SOFTENG710/COMPSCI 711

Transcription

1 Steps on accessing a web site The time for DNS server to resolve the web server s name to IP address (if necessary). Not shown in the diagram The time for the client to set up a TCP connection with the web server The time for the request transmitted from the client to the web server The time for the web server to parse the request and generate a web page The time for the response transmitted from the host to client The time when all the information are received by the client The time for the client browser to render the page Not shown in the diagram 1 2 How to reduce the response time Reduce the network latency (i.e. transmission delay) Place servers closer to the clients Could be significant Reduce the amount of information that have to be sent to the clients Benefits the users that connect to the servers through low- bandwidth networks For a given bandwidth, less information to be transmitted means less time is needed for transmission Reduce the load on the web server Generate the response faster How to achieve these? Proxy server, caching server A proxy server normally caches web contents. But, web contents can be cached on other types of servers as well. Web Cache A web cache usually sits between the web server and clients Watches requests come by, and saves copies of the responses for them. When there is a coming request for the same URL resource, the web cache can retrieve the cached response and send it back to the client without asking for the web server to serve this request again

2 Advantages of Web Caching Reducing network latency There is a distance between clients and web servers. Caching web object locally can dramatically reduce transmitting latency Reducing bandwidth consumption Cache the web objects close to the client side Reponses to the same requests do not need to be transferred again between the proxy server that caches the web objects and the origin web server. Reducing server load Some of the requests do not need to be handled by the origin server. As some requests can be served from proxy servers, the origin web server can direct more capability to deal with other requests. Improve the response time for the non-cached contents. 5 6 Browser Categories of Web Caching Many web browsers support caching. When a web page is first requested, the browser saves the web page in the disk. If the page is requested again or the user clicks the Back button, the cached page might be used. In some cases, the browser might need to check with the remote web server to see whether the cached page is still valid. If it is valid, the page in the local disk is used. Examples: Firefox, IE. Proxy Server There are two types of proxy servers, i.e. forward proxy and reverse proxy A forward proxy is an intermediate system that enables a browser to connect to a remote network to which it normally does not have access. A forward proxy can also be used to cache data, reducing load on the networks between the forward proxy and the remote web server. A reverse proxy (http accelerator) is installed in front of web servers. All connections coming from the Internet addressed to one of the web servers are routed through the proxy server, which may either deal with the request itself or pass the request to the web server behind

3 Proxy servers are located in various locations throughout the Web. hierarchical caching structure Each cache serves many clients, which may be users or other caches. When the request is not served by a local cache, the request is passed on to a higher h level l in the hierarchy, until reaching a root cache, which, having no parent, would request the object from the origin server. 9 cooperative caching architecture Caches communicate with each other as peers (e.g., using an inter-cache protocol). On a miss, a cache would ask a predetermined set of peers whether they already have the missing object. If so, the request would be routed to the first responding cache. Otherwise, the cache attempts to retrieve the object directly from the origin server. This approach can prevent storage of multiple copies and reduce origin server retrievals Suffers from increased inter-cache communication. Instead of asking peers directly about each request, a peer periodically gives peers a summary of the contents of the cache 10 Database Server server-side cache When queries are made to the back-end database, the result of such queries is cached as materialized views at the database server. When the same query is made again to the database, the database server can use the cached view for it, which h reduces the computation cost on the database server

4 Database Accelerator It caches the common data sets for the database to accelerate the processing of the database query operations. When the application server does database queries, it can query on these common data sets without passing the queries to the database server. Reduce the communication between application server and database server Reduce the workload for the back-end database. Examples: Oracle Application Server Database Cache, TimesTen Web Application Server Application servers cache the result of dynamic generated web page. Reduces the workload on the application server and back-end database. Changes in the back-end database can invalidate the cached page. Examples: BEA WebLogic, IBM WebSphere Content Delivery/Distribution Network (CDN) CDN is designed to place of content caching servers at the edge of network around the world. It reduces the traffic congestion on the network and the latency perceived by users. The caching servers at the edge side of network are called edge servers which can cache HTML pages, images, audio and video files. Examples: Akamai, Digital Island When the client requests a web page, the request is firstly sent to a forward proxy. The forward proxy then decides which edge server to communicate with. If none of the edge servers cache this page, the request is delivered to the web server. Otherwise, one of the edge servers, which is closest to the client, will serve the request. Examples: Akamai EdgeSuite, IBM WebSphere Edge Server

5 Potential problems The end user see stale (e.g., old or out-of-date) content, compared to what is available on the origin server (i.e., fresh content). HTTP does not ensure strong consistency Caching tends to improve the response times only for cached responses that are subsequently requested (i.e., hits). Misses that are processed by a cache generally have decreased speed. Thus, a cache only benefits requests for content already stored in it. Caching is limited it by a high h rate of change of source data. Some responses cannot or should not be cached Web server will set and send the HTTP headers that determine cacheability Dynamic Data Page Dynamic data pages are generated by applications according to the underlying frequently changing data sources such as databases and files. Dynamic data pages are usually generated on the demand of user s request. Dynamic pages have two characteristics: high volatility and high variation It is volatile because not only it changes more frequently but also its change is unpredictable. It is various because it is often customized or personalized from a set of content fragments. Compared to a static page, a dynamic data page is expensive to create. more valuable to cache a dynamic data page 19 Proxy-based Dynamic Page Caching Proxy-based caching approaches can be classified into three broad categories page-level caching fragment-level l caching data-centric caching In the page-level caching, the proxy server caches full- page outputs. Many commercial products have applied this approach. Compared to no-caching, the page-level caching has the following advantages: reduce the page generation delay reduce the delay associated with packet filtering and other firewall-relatedrelated delays reduce the bandwidth required to transmit the content from the web server to the proxy 20 5

