Active Cache: Caching Dynamic Contents on the Web Presented by Rahul K S & Balaji N V

Abstract Dynamic documents constitute an increasing percentage of contents on the Web, and caching dynamic documents becomes an increasingly important issue. Active Cache scheme to support caching of dynamic contents at Web proxies has been proposed in this paper. The scheme allows servers to supply cache applets to be attached with documents, and requires proxies to invoke cache applets upon cache hits to furnish the necessary processing without contacting the server. This scheme can result in significant network bandwidth savings at the expense of moderate CPU costs.

WWW - The Inside Story n : –Tim Berners-Lee develops Web at CERN. HTML, HTTP, line mode browser. –Countries from all over world connect to NSFNet backbone network. n 1993 –NCSA release first alpha version of Marc Andreesen's ” Mosaic for X". –Over 200 Web servers in operation. –TCP/IP is integrated in Microsoft and Apple operating systems. 98mcs011: CERN:European Union of Particle Physiscs 98mcs011: CERN:European Union of Particle Physiscs 98mcs011: NCSA : National Center for Supercomputing Applications 98mcs011: NCSA : National Center for Supercomputing Applications 98mcs011: NLANR -- National Laboratory for Applied Network Research 98mcs011: NLANR -- National Laboratory for Applied Network Research

n 1994: –1st WWW conference –Andreesen forms Mosaic Communication –First W3C meeting n 1995 –network providers (MCI, Sprint, etc.) –Internet open for commercial use. –Web traffic becomes dominant Internet traffic. –Compuserve, America Online, Prodigy provide Internet/Web service. –Netscape goes public. –NLANR begins to build a network of Web caches. n 1998 – Over two million Web servers in less than ten years!

Why Caching in the Network? n Reduce latency by avoiding slow links between client and origin server –Low bandwidth internet connections –Congested links n Reduce traffic on links –Low bandwidth lines can be avoided n Spread load of overloaded origin server to caches. –Inexpensive servers n Avoid protocol overheads –Slow start experienced by every object can be avoided

Caching: Motivation and Strategies n Temporal Locality of Reference –User often visits a site Store the web page in the cache n Spatial Locality of Reference –User is likely to visit the links provided in the current page Pre-fetch the pages and cache n Hit Intensive Sites –Popular web sites are likely to be accessed by many. Keep a common cache for all the users

Current Forms of Caching n Client Caches –Caches maintained at clients. Managed by browsers. n Hierarchical Caching –Cache is maintained at every level of the Hierarchy. At the Client, Local ISPs, Regionsl ISPs etc... –More Cache in the Servers higher in the hierarchy –Cache is maintained at proxy servers n Chain of Caches – caches collaborate within themselves. For eg. National level ISPs can agree to collaborate

Shortcomings of Present Cache Schemes n Only static pages are cached –Current caches do not store dynamic web pages: dynamic web pages are becoming very widely used techinique to personalize the internet e.g my.yahoo.com n No efficient prefetching algorithms are used –Only brute-force strategy is used e.g. web accelarators

Active Cache Scheme n A Cache Applet is included with every URL or set of URLs –Cache Applets are server supplied code –Written in platform-independent programming language such as Java –migrates parts of server processing on each user request to the caching proxy –On every hit the applet is run and the results are sent to the client

Features and Benefits of Active Cache n Can perform a variety of functions, for example, logging user accesses, rotating advertising banners, checking access permissions, constructing client- specific Web pages, etc. n They promote caches from static storage of pages to storage of objects – data with a method is invoked on hits

Working of Active Cache 1. The cache server gets the URL page along with its cache applet on the first accces 2. On hit on cached document, either cache applet is invoked or the request is sent to the server –cache server has the freedom to invoke or send the request to the server depending on cache applet’s resource usage 3. If the applet's execution fails due to any reason, the request is sent to the server 4. if the applet's execution succeeds, the proxy will take the appropriate action based on the return value of the FromCache method 5. Each applet can deposit information in a special log object and the proxy will send the log object back to the server periodically.

More About Active Cache n Specification n Interface n Security Mechanism n Resource Management Policies

Active Cache : Protocol Specification n entity header, ``CacheApplet” : CacheApplet: code = ``code.class'', archive = ``code.jar'', codebase = ``codebase_url'' – HTTP/1.1 requires that if a proxy does not recognize an entity header, it should forward the header. So the server can be assured that Active-Cache enabled proxies will receive the header even if they have parent proxies. n Active-Cache enabled proxies work as mentioned earlier

ActiveCache Protocol: Interface n The cache applet must implement an interface called “ActiveCacheInterface “ –The Applet must be written in Java –It should have a function called “ FromCache “ The function is called when an access hits in the document arguments of the function call include the user request, the IP address of the client machine, the name of the client machine, the document in cache, and a new file descriptor for the applet to write when constructing a new Web page –The cache applet can only call the ActiveProxy class provides the native methods for file access, cache query, locking and unlocking as well as sending requests to servers.

ActiveCache Protocol: Security Mechanisms n Illegal Acccess : –rely on a type-safe language.like Java, with built-in security mechanisms, a well-defined security interface, n Denial of Service Attacks: –The proxy can keep track of an applet's resource consumption in the following five aspects: storage size Disk Bandwidth Consumption Network Bandwidth Consumption CPU Usage Virtual Memory Size –Proxy can impose upper limit on the above resources and purge the applet whose resource usage exceeds the upper limit

ActiveCache protocol: Resource Management Issues: n Cache the Document or Not n Invoke the applet or forward the request to the server Category of applets and solutions: n Un-negotiated objects –Estimate the benefits of caching. If the benefits are more cache otherwise do not cache –Impose upper limit on resource usage. Invoke the applet if the resource usage is less than the upper limit n Negotiated objects –Negotiate the resource utilisation through negotiate protocol Cache the document and invoke each time the applet in case of successful negotiation

References 1. Pei Cao, Jin Zhang and Kelwing Beach, “Active Cache: Caching Dynamic Contents on the Web”, Proceedings of Middleware’98. Available cache/active-cache.html 2. Tutorial on web cache Available in the link Online Presentations:Distribution of Stored Information in the Web: