Reliable Caching-A Novel Approach to Caching Web Pages

Transcription

1 Mala C, R.Shriram, Shashank Agrawal, K.Geetha and Narendran Rajagopalan Reliable Caching-A Novel Approach to Caching Web Pages C.Mala 1, R.Shriram 2, Shashank Agrawal 2, K. Geetha 3, Narendran Rajagopalan 3 1 Associate Professor and Head 2 Student 3 Research Scholar Department of Computer Science and Engineering National Institute of Technology Trichy , India Id: mala@nitt.edu, shri_ram_r@yahoo.com, shashankagarwal1706@gmail.com, geetha@nitt.edu, narenraj1@gmail.com Abstract The World Wide Web (WWW), since its inception in 1964 has been growing in leaps and bounds. Over the last decade, the WWW has become a warehouse for the information domain. Web servers are available in different parts of the web to provide the pages requested by the users. A Cache server placed between the web server and the user reduces the latency in accessing the pages, minimizes the bandwidth utilization, and also the possibilities of congestion in the network As caching every page found in the WWW is costly and consumes lot of memory, only those pages that are frequently referred are loaded into the cache. There are three different types of pages in the web viz., static pages (no change in page content with time), dynamic (page content changes with time), and highly dynamic (frequency of change in the page content is very high). When a page is provided to the user, it should be always the latest version of the page. The different caching schemes in use namely Static caching, Periodic caching, Active Caching, and Dynamic Caching fail in providing the latest version of the page at some point in time. As such, it is possible that the user is provided with the outdated and invalid information. The Reliable Caching scheme proposed in this paper solves this problem by capitalizing on the nature of web pages and always provides the recent pages to the user thereby enabling the users to get only valid documents from the web than those provided by conventional schemes under the same circumstances. It also identifies the un cacheable pages and discards them. The simulation results show that even in a heavily loaded user network, the Reliable Caching scheme significantly reduces congestion and the load on the web server whose pages are being accessed. Keywords WWW - Web server - pages - caching schemes - reliable. 1. INTRODUCTION With the Internet becoming the main source of information in day-to-day life, the number of web users is increasing and so is the demand to provide the requested document to the user at the earliest[1-8]. The critical issue today in Internet is the latency, the time interval between the user placing a request and getting a response from the server. Hence it is required that the frequently accessed pages be placed in a web server closer to the user[7], thereby reducing the distance and time for popular documents to travel to the client. A Cache server placed in between the user and the web server not only minimizes the response time but also minimizes the congestion in the network. As the cache stores the frequently requested pages, 66

2 Reliable Caching-A Novel Approach to Caching Web Pages whenever a cache hit occurs it serves the page request from the cache store. The page requested by a client can be static[5,10], regularly changing[6-7] or highly dynamic[8,11-14]. It might also involve dynamic content, user specific information, active components, etc. If the client has requested a static page, then the client will get the actual copy. But in today s scenario, most of the frequently requested pages change with time, either periodically or in random fashion. When a cache hit occurs for such pages, sending the page as such from the cache will not give the latest updated page but its earlier version. This leads to inconsistency in the pages made available to the client. There are many caching schemes in use[1-9]. These schemes in spite of reducing the response times and load in web servers do not take into account the nature of the pages cached. Most caching schemes use a form of periodic refreshment policy irrespective of the nature of the page[6,7]. It may so happen that a cached page never changes its contents. Refreshing such a page is not required as it only consumes bandwidth and time. If a page changes very frequently then caching it to provide a latest copy is not possible as the page might have changed during caching itself. So when a user requests for a page which changes with time, it is possible that the user is not given the latest update of the document, if the cache server were to follow one time caching policy[5] or a periodic refreshment policy[6,7]. To overcome this problem, this paper proposes a Reliable caching scheme, which first classifies the web pages depending on their frequency of change, and always caches the latest update of the document. The remainder of this paper is structured as follows: Section 2 discusses the existing caching schemes, Section 3 gives the motivation for the paper, Section 4 discusses the proposed technique, the proposed algorithm and its performance, Section 5 concludes the paper. 2. EXISTING SCHEMES One of the simplest caching schemes is Static Caching[5]. In this scheme, the network traffic is monitored for a fixed period. From the traffic logs, the popular pages are identified using some heuristics. These popular pages are pre fetched and future requests for those pages are served from the cache. The cache is generally emptied after some period of time and reloaded again. Though this use of cache server provides faster access to the documents, the nature of the document, whether static or dynamic is not considered here and so there is always a possibility that the user is not getting the latest update of the document. A more adaptive scheme called Periodic Caching[6,7] makes use of periodic pre fetching. Instead of emptying the cache, this scheme refreshes the documents in the cache periodically, thereby improving the chances of serving a more recent page than the static caching scheme. Though this scheme periodically refreshes the cache contents, it is not taking into account the static pages, for which refreshing is not required and also dynamic pages which differ in their update rate. As a consequence of this discrepancy, there still exists a greater possibility of the user not being served with the latest copy of the document. In the Internet, dynamic documentation constitutes a significant percentage of services given to the users. To enable caching of dynamic pages, Active cache[8] scheme makes use of cache applets, which is attached to one URL or a collection of URLs. Upon every cache hit, the applet connects to the host web server of the requested document to check if the cached copy is the same as the copy on the server. If International Journal of the Computer, the Internet and Management Vol.18.No.3 (September - December, 2010) pp