6 The page-level caching has the following limitations i i There is often very little reusability of full HTML pages A personalised page displaying the name of the user at the top of the page: the cached page in the proxy is only reusable if the same user accesses to the same page Unnecessary invalidation If there is only one element of the page becomes invalid, then the whole page needs to be invalidated in the cache. 21 Fragment-level caching Fragment-level caching requires establishing a template for each dynamically generated page. The template file specifies the contents and layout of the page. Essentially, each page is factored into a number of fragments of differing cacheability profiles and different caching expiration time. The fragments and the templates of the pages are maintained as separate elements on the origin servers and the proxy servers. A web page is assembled at proxy caching server when the page is requested. Popularized by Akamai as part of the Edge Side Includes (ESI) initiative 22 Edge Side Includes (ESI) Edge Side Includes (ESI) is a simple markup language used to define web page fragments of differing cacheability profiles and different caching expiration time. ESI defines some tags that can be used in the template files. A template file is a normal HTML file with ESI tags. Amongst the ESI tags, esi: include is used to include a fragment. <html> <body> <esi:include src= /> <esi:include src= /> <esi:include src= /> <esi:include src= /> </body> </html> advertise navigation i personalize news

7 Compared with page-level caching approach, fragment-level l caching approach can improve hit rates as it can specify granular caching capability. According to the properties of different fragments, the fragment can be set with different caching expiring time and cacheability. Further reading Data-centric caching Fragment-level caching requires the web pages to be factored into a number of fragments Data-centric caching focuses on the contents of the data being transmitted. Carried out by the underlying mechanism. Does not require the cooperation of the web page authors We will look at two schemes: Value-Based Web Caching S. C. Rhea, K. Liang and E. Brewer, Value-Based Web Caching, Proc. of the 12th International Conference on World Wide Web,,pp , 2003 Delta encoding Jeffrey C. Mogul, Fred Douglis, Anja Feldmann, Balachander Krishnamurthy, Potential benefits of delta encoding and data compression for HTTP, 1997 Value-Based Web Caching A lot of information being transmitted over the Internet are redundant, since they are identical % text-based responses were caused by resource modifications. Many modifications only change the contents of the information being transmitted by a small amount. Aliasing, which means two distinct URIs reference the same data, also results in redundant data being transmitted over the network. Aliased data account for 36% of the data transferred Value-based web caching exploits similarities between data to eliminate redundant data transmission caused by resource modification and aliasing

8 A page is broken into blocks of approximately 2KB each, and each block is named by its fingerprint (i.e. digest) calculated using a secure hash function MD5. According to the properties p of MD5, it is highly unlikely that two blocks containing different contents have the same fingerprint. Two blocks are regarded as identical if their fingerprints are the same. The cached blocks are indexed using their fingerprints. If the responses to two requests contain identical blocks, the identical blocks do not need to be sent again in the second response. The scheme was proposed to reduce the latency of the users connecting to the Internet through h low-speed connection The scheme uses two kinds of proxies. One resides on the user s PC, called child proxy; The other one resides on the user s ISP, called parent proxy. The child proxy caches the contents of the blocks and their fingerprints. The parent proxy only records the fingerprints of the blocks that are cached on the child proxy. The origin server is responsible for partitioning pages into blocks and computing the fingerprints. When a parent proxy receives the blocks that constitute the response to a user s request, the parent proxy determines which blocks the child proxy already has by checking the fingerprints of the received blocks. The parent proxy only sends the blocks that the child proxy does not have to the child proxy. The child proxy assembles the page according to the received/cached blocks and delivers the page to the user

9 Instead of using a fixed block size, e.g. 2KB per block, the scheme uses Rabin functions to determine how to partition a page into blocks. The benefit of this approach is that modifications to a page would only affect the block(s) in which the modifications occur. Determine the boundary according to the value of the blocks. Let f be a function mapping n one-byte inputs uniformly and randomly to the set {0,, 2047}. Place a block boundary before byte i in a resource if the value of f on the n bytes proceeding byte i is 0. Since f is uniform and random, we expect to evaluate it on average 2048 times before finding a zero; thus the expected block size from this technique is 2 kb Delta encoding In delta encoding, the origin server generates many different base files. When a user s request arrives, the server finds the base file that is closest to the response to be sent back to the user. The difference ( delta ) between the response and the chosen base file is computed. Then, the delta and a reference to the base file are sent back to the user. The bandwidth savings stem from the fact that the base file is commonly used and the user has a local copy of the base file. Thus, the user can obtain the requested page by applying the delta to the base file

10 client request V1 V1 server 734 exam is cancelled. V2 734 exam is not cancelled. V3 734 exam is on