Transcript Overview on Web Caching

Overview on Web Caching
COSC 513 Class Presentation
Instructor: Prof. M. Anvari
Student name: Wei Wei
ID: 103765
Web Caching: A Key
Technique for Internet
1.What is Web Cache and what is Web
Caching?
 Web Cache: sits between server(s) and
client(s);
 Web Caching: technique to use cache
concept on the web.
2.Why use Web Cache?
 Reduce latency;
 Reduce traffic.
3.Kinds of Web Caches:
 Browser Caches:
1) use disk and memory (size 5~50MB);
2) local to machine: private cache.
 Proxy Cache:
1) an application-layer network service;
2) operated on dedicated hardware (5~50
GB hard disk, 64~512MB RAM);
3) shared by many users.
4.Any difference between proxy and cache?
 Proxy refers to an important aspect of design;
 Strictly speaking, proxy does not always cache
the replies pass through it;
 Use term proxy cache to mean Web Cache
which is implemented as a HTTP proxy.
5.Advantages of Web Caching:
 objects delivered to clients with higher
throughput and lower latency;
 cache hits reduce WAN traffic;
load on original servers reduced;
 isolate end-users from network failure.

6.Disadvantages of Web Caching:
 lose precious access counts when cache hits
are served;
 manual configuration often required;
 cannot be implemented totally transparently;
 received stale information;
 require additional resources;
 single point of failure.
7.Other features of caching:
 Depending on specific perspective ,the
following features might be considered
either good or bad:
 servers cannot see client IP;
 log file data can be used in different way;
 related to above issues, cache can be
used to block certain requests.
8.Web Caching effectiveness measures:
 hit ratio (how about 304 Not Modified);
 byte hit ratio (reply but not request);
service time (underestimate at least 2
network RTTs).
9.How Web Cache work?
 Have to determine when to serve an object
from the cache, if it’s available:
 rules are set in protocols (HTTP 1.0 & 1.1);
 rules set by administrators of cache (either
browser user or proxy administrator).

Common rule:

1) If object’s headers tell cache not to keep
the object, it won’t. Also , if no validator is
present, most cache will mark the object
uncacheable.
2) If the object is authenticated or secure, it
won’t be cached.
3) A cached object is considered fresh (that is,
able to be sent to a client without checking
with the origin server) If:
 it has an expire time or other age-controlling
set, and is still within the fresh period.
 If a browser cache has already seen the
object , and has been set to check once a
session.

If a proxy cache has seen the object
recently, and it was modified relatively long
ago.
 Fresh documents are served from cache
directly without checking with origin servers.

4) If an object is stale, the origin server will be
asked to validate the object , or tell the
cache whether the copy that it has is still
good.
Together, freshness and validation are
the most important ways that caches
work with content.
 Fresh object available instantly from
cache;
 Validated object will avoid 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.