Web Cache

1. Web Cache Yee Vang

2. Introduction • Internet has many user • Issues with access latency (lag) • Server crashing • How to solve? • One solution, Web Cache

3. Web Cache • What is web cache? • Cache “a place of storage” • Web cache – “a place to store websites or web objects”

4. Web caching • Web Caching • Technique that can: • Reduce access latency • “the time it takes for a request to be completed” • Network congestion • “occurs when a link or node is carrying so much data that its quality of service deteriorates”

5. Web Caching • How does it reduce user access latency and network congestion? • No cache example • Movie Storage Room in the next building • Contain one copy of every movie • One worker

6. Web Caching • Cache example • The same as the previous example • Movie Storage Room in the next building • Contain one copy of every movie • One worker • A Movie rack that can hold five movie at a time, to simulate a movie cache.

7. Web Caching • In the example • Customer -> User • Movie -> Web Pages • Worker -> ISP • Movie Storage Room -> Origin Server • Movie Rack -> Web Cache

8. Cache Hit/Cache hit rate • Cache hit • Occurs when a request can be satisfied by the web cache. • In the movie store example • Hit? • Cache hit rate • Is the percentage that a previously cached object will score a cache hit

9. Cache Miss • Cache miss • Occurs when a request cannot be satisfied by the web cache. • In the movie example • Miss?

10. Web Caching • Pros • Can reduce internet bandwidth • If a request can be satisfied by the web cache • Reduce the work load of the origin server • By storing previously requested web objects in a web cache • Reduce user access latency • When a cache hit occurs

11. Web Caching • Cons • Not every web objects are cacheable • Website that generate dynamic data • Requires an active connection • https:// • Stale Cache • Cache that are out of date • Bottleneck at the proxy server (in proxy caching)

12. Types of web cache • Browser Cache • Proxy Cache • Reverse Proxy Cache

13. Browser Cache • Cache stored at client level • Meaning the cache is actually stored on the user’s computer • i.e. Temporary internet files, [http://www.holgermetzger.de/pdl.html]

14. Browser Cache • Advantages of Browser Cache • Stored Locally • On cache hit it saves bandwidth • Increase in access latency • User pattern • The same user has a higher probability of browsing the same website each day.

15. Browser Cache • Disadvantages of Browser Cache • Takes up hard drive space • Stale object • Always risk running into stale object with caching. • Stored Locally • Only serves one computer.

16. Proxy Cache • Cache are stored at a proxy server • The proxy server usually serves more than one user • Acts as a gateway to the internet for large company or institution http://www.codeproject.com/KB/web-cache/ExploringCaching/cache_array.jpg

17. Proxy Cache • Request are directed to the proxy server instead of the origin server. • On cache hit • Returns the requested object to the user. • On cache miss • Request is then forwarded to origin server.

18. Proxy Cache • Advantages • Serves more than one client • Cache hit can occur even if different user makes the same request. • Gateway • Companies can limit what user can access. • Disadvantages • Serves more than one client • Can be overloaded. • Gateway • When the proxy server is down all the users are disconnected from then internet.

19. Reverse proxy Cache • Serves, origin server • Basically a proxy server that sits in front of the origin server. http://odino.org/images/proxy-cache.jpg

20. Reverse Proxy Cache • When a request is made? • Directed to the reverse proxy cache server • On cache hit • Object is returned to user • On cache miss • Request is forwarded to the origin server • A copy is stored on the Reverse proxy server • A copy is sent back to the user

21. Reverse Proxy Cache • Advantages • Reduces workload off of the origin server • Requested object can be requested once, cached on the reverse proxy server, and server many clients without contacting the origin server again • Static files can be cached • i.e. CSS files, java scripts, logos • Allows the origin server to better process dynamic contents

22. Reverse Proxy Cache • Disadvantages • Bottleneck • Many users making requests at the same time • Stale Cache/old files • Risk of cache hits on stale object, also static files can be outdated

23. Web Caching Architecture • Two main web caching architecture • Hierarchical • Distributed • They both utilizes the network shown below [3]

24. Hierarchical Caching Architecture • There are more than one level of cache between the users and the origin server • Typically employs more than one types of cache • There are parents, child and sibling relationships between caches.

