Web Caching Schemes1 A Survey of Web Caching Schemes for the Internet Jia Wang.

Web Caching Schemes 1

A Survey of Web Caching Schemes for the Internet

Jia Wang


AgendaThe World Wide WebProblem and solution (caching)Proxy serversAdvantages of web cachingDisadvantages of web cachingElements of A WWW caching systemDesirable properties of WWW caching systemProblems in designing caching systems for the WWWCaching architecture


The World Wide Web

The WWW can be considered as a large distributed information system.

Exponential growth in size.

On may 1999 included 600 millions of static web pages.

Increases 15% per month.

Very popular.


0

100

200

300

400

500

600

Jun-97 Nov-97 Mar-98 May-99

SIZE OF DISTINCT STATIC WEB PAGES


The World Wide Web

Usage is relatively inexpensive

Accessing information is very fast

Documents appeal to a wide range of interests

But…..


The World Wide Web

Network congestion

Server overloading


Problem

Internet backbone capacity increases 60% per year.

Bandwidth is not growing fast enough.

