further usage and send it to the users [2]. From user perspective, as aresult of cache-hit, the response time is faster than when cache-miss is happened, becausewhen the cache-miss happened the proxy server has to perform task to find the fresh datato origin server and send it to the users. 3
  4. 4. 3.3. Web caching techniqueThe problem that would be faced in web cache technology is the cache interconnected tothe other cache. The web caching technique has been introduced including: InternetCache Protocol (ICP) and Cache Array Routing Protocol (CARP) [4]. ICP performsusing three basic messages: ICP Query, ICP_HIT and ICP_MISS for inter-cachecommunication between cache and cache [4]. CARP performs HASH function to mapsstring into integer [4]. By performing CARP, mirroring effect that occurs in ICP can besolved [4].4. Response time comparisonFigure 3.3 – Graphic cumulative distribution of request versus response time [7]Recent study about web caching technology by Davison [7] uses NCS (Network CacheSimulator) as a tool to get the response time accurately has been done. The experimentwas using a model simple topology of web cache proxy server, where the server attachedto the Internet with bandwidth 1,544 Mbps (T1) and the network latency is equal to 50ms second, and 30 ms per-request overhead, whereas the client is connected to bandwidth27,4 kbps with 200 ms of latency and requested 10000 Byte of files shows that in non-persistence connection, the mean of response time without using a web cache proxyserver was equal to 5.5 second and with using a web cache proxy server the mean ofresponse time equal to 4.6 second [7]. This means there is a significant improvement thatequal to 0.9 second of response time using a web cache proxy server as it shows on thefigure 3.3. In addition, using persistent connections improves the mean of response timein the condition where a web cache not in used drop to 5.1 second whereas with a web 4
  5. 5. cache the mean of response time drops to 2.4 second [7]. This means there is a significantimprovement that equal to 1.7 second of response time using a web cache proxy server inpersistent condition. Thus, using a web cache proxy will improve response timesignificantly.5. ConclusionIn conclusion, implementing a web proxy server is essential as it can reduce responsetime. This because a web proxy server serves the client by placing a popular copy of webobjects as near as possible to the users. So, every time user requests data to the proxyusing protocol HTTP, proxy servers can response immediately.Reference:[1] IMimic, 2001, “A primer on web caching technology and benefits”, p. 1- 5, [Online]Available: http://www.imimic.com/documents/WPBackgroundTechBenefits.pdf accessed:04/04/2005.[2] Dutkiewicz E., 2005, “3rd Week lecture materials of content servers and cachingtechnologies”, School of Electrical, Computer, and Telecommunication Engineering, Universityof Wollongong, p. 4- 40.[3] Dutkiewicz E., 2005, “4th Week lecture materials of content servers and cachingtechnologies”, School of Electrical, Computer, and Telecommunication Engineering, Universityof Wollongong, p. 4- 40.[4] Wooster R. P., 1996, “Optimizing response time, rather than hit rates of www proxy caches, ”(Master of Science, Virginia Polytechnic Institute and State University), [Online] Available:http://scholar.lib.vt.edu/theses/available/etd34131420119653540/unrestricted/etd.pdf accessed:04/07/2005.[5] Powell J., 1997, “Web Caching: questions and answers,” p. 1- 3, [Online] Available:http://scholar.lib.vt.edu/digilib/reports/dlcachetalk.pdf accessed: 04/07/2005.[6] Yang J. and Li X, 2000, “Pachyderm: The web proxy that never forgets,” p. 1- 12, [Online]Available: www.cs.wisc.edu/~junfengy/submit/Pachyderm.pdf accessed: 04/06/2005.[7] Davison B. D., 2001, “NCS: Network and cache simulator An Introduction’, p. 1- 13, [Online]Available: http://www.cse.lehigh.edu/~brian/pubs/2001/dcs-tr-444/dcs-tr-444.pdf accessed:04/07/2005 5