3 Mala C, R.Shriram, Shashank Agrawal, K.Geetha and Narendran Rajagopalan not, the changes are transmitted to the proxy. The problem with this method is that the proxy server will not service a cache hit, if the corresponding applet is not in the cache. Moreover it is possible that the applet consumes too many resources thereby reducing the number of documents that can be cached. The Active caching proposed in [9] makes use of an auto organizing multicast tree mechanism to ensure user get valid documents if the documents are fast changed. This method is difficult and it takes into account only the dynamic documents. To reduce the burden on the cache server, a peer to peer client web caching system is proposed in [15]. This system does not take into account the type of pages that are cached. Dynamic Caching[6,7] is another technique which addresses the problem of caching pages from servers serving dynamic or client specific pages. It makes use of a dynamic content accelerator which identifies and caches individual components of a page in a ready to display HTML format. Though this method is cost effective and fast for caching dynamic pages, it involves a lot of processing overhead for other types of pages. To overcome these problems, a novel Reliable caching scheme is proposed in this paper. 3. MOTIVATION The methods in [5] and [6] in spite of placing a cache server between the web server and the user to reduce the latency involved in accessing the document, they do not guarantee that the document given to the user is the latest document. Though the schemes [7] and [8] attempt to cache pages with active components or user specific information, the rapidly changing behaviour of dynamic web pages cannot be discriminated by [1-9]. This motivated us to propose a Reliable Caching scheme which first classifies the web pages as static page, periodically changing page and highly dynamic page depending on the frequency of the update of the web page as opposed to the other techniques [1-9], then automatically refreshes the pages in the cache based on their frequency of update, and always provides the user with the latest update of the web page. 4. PROPOSED TECHNIQUE The proposed Reliable Caching technique when used by the cache placed between the web server and the user reduces the page access time. The cache always stores the frequently accessed pages. In Reliable caching scheme, when a page is added to the cache, its period of change is determined by the proposed Algorithm 1. The page is periodically retrieved from the web server after that time interval. If this time interval is too low i.e. if the page is highly dynamic, then the page is monitored for some time. If the page continues to be too dynamic then it is rejected from the cache. When the period of change is too high i.e. if the page happens to be static, then the page is retrieved after a fixed time interval. This time interval is progressively increased if the page is found to remain unchanged during each fetching. In case of a dynamic page, the frequency of change is determined and a timer is set to this value. At the expiry of this timer, the latest updated copy of the page is automatically retrieved from the web server and loaded into cache from where it is given to the user upon request. 68

4 Reliable Caching-A Novel Approach to Caching Web Pages Fig. 1. A detailed schematic of the operations inside a Web Server using Reliable Caching Scheme 4.1. Block Schematic of Reliable Caching Scheme Fig.1 shows the operations of a server using the Reliable Caching Scheme. This scheme makes use of the following modules: 1. Page Store Unit stores the cached pages. 2. Cache Maintenance Unit checks the cache periodically for less frequently used pages and removes them. It also removes highly dynamic pages from page store upon receiving request from Refreshing Unit. It responds to a Server s query to check if a page is in cache and if present, returns the page to the Server. 3. Refreshing Unit identifies the nature of the page and refreshes them depending on their frequency of change. If the page happens to be highly dynamic, then the refreshing unit sends a request to the cache maintenance unit to remove that page from the page store. When the request for a page arrives to the server from the client, the server first checks if that page is available in the cache. If the page is available in the cache it is directly served from the cache. If the page is not available in the cache it is fetched and served, at the same time a copy of the page is also loaded in the cache. To find out whether the page is static, dynamic or highly dynamic, the Reliable caching scheme sets a timer to a default time interval which is less than the minimum allowable period of change of a page. The moment a page is loaded into the cache, the last modified date of change of the page is first noted. The page is then retrieved after the default time interval and is checked to see if it was modified after being refreshed. If the page is not modified after the first refreshment, the default time interval is progressively increased, and if the page is not found to change during successive intervals, then the page is International Journal of the Computer, the Internet and Management Vol.18.No.3 (September - December, 2010) pp

