html5-img
1 / 62

Web Caching

Web Caching. By Amisha Thakkar Alpa Shah. Overview. What is a Web Cache ? Caching Terminology Why use a cache? Disadvantages of Web Cache Other Features Caching Rules. Overview. Caching Architectures Comparison of Architectures Cache Deployment Scheme Client Side Cache Cooperation

gloria
Télécharger la présentation

Web Caching

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Web Caching By Amisha Thakkar Alpa Shah Web Caching

  2. Overview • What is a Web Cache ? • Caching Terminology • Why use a cache? • Disadvantages of Web Cache • Other Features • Caching Rules Web Caching

  3. Overview • Caching Architectures • Comparison of Architectures • Cache Deployment Scheme • Client Side Cache Cooperation • Active Caching Web Caching

  4. What is a Web Cache ? • Cache is a place where temporary copies of objects are stored • Cached information is generally closer to the requester than the permanent information is • Objects -HTML pages, images, files Web Caching

  5. What is a Web Cache? Web Caching

  6. Caching Terminology • Client - An application program that establishes connections for sending requests • Server- An application program that accepts connection to service requests by sending back responses • Origin Server-The server on which the given resource resides or is to be created Web Caching

  7. Caching Terminology • Proxy- An intermediary program which acts both as a server and a client which requests on behalf of the other clients • Proxy is not necessarily a cache * Proxy does not always cache the replies passing through it * It may be used on a firewall to monitor accesses Web Caching

  8. Whyuse a cache ? • To reduce latency • To reduce network traffic • Load on origin servers will be reduced • Can isolate end users from network failures Web Caching

  9. Disadvantages of Web cache • With cached data there is always a chance of receiving stale information • Content providers lose access counts when cache hits are served • Manual configuration is often required • Operation of cache requires additional resources • In some situations the cache can be a single point of failure Web Caching

  10. Other Features • Depending on the perspective the following may be good or bad * Cache requests on behalf of clients ; the servers never see the clients IP addresses * Cache provides an easy opportunity to monitor and analyze browsing activities * Cache can be used to block certain requests Web Caching

  11. Types of Web Caches • Proxy caches * Serve a large number of users * Large corporations and ISP’s often set them up on the firewalls * They are type of shared caches • Browser caches * Use a section of the computer’s hard disk to store objects that you have seen Web Caching

  12. Caching Rules • Rules on which caches work - * Some of them set in protocols * Some are set by cache administrator • Most common rules : * If the object is authenticated or secure it won’t be cached * Object’s headers indicate whether the object is cacheable or not Web Caching

  13. Caching Rules * Object is considered fresh when -  It has an expiry time or other age controlling directive set & is still within the fresh period  If the browser cache has already seen the object & has been set to check once a session Web Caching

  14. Caching Rules  If a proxy cache has seen the object recently & it was modified relatively long ago Fresh documents are served directly from the cache without checking with the origin server Web Caching

  15. Caching Rules * For a stale object , the origin server will be asked to validate the object , or tell the cache whether the copy is still good * The most common validator is the time that the object was last changed Web Caching

  16. Caching ArchitecturesHierarchical /Simple Cache • Browser-cache interaction is same as browser -host interaction, i.e. a TCP connection is made & item requested • If not found send request to parent cache • Hierarchy built up - each level serving indirectly a wider community of users Web Caching

  17. National Network National Network RegionalNetwork RegionalNetwork Institutional Network Institutional Network Institutional Network Institutional Network Caching ArchitecturesHierarchical /Simple Cache Web Caching

  18. Caching ArchitecturesDistributed /Co-operating Cache • Decentralized(Cache Mesh) • Multiple servers cooperate in such a way that they share their individual caches to create a large distributed one • Simply put caching proxies communicating with each other to serve different users • On a cache miss, it checks with other proxy caches before contacting the origin server Web Caching

  19. Caching ArchitecturesDistributed /Co-operating Cache • Caches communicate amongst themselves using a protocol like ICP (Internet Cache Protocol) • Caches can be selected on the basis of * Distances from the end user * Specialize in particular URLs(location hint). Web Caching

  20. Caching ArchitecturesDistributed /Co-operating Cache • Why Distributed - limitations of hierarchy * Width of cache in hierarchy: caches at same level are inaccessible to each other * LRU policy implies sufficient disk space * Cost in replication of disk storage * Amount of disk space reqd. depends on number of users served & breadth of reading Web Caching

  21. Caching ArchitecturesDistributed /Co-operating Cache More the users  more disk space higher in the hierarchy * Exponential growth of number of documents on WWW Web Caching