25. Hierarchical Caching Architecture • First level of cache – Institutional Network • Second level of cache – Regional Network • Third level of cache – National Network • Parents? Child? Siblings? [3]

26. Hierarchical Caching Architecture • When a request is made • Its sent to the level one cache • If the level one cache cannot satisfy the request • Then its forwarded to the level two cache • If the level two cache cannot satisfy the request • Then its forwarded to the next level. • Once it reaches the last level, and still not be satisfied, then the request is forwarded to the origin server

27. Hierarchical Caching Architecture • Advantages • Different level of cache offers more chance for a cache hit • Leads to decrease access latency • Also reduce workload on the origin servers • Disadvantages • Every level added to the hierarchy adds delay • On cache miss there is a slight increase in latency • Higher level cache servers are expensive

28. Distributed Caching Architecture • Cache are stored at the Institutional Level • Regional and national level are eliminated • Each institutional network in the distributed system are siblings to each other. [3]

29. Distributed Caching Architecture • What is special in the distributed caching architecture? • Each institutional cache can contact its sibling cache • So each cache can knows what is in the other cache • They can receive objects from their sibling

30. Distributed Caching Architecture • When a request is made? • Query-Based Approach – Internet Caching Protocol • Request sent to configured institutional cache server • On cache miss, the request is broadcasted to the institutional cache’s sibling cache. • If a sibling cache contains the requested object, the sibling cache sends the object to the immediate institutional cache. The immediate institutional cache then stores a copy in itself, and sends the client another copy • If no sibling contains the requested object, a timeout will occur. • At which point the immediate institutional cache will then forward the request to the origin server.

31. Distributed Caching Architecture • When a request is made? • Directory-Based Approach – Cache Digest (Squid) • In this approach metadata is used. • Each cache is aware of it’s siblings content. • When a request is made, its sent to the immediate institutional cache. • On cache miss, the institutional cache checks its metadata to see if any of it’s sibling cache contains the requested object. • If not, then it forwards the request to the origin server

32. Distributed Caching Architecture • Advantage • Sibling cache servers share common interests • More chance of cache hit • Sibling cache servers are assigned based on proximity • Faster response time

33. Distributed Caching Architecture • Disadvantage • Sibling cache servers share common interests • If the servers are too far apart • Increase in access latency • Sibling cache servers are assigned based on proximity • Servers may not share common interest • Less chance of cache hit

34. Web Cache Coherency • Web cache coherency • Is the cache up to date? • Web cache coherency mechanism • Validation check • When a web object is first received • It gets time stamped • When the cached object is used, the cache server makes a validation check, by sending the time stamp to the origin server

35. Web Cache Coherency • Web cache coherency mechanism • Callback • When a web object is cached, it receives a callback promise for the object, from server. • Callback promise – a promise that the origin server will notify the cache server if the object has been updated • So the cache object is up to date if the cache server have not received a notification from the origin server

36. Web Cache Coherency • Web cache coherency mechanism • Expiration • When an object is cache an expiration date is assigned to it • Object is valid until expiration date • The first request for the object after its expiration date is requested from the origin server again. • At this time a new expiration date is assigned to the object

37. Cache Placement and Replacement Policies • How cache are replaced • Random • A random cache is replaced. • Size • Largest cache is replaced first • FIFO – First In First Out • Oldest cache is eliminated first

38. Cache Placement and Replacement Policies • LRU – Least Recently Used • Cache that has not been requested for the longest time is eliminated first • LRU/MIN – Least Recently Used Minimum • The first document whose size is larger than or equal to the size of the new document is removed • HLRU – History Least Recently Used • Record how many times each cached object is used • Elimination based on • LRU • Least Used

39. Cache Placement and Replacement Policies • LFU – Least Frequently Used • Cache are sorted based on how frequently it is used • On cache hit, the counter for the hit object is incremented by one. • List is then re-ordered • The web object with the lowest count is replaced first • LFU – Aging • Same as LRU • The Average count of all cached object is monitored • When the average count reaches a threshold, all counts are reset back to zero

40. Cache Placement and Replacement Policies • LRV – Lowest Relative Value • Each cached object is assigned a cost value • Object with the lowest cost value are replaced first • GD – Greedy Duel • Each cached object is assigned a cost value • Lowest cost object are replaced first • Then all cached object has their cost lowered by the replaced object’s cost • Each time a cache is accessed its cost is reset back to its original cost

41. Conclusion • Web cachinghelps reduce: • Network Congestion • User access latency • Performance of origin server

42. Questions? • Questions?

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45. Reference • [11]Vakali, A. (2000). Lru-based algorithms for web cache replacement. In K. Bauknecht, S. Kumar Madria & G. Pernul (Eds.), Electronic Commerce and Web Technologies, First International Conference (p. 409-418). Retrieved from http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.59.5504&rep=rep1&type=pdf