5 Mala C, R.Shriram, Shashank Agrawal, K.Geetha and Narendran Rajagopalan declared to be a static page. But, if the page is not modified during the first refreshment, but on subsequent refreshments if it is getting modified in a random fashion, then the page is declared to be a dynamic page. The exact period of change is found out by the proposed Algorithm 1 in Section 4.2 and the page is refreshed as per this time interval. Alternatively, if the page happens to be modified after the first refreshment, then the default time interval is gradually reduced to a value less than the minimum allowable period of change. And if the page is found to be modified after each retrieval, then the Reliable caching scheme declares the page to be highly dynamic page and from then on it is no longer cached. Any subsequent request to the highly dynamic page is served directly from the web server itself and not from cache server Proposed Algorithm The following are the parameters used: LM : Last Modified Time of Page DTI : Default time interval of retrieval of page MIN Limit : Minimum refresh time interval MAX Limit : Maximum refresh time interval NoC : Page not changed (during a particular refresh) counter (initialized to 0) 70

6 Reliable Caching-A Novel Approach to Caching Web Pages PoC : Period of change of page.the page is refreshed after every PoC. ZERO Limit : the max number of times a page is refreshed after PoC even if it has not changed. API : Auxiliary Period Interval (to be added to PoC when a NoC for a page exceeds ZERO Limit LMIN : the number of times a page's PoC is allowed to go below MIN Limit before it is removed from cache L count : incremented when a page's PoC goes below MIN Limit (initialized to 0) 4.3. Performance Analysis A web server is simulated in our institute s local area network with a working set of 100 pages, with an average page size of 6kb, and with a cache of size 300kb. The Algorithm 1 for Reliable caching scheme was implemented and tested in our institute s network. The performance of the Reliable caching scheme and the other schemes[1-8] for different sets of requests is observed at different time instants. The number of valid pages available in the cache, with each of these schemes, depending on the type of page is observed at every half an hour and the results are tabulated in Table 1, Table 2 and Table 3 respectively. Fig.2 gives the plot of Number of valid pages Vs Time for Static pages, Fig.3 gives the plot of Number of valid pages Vs Time for Dynamic pages, and Fig.4 gives the plot of Number of valid pages Vs Time for Highly dynamic pages. From the graph in Fig.2, it is inferred that, for static type of pages, the number of valid pages available in cache is almost same by each of the caching schemes. It is inferred from the graph in Fig.3 that for dynamic pages, as Reliable caching scheme refreshes the cached pages based on their frequency of update, the number of valid pages present in cache is relatively high compared to Static caching, Periodic caching, Active Caching and No caching schemes. In the case of highly dynamic pages, the pages in spite of their dynamism are retained in the cache by the Static caching, Periodic caching, Active Caching and No caching schemes. So, the user may be provided with an old invalid copy of the page which may lead to misinterpretation of data by the user if the user is accessing pages related to concurrent database applications, viz., online banking, online reservation systems, stock market, etc., But, from Fig.4, it is inferred that though Reliable caching scheme initially caches the highly dynamic pages, once it has identified their high frequency of update, it no longer caches these pages and for subsequent requests of these pages, it fetches these pages from the web server itself and not from the Cache. This is shown by the sharp reduction in the number of valid pages in the cache in Fig.4. The amount of bandwidth utilized by Reliable caching, Static caching, Active caching, Dynamic caching and No caching schemes is also observed at an interval of every half an hour, every hour, and on a per day basis. The results are tabulated in Table 4, Table 5, and Table 6 respectively. Fig.5, Fig.6 and Fig.7 give the plot of Bandwidth utilization Vs Time for the different schemes at an observation interval of every half an hour, every hour, and on a per day basis. From the graph in Fig.5, Fig.6, and Fig.7, it is inferred that the bandwidth utilized by the Reliable caching scheme is low compared to the Periodic caching scheme. To refresh the pages in the cache based on their frequency of update and to always provide valid pages to the user from the cache, it consumes more bandwidth than Active caching, Static caching and No caching schemes. International Journal of the Computer, the Internet and Management Vol.18.No.3 (September - December, 2010) pp

7 Mala C, R.Shriram, Shashank Agrawal, K.Geetha and Narendran Rajagopalan Table 1. Observation time of Hour for STATIC PAGES Time Caching Nil Static Periodic Active Reliable Table 2. Observation time of Half an Hour for DYNAMIC PAGES Time Caching Nil Static Periodic Active Reliable Table 3. Observation time of Half an Hour for HIGHLY DYNAMIC PAGES Time Caching Nil Static Periodic Active Reliable Table 4. Observation time : Every Half an Hour Time Caching Nil Static Periodic Active Reliable Table 5. Observation Time An Hour for Static Pages Time Caching Nil Static Periodic Active Reliable

