Web Caching1 By Amisha Thakkar Alpa Shah

Web Caching2 Overview What is a Web Cache ? Caching Terminology Why use a cache? Disadvantages of Web Cache Other Features Caching Rules

Web Caching3 Overview Caching Architectures Comparison of Architectures Cache Deployment Scheme Client Side Cache Cooperation Active Caching

Web Caching4 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 Caching5 What is a Web Cache?

Web Caching6 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 Caching7 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 Caching8 Why use a cache ? To reduce latency To reduce network traffic Load on origin servers will be reduced Can isolate end users from network failures

Web Caching9 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 Caching10 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 Caching11 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 Caching12 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 Caching13 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 Caching14 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 Caching15 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 Caching16 Caching Architectures Hierarchical /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 Caching17 Caching Architectures Hierarchical /Simple Cache National Network Regional Network Institutional Network

Web Caching18 Caching Architectures Distributed /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 Caching19 Caching Architectures Distributed /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 Caching20 Caching Architectures Distributed /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 Caching21 Caching Architectures Distributed /Co-operating Cache More the users  more disk space higher in the hierarchy * Exponential growth of number of documents on WWW

Web Caching22 Caching Architectures Distributed /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 Caching23 Caching Architectures Distributed /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 Caching24 Caching Architectures Distributed /Co-operating Cache * UDP reply from cache can indicate a. Presence b. Speed c. Availability of requested documents

Web Caching25 Caching Architectures Hybrid Cache Note: ICP

Web Caching26 Comparison of Architectures Hierarchical : caches placed at multiple levels Distributed :caches only at bottom level; no intermediate caches

Web Caching27 Comparison of Architectures Performance parameters.  Connection time (T c )is defined as the time since the document is requested & first data byte is received  Transmission time (T t )is defined as the time taken to transmit the document  Total latency = T c +T t.  Bandwidth usage

Web Caching28 Comparison of Architectures Fig 3 -Connection time for different document’s popularity

Web Caching29 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 Caching30 Comparison of Architectures Fig 4 Network traffic generated

Web Caching31 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 Caching32 Comparison of Architectures Fig 5 a, Not congested national network

Web Caching33 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 Caching34 Comparison of Architectures Fig 5 b Congested National Networks

Web Caching35 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 Caching36 Comparison of Architectures Fig 6 Average total latency

Web Caching37 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 Caching38 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 Caching39 Comparison of Architectures With Hybrid Scheme Fig 7 connection time

Web Caching40 Comparison of Architectures With Hybrid Scheme Fig 8.

Web Caching41 Comparison of Architectures With Hybrid Scheme In the hybrid scheme if the number of cooperating caches (k c ) 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 Caching42 Comparison of Architectures With Hybrid Scheme Fig 9 Transmission time

Web Caching43 Comparison of Architectures With Hybrid Scheme For un-congested n/w the no.of coop caches (k t ) at every level hardly influences T t If no. of coop caches is very small, high T t & vice -versa If the no increases above the threshold the T t increases Optimum no. of caches depends on the no of caches reachable avoiding congested links

Web Caching44 Comparison of Architectures With Hybrid Scheme Fig 10

Web Caching45 Comparison of Architectures With Hybrid Scheme Fig 11 total latency

Web Caching46 Comparison of Architectures With Hybrid Scheme The no. of coop caches(k opt ) at every level depend on the document size to minimize the total latency For small documents the optimum no. is closer to k c For large documents the the optimum no. is closer to k t

Web Caching47 Comparison of Architectures With Hybrid Scheme Fig 12

Web Caching48 Comparison of Architectures With Hybrid Scheme For any document the optimum k opt that minimizes the total latency is such that k c  k opt  k t

Web Caching49 Cache Deployment Schemes Proxy caching

Web Caching50 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 Caching51 Cache Deployment Schemes Disadvantages  Every browser must be configured to point to the cache  Each client can hit only one cache  Single point of failure  Unnecessary duplication of data  Bottleneck in cases where content is otherwise available in LAN

Web Caching52 Cache Deployment Schemes Transparent Proxy caching

Web Caching53 Cache Deployment Schemes Advantages  No browser configuration  Cost of upgrading h/w & s/w is limited  No administration of intermediate systems required

Web Caching54 Cache Deployment Schemes Disadvantages  Each client can hit only one cache  If cache goes down internet as well as intranet access lost  Negative impact on non web traffic  Cache has to route non web traffic  Routing,packet examination & n/w addr. translation steal CPU cycles from the main cache serving function

Web Caching55 Cache Deployment Schemes Transparent proxy caching with web cache redirection.

Web Caching56 Cache Deployment Schemes Advantages  Switch/ router examines the packets  Minimal impact on non-web traffic  Frees up CPU cycles for the web cache  Allows client load to be dynamically spread over multiple caches  Eliminates single point of failure especially if redundant redirectors are used

Web Caching57 Cache Deployment Schemes Disadvantages  Additional intermediate systems must be deployed  Increases expense

Web Caching58 Client Side Cache Cooperation.

Web Caching59 Active Caching Current problem unable to cache dynamic documents Caching Dynamic contents on the web using active web Cache applet is server supplied code that is attached with an URL, or collection of URLs Applet is written in platform independent language

Web Caching60 Active Caching On a user request the applet is invoked by the cache The applet decides what is to be sent to the user Other functions of the applet- * Logging user accesses * Checking access permissions * Rotating advertising banners

Web Caching61 Active Caching The proxy has the freedom to not invoke the applet but send the request to the server Proxy promises to not send back a cached copy without invoking the applet If applet too huge,send request to server Proxy not obligated to cache any applet, in that case agrees to not service the request for that document

Web Caching62 Active Caching Proxy can devote resources to the applets associated with the hottest URLs to its user Proxy that receives the request is typically the proxy closest to the user, the scheme automatically migrates the server processing to the nodes that are close to users Thus increasing the scalability of web based services