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Caching Solutions to increase availability of Web Content

Caching Solutions to increase availability of Web Content. Krithi Ramamritham IIT Bombay krithi@cse.iitb.ernet.in. Web sites have traditionally served static content But, dynamic content generation has come into vogue

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Caching Solutions to increase availability of Web Content

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  1. Caching Solutions to increase availability of Web Content Krithi Ramamritham IIT Bombay krithi@cse.iitb.ernet.in

  2. Web sites have traditionally served static content • But, dynamic content generation has come into vogue • generated on the fly by running dynamic scripts, e.g., Active Server Pages (ASP), Java Server Pages (JSP), Servlets • allows generation of different content for the same request Web Content

  3. Ad Component Headline Component Headline Component Navigation Component Headline Component Headline Component Personalized Component Dynamic Web Pages… Web Page A News content site

  4. IIT Bombay’s aAQUA Community Forum Farmers get information and get their questions answered -- In the local context -- In their local language Capitalizes on existing human and infrastructural resources: Agri-extension center – KVK, Baramati NGO – Vigyan Ashram, Pabal Corporate infrastructure -- ITC e-chaupal Government – MCIT www.aAQUA.org

  5. Users Typical End-to-end Web Site Architecture Web Server Cluster Application Server Cluster Data . . . .

  6. Web servers • Do well defined and quantifiable local work • e.g., processing HTTP headers, serving static content • Application servers • Run multi-layer programs • e.g., scripts involving calls to backends WS vs. AS

  7. DATABASE Dynamic Content Accelerator APPLICATION SERVER HTTP (Internet) WEB BROWSER CLIENT zzzzz CORE FUNCTIONALITY LEGACY APPLICATIONS WEB SERVER with plug-in Presentation Logic Business Logic Connectors COM+ CORBA EJB ASP JSP Servlets ADO JDBC ODBC RMI/IIOP DIRECTORY SERVICES (LDAP) JAVA CLIENT VALUE-ADDED SERVICES Commerce Content Management Personalization Application Layer Details Servlets

  8. Causes of page generation delays include (in addition to pure processing overhead): • Remote database accesses: Heavy I/O loads, Network delays • XML-HTML transformations: Extensive processing delays • Personalization logic: e.g., Broadvision, Vignette, etc. • Interaction bottlenecks: e.g., database connection pools => serious performance and scalability problems for web sites due to increased load on server-side infrastructure The Problem: Page Generation Delays

  9. Approaches fall into 3 broad categories: • Database caching • Page level caching • Fragment level caching Reducing delays

  10. Sources Repositories Clients Alternative: CDNs Content Distribution Networks

  11. Sources Cooperating Repositories Clients • Resilient and efficient content distribution network (CDN) for dynamic data. • Existing CDNs : Akamai, Dynamai Push Based Core Infrastructure

  12. Generic Architecture wiredhosts sensors Network Network mobile hosts servers Data sources End-hosts

  13. Generic Architecture wiredhost sensors Network Network servers Proxies /caches mobile host Data sources End-hosts

  14. Proxy registers the data item of interest and the coherency requirement with the server Server pushes interesting changes + Achieves Strong Consistency + Keeps network overhead minimum -- Poor Scalability (has to maintain state and has to keep connections open) -- Low Resiliency Server Proxy User Push Push The Push Approach

  15. Proxy Pulls after Time to Live (TTL) Time To next Refresh (TTR / TNR) + Can be implemented using the HTTP protocol + Stateless and hence is generally scalable with respect to state space and computation Weak cache consistency Heavy polling for stringent coherence requirement or highly dynamic data Network overheads higher than for Push Server Proxy User Pull Push The Pull Approach

  16. Two broad types: • Query result caching • Middle tier database caching • caching database tables in main memory Database Caching

  17. Many application server products offer this feature • [Luo et. al., 2000] proposed query result caching at Web proxy caches -- mitigates only local database access latency -- only a subset of query results may be reused in page generation -- page fragments may not all be from databases Query result caching

  18. Caching database tables in main memory Oracle 9i Cache Main-memory databases, e.g., TimesTen -- mitigates only database access latency -- caching at table granularity results in poor cache utilization -- main-memory databases are difficult to integrate and maintain and can be expensive Middle tier database caching

  19. Dynamically generated HTML pages are cached [Iyengar & Challenger, 1997; Zhu & Yang, 2000] • Several commercially available products follow this approach, e.g., SpiderCache, Xcache, Dynamai + Can completely offload work from web/app server • Low reusability for highly personalized web pages • URL may not uniquely identify a page -- increasing the risk of delivering incorrect pages • Often introduces excessive invalidations -- e.g., even if a single element on the page changes Page Level Caching

  20. Approaches fall into 3 broad categories: • Database caching • Page level caching • Fragment level caching Reducing page generation delays

  21. Code block HTML sent to user Code block HTML Buffer How Dynamic Scripting Works Page generation script Write to Out Write to Out . . .

  22. Code block Application logic Code block Database calls HTML formatting Code Blocks Perform Work Page generation script Write to Out Write to Out . . . . . .

  23. Code block Code block Code Blocks <-> Components Page generation script Web Page Ad Component Write to Out Headline Component Headline Component Navigation Component Headline Component Headline Component Write to Out . . . Personalized Component (Example: News content site) Certain components can be cached

  24. Start tag End tag DCA: Our Solution Page generation script Code block Request Dynamic Content Accelerator Code Block Output Application logic Code block Work bypassed Database calls HTML formatting . . .

  25. Users • A single instance of the DCA serves a rack of application servers • Application servers communicate with DCA through a lightweight API DCA in a Typical End-to-end Web Site Architecture Web Server Cluster Application Server Cluster Data Dynamic Content Accelerator

  26. A critical aspect of any caching solution • DCA supports novel cache management strategies: • Prediction-based cache replacement • Observation-based cache invalidation Cache Management

  27. Site Graph News Sports Hockey Schedules Scores Players Teams Cache Replacement • Prediction-based replacement • fragments having lowest probability of access replaced • Least-Likely-to-be-Used (LLU) • Access probabilities based on: • Current user navigational patterns over site graph • (in the form of clickstreams) • Historical user navigational patterns over site graph • (in the form of association rules) (News, Sports, Hockey)  Schedules = 20% (News, Sports, Hockey)  Players = 15% LLU (News, Sports, Hockey)  Teams = 10% (News, Sports, Hockey)  Scores = 55%

  28. DCA supports common cache invalidation techniques: • Time-based: Each cache element assigned a TTL • Event-based: Updates to the database send an invalidation message to the cache • On demand: Manual invalidation of selected elements • DCA supports additional invalidation techniques…. Cache Invalidation

  29. Other invalidation techniques supported: • Observation-based • User-initiated updates are observed in scripts; each such update sends an invalidation message to the cache • Most appropriate for auction sites, online trading sites • Invalidation does not require communication with the databases • Keyword-based: • Elements can be associated with keywords; e.g., a retailer may wish to invalidate all “seasonal” items • Regular expression-based: • Elements can be invalidated based on regular expression matching Cache Invalidation…

  30. Other Fragment Level Caching… • app servers (e.g., BEA’s WebLogic, IBM’s WebSphere) cache fragments produced by JSP scripts Application Server Cluster • + can offload presentation layer tasks • runs in the application server process space • => competes for server resources • application server cluster • => multiple cache instances, • duplication of content, • additional synchronization overhead

  31. Weave system [VLDB 2000] caches XML fragments, as well as query results and HTML pages • Requires use of declarative web site specification language Other Fragment Level Caching….

  32. Metric: • Average page generation time time required to construct HTML page Performance Study

  33. Test Site • Fictitious online retail site, allows browsing of product catalog • Pages generated using JSP scripts • Site content stored in Oracle database • Database schema based on Dublin Core Metadata Open Standard • Contains 200,000 products and 44,000 categories • Each page consists of 3 components, each involving a database call Performance Study…

  34. Test Setup • Content Database Server: Oracle 8.1.6 • Web/Application Server: WebLogic 6.0 running on cluster of 2 machines • Server machines: have 1 GB RAM, dual P III-933 Mhz processors run Windows 2K Advanced Server Performance Study…

  35. DCA compared to 2 middle tier caching solutions: Main Memory Database: TimesTen used to cache the content database (entire database is cached, runs on database server machine) Application Server Cache: WebLogic Server JSP caching (WLS Cache) • For both WLS and DCA, 2 (of 3) page components are cached • Usually, DCA runs on a separate machine (512 MB RAM, P III-600Mhz processor, running Windows 2K Advanced Server) Testing Methodology

  36. Baseline Parameters: • Cache Size, i.e., percentage of fragments that fit into cache: 75% • Cache replacement policy: LLU for DCA • User load is varied by sending requests from client machines running Radview’s WebLoad • Simulated users navigate site according to Zipf 80-20 distribution (i.e., 80% of users follow 20% of navigation links) Testing Methodology...

  37. Page Gen. Times vs. Number of Users TimesTen vs. DCA -- 3x to 9x improvement TimesTen only mitigates local database access latency -- still requires query processing, formatting operations

  38. Page Generation Times... WLS vs. DCA -- 2x to 5x improvement WLS runs in application server process space, competes for server resources WLS utilizes multiple caches, causing redundant caching DCA runs as single, standalone logical cache

  39. Sensitivity to Cache Size As expected, performance improves as cache size increases Since cached elements are typically quite small (e.g., a few hundred bytes), larger cache sizes are feasible in practice

  40. Increased use of dynamic page generation technologies => increases load on application servers => serious performance and scalability problems for e-business sites • DCA (Dynamic Content Acceleration) => significantly reduces the load on the server side infrastructure, allows e-business sites to scale => significantly outperforms existing middle tier caching solutions Conclusion

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