8 Reliable Caching-A Novel Approach to Caching Web Pages Table 6. Observation Time : Every Day Time Caching Nil Static Periodic Active Reliable Fig. 2. Observation time of ½ Hour for STATIC PAGES Fig. 3. Observation time of ½ Hour for DYNAMIC PAGES International Journal of the Computer, the Internet and Management Vol.18.No.3 (September - December, 2010) pp

9 Mala C, R.Shriram, Shashank Agrawal, K.Geetha and Narendran Rajagopalan Fig. 4. Observation time of ½ Hour for HIGHLY DYNAMIC PAGES Fig. 5. Bandwidth Utilized Vs. Time 74

10 Reliable Caching-A Novel Approach to Caching Web Pages Fig. 6. Bandwidth Utilized Vs. Time Fig. 7. Bandwidth Utilized Vs. Time International Journal of the Computer, the Internet and Management Vol.18.No.3 (September - December, 2010) pp

11 Mala C, R.Shriram, Shashank Agrawal, K.Geetha and Narendran Rajagopalan 5. Conclusion As the Internet has become as indispensable resource in everyone s life, the number of Internet users increase day by day. So, it is essential to provide them with fast access to the internet as it saves both time and cost involved in access. The Reliable caching scheme proposed in this paper provides a fast and reliable access to the frequently accessed pages in the internet by segregating them as static, dynamic, and highly dynamic pages. The simulation results also shows that Reliable caching scheme performs better than the existing caching schemes as it consumes less bandwidth and takes less time to access the pages from the Internet. Moreover, this scheme inherently reduces the load on the network and web server thereby reducing the possibilities of congestion compared to other caching schemes. References [1] Hai Liu and Maobian Chen (2010), Evaluation of web caching consistency, 3rd International Conference on Advanced Computer Theory and Engineering (ICACTE). [2] Yanjun Liu, Yueping Wang and Huilin Du (2010), Strong cache consistency on World Wide Web, 3rd International Conference on Advanced Computer Theory and Engineering (ICACTE). [3] Chetan Kumar and John B. Norris (2008), A new approach for a proxylevel web caching mechanism, Decision Support Systems, Vol. 46, Issue 1, pp [4] Ronny Lempel and Shlomo Moran (2004), Competitive caching of query results in search engines, Theoretical Computer Science, Vol. 324, Issues 2-3, pp [5] Igor Tatarinov, Alex Rousskov and Valery Soloviev (1997), Static Caching in Web Servers, Proceedings of 6th International Conference on Computer communications and Networks, pp [6] G. Govatos (2000), Speed up Your Dynamic Web Content, Network World Fusion News, R. Loren, Foundations of resource development. [7] Mazen Zari, Hossein and Saiedian(2001), Understanding and reducing Web delays, IEEE Computer vol. 34, Issue 12, pp [8] Pei Cao, Jin Zhang and Kevin Beach(1998), Active Cache: Caching Dynamic Contents on the Web, Proceedings of Middleware. [9] Xu Du, Zongkai Yang, Jiaqing Huang and Wenqing Cheng(2002), An Active Caching Scheme, IEEE Proceedings of International Conference on Communication, circuits and systems, pp [10] M.Abrams, C.Stanbridge, G.Abdulla, S.Williams and E.Fox (1995), Caching Proxies : Limitations and Potentials, Proceedings of 4th International conference on WWW. [11] J. Almeida, V. Almeida and D.Yates (1997), Measuring the behaviour of a Worlds wide Web server, Proceedings of 7th IFIP conference on High performance networking, pp [12] Ramon Caceres, Fred Douglis, Anja Feldmann, Gideon Glass and Michael Rabinovich(1998), Web proxy caching, Proc. of SIGMETRICS Internet Server Performance Workshop. [13] Pei Cao and Sandy Irani(1997), Costaware WWW proxy caching algorithms, Proceedings of the USENIX Symposium on Internet Technology and Systems. 76

12 Reliable Caching-A Novel Approach to Caching Web Pages [14] B.D.Davison(2001), A Web Caching Primer, IEEE Internet computing, vol 5, Issue 4, pp [15] Jiangchuan Liu, Xiaomen Chu and Ke Xu(2005), On Peer to Peer Client Web Cache Sharing, Proceedings of IEEE International Conference on Communications, vol. 1, pp International Journal of the Computer, the Internet and Management Vol.18.No.3 (September - December, 2010) pp