Overview on Web CachingOverview on Web Caching

COSC 513 Class PresentationCOSC 513 Class Presentation

Instructor: Prof. M. AnvariInstructor: Prof. M. Anvari

Student name: Wei WeiStudent name: Wei Wei

ID: 103765ID: 103765

Web Caching: A Key Web Caching: A Key Technique for InternetTechnique for Internet

1.What is Web Cache and what is Web 1.What is Web Cache and what is Web Caching?Caching?

Web Cache: sits between server(s) and Web Cache: sits between server(s) and client(s); client(s);

Web Caching: technique to use cache Web Caching: technique to use cache concept on the web.concept on the web.

2.Why use Web Cache?2.Why use Web Cache? Reduce latency;Reduce latency; Reduce traffic.Reduce traffic.

3.Kinds of Web Caches:3.Kinds of Web Caches: Browser Caches:Browser Caches:

1) use disk and memory (size 5~50MB);1) use disk and memory (size 5~50MB);

2) local to machine: private cache.2) local to machine: private cache. Proxy Cache:Proxy Cache:

1) an application-layer network service;1) an application-layer network service;

2) operated on dedicated hardware 2) operated on dedicated hardware (5~50 GB hard disk, 64~512MB (5~50 GB hard disk, 64~512MB RAM);RAM);

3) shared by many users.3) shared by many users.

4.Any difference between 4.Any difference between proxy proxy and and cachecache?? ProxyProxy refers to an important aspect of refers to an important aspect of

design;design; Strictly speaking, Strictly speaking, proxy proxy does not always does not always

cache the replies pass through it;cache the replies pass through it; Use term Use term proxy cacheproxy cache to mean Web Cache to mean Web Cache

which is implemented as a HTTP proxy.which is implemented as a HTTP proxy.

5.Advantages of Web Caching:5.Advantages of Web Caching: objects delivered to clients with higher objects delivered to clients with higher

throughput and lower latency;throughput and lower latency; cache hits reduce WAN traffic;cache hits reduce WAN traffic;

load on original servers reduced;load on original servers reduced; isolate end-users from network failure.isolate end-users from network failure.

6.Disadvantages of Web Caching:6.Disadvantages of Web Caching: lose precious access counts when cache lose precious access counts when cache

hits are served;hits are served; manual configuration often required;manual configuration often required; cannot be implemented totally cannot be implemented totally

transparently;transparently; received stale information;received stale information; require additional resources;require additional resources; single point of failure.single point of failure.

7.Other features of caching:7.Other features of caching: Depending on specific perspective ,the Depending on specific perspective ,the

following features might be considered following features might be considered either good or bad:either good or bad:

servers cannot see client IP;servers cannot see client IP; log file data can be used in different log file data can be used in different

way;way; related to above issues, cache can be related to above issues, cache can be

used to block certain requests.used to block certain requests.

8.Web Caching effectiveness measures:8.Web Caching effectiveness measures: hit ratio (how about 304 Not Modified);hit ratio (how about 304 Not Modified); byte hit ratio (reply but not request);byte hit ratio (reply but not request);

service time (underestimate at least 2 service time (underestimate at least 2 network RTTs).network RTTs).

9.How Web Cache work?9.How Web Cache work? Have to determine when to serve an object Have to determine when to serve an object

from the cache, if it’s available: from the cache, if it’s available: rules are set in protocols (HTTP 1.0 & 1.1);rules are set in protocols (HTTP 1.0 & 1.1); rules set by administrators of cache (either rules set by administrators of cache (either

browser user or proxy administrator).browser user or proxy administrator).

Common rule:Common rule: 1) If object’s headers tell cache not to keep 1) If object’s headers tell cache not to keep

the object, it won’t. Also , if no validator is the object, it won’t. Also , if no validator is

present, most cache will mark the object present, most cache will mark the object

uncacheable.uncacheable.

2) If the object is authenticated or secure, 2) If the object is authenticated or secure, it won’t be cached.it won’t be cached.

3) A cached object is considered fresh (that 3) A cached object is considered fresh (that is, able to be sent to a client without is, able to be sent to a client without checking with the origin server) If:checking with the origin server) If:

it has an expire time or other age-it has an expire time or other age-controlling set, and is still within the fresh controlling set, and is still within the fresh period.period.

If a browser cache has already seen the If a browser cache has already seen the object , and has been set to check once a object , and has been set to check once a session.session.

If a proxy cache has seen the object If a proxy cache has seen the object recently, and it was modified relatively recently, and it was modified relatively long ago.long ago.

Fresh documents are served from Fresh documents are served from cache directly without checking with cache directly without checking with origin servers.origin servers.

4) If an object is stale, the origin server 4) If an object is stale, the origin server will be asked to will be asked to validatevalidate the object , or the object , or tell the cache whether the copy that it tell the cache whether the copy that it has is still good.has is still good.

Together, freshness and validation Together, freshness and validation

are the most important ways that are the most important ways that caches work with content.caches work with content.

Fresh objectFresh object available instantly available instantly from cache;from cache;

Validated objectValidated object will avoid will avoid resending if not changed.resending if not changed.

Conclusion

As Web service becomes more popular, users are suffering network congestion and server overloading. Web caching is viewed to be one of the efficient techniques to alleviate server bottleneck and network traffic, thereby minimize the user access latency.Note: This presentation only covers a small part of the final report.