  22. Caching ArchitecturesDistributed /Co-operating Cache • Caching close to user - more effective, higher the level lower the efficiency • Can be created for load balancing • Most effective when serving a community of interests Web Caching

  23. Caching ArchitecturesDistributed /Co-operating Cache • First an UDP packet sent for cache inquiry. • Cache selection decision is determined by RTT • Potential problem -network congestion because of UDP • In favor- * UDP exchange :2 IP packets, TCP :at least 8 packets Web Caching

  24. Caching ArchitecturesDistributed /Co-operating Cache * UDP reply from cache can indicate a. Presence b. Speed c. Availability of requested documents Web Caching

  25. Caching ArchitecturesHybrid Cache Note: ICP Web Caching

  26. Comparison of Architectures • Hierarchical : caches placed at multiple levels • Distributed :caches only at bottom level; no intermediate caches Web Caching

  27. Comparison of Architectures • Performance parameters.  Connection time (Tc)is defined as the time since the document is requested & first data byte is received  Transmission time (Tt)is defined as the time taken to transmit the document  Total latency = Tc +Tt .  Bandwidth usage Web Caching

  28. Comparison of Architectures • Fig 3 -Connection time for different document’s popularity Web Caching

  29. Comparison of Architectures • For unpopular documents high connection time • No of requests increases  avg.. connection time decreases • For extremely popular documents distributed has smaller connection times Web Caching

  30. Comparison of Architectures • Fig 4 Network traffic generated Web Caching

  31. Comparison of Architectures • On lower levels, distributed caching practically double the network bandwidth usage • Around the root node in national network, the network traffic is reduced to half • Distributed caching uses all possible network shortcuts between institutional caches, generating more traffic in the less congested low network levels Web Caching

  32. Comparison of Architectures • Fig 5 a, Not congested national network Web Caching

  33. Comparison of Architectures • The only bottleneck on the path from the client to the origin server is the international path. Hence transmission times are similar for both Web Caching

  34. Comparison of Architectures • Fig 5 b Congested National Networks Web Caching

  35. Comparison of Architectures • Both have higher transmission times compared to the previous case • Distributed caching gives shorter transmission times than hierarchical because many requests travel through lower network levels Web Caching

  36. Comparison of Architectures • Fig 6 Average total latency Web Caching

  37. Comparison of Architectures • For large documents transmission time is more relevant than connection times • Hierarchical caching gives lower latencies for documents smaller 200 KB due to lower connection times • Distributed caching gives lower latencies for larger documents due to lower transmission times Web Caching

  38. Comparison of Architectures • The size- threshold depends on the degree of congestion in national network • Higher the congestion, lower is the size- threshold • Distributed caching has lower latencies than hierarchical Web Caching

  39. Comparison of ArchitecturesWith Hybrid Scheme • Fig 7 connection time Web Caching

  40. Comparison of ArchitecturesWith Hybrid Scheme • Fig 8. Web Caching

  41. Comparison of ArchitecturesWith Hybrid Scheme • In the hybrid scheme if the number of cooperating caches (kc) is very small , the connection time is high • When number of cooperating caches increases, the connection times decreases up to a minimum • If the number increases over the threshold , the connection time increases very fast Web Caching

  42. Comparison of ArchitecturesWith Hybrid Scheme • Fig 9 Transmission time Web Caching

  43. Comparison of ArchitecturesWith Hybrid Scheme • For un-congested n/w the no.of coop caches (kt) at every level hardly influences Tt • If no. of coop caches is very small , high Tt & vice -versa • If the no increases above the threshold the Tt increases • Optimum no. of caches depends on the no of caches reachable avoiding congested links Web Caching

  44. Comparison of ArchitecturesWith Hybrid Scheme • Fig 10 Web Caching

  45. Comparison of ArchitecturesWith Hybrid Scheme • Fig 11 total latency Web Caching

  46. Comparison of ArchitecturesWith Hybrid Scheme • The no. of coop caches(kopt) at every level depend on the document size to minimize the total latency • For small documents the optimum no. is closer to kc • For large documents the the optimum no. is closer to kt Web Caching

  47. Comparison of ArchitecturesWith Hybrid Scheme • Fig 12 Web Caching

  48. Comparison of ArchitecturesWith Hybrid Scheme • For any document the optimum kopt that minimizes the total latency is such that kc koptkt Web Caching

  49. Cache Deployment Schemes • Proxy caching Web Caching

  50. Cache Deployment Schemes • Advantages  Clients point all web requests directly to cache : no effect on non web traffic Cost of upgrading h/w & s/w is limited  Administration on caches limited to basic configuration Web Caching

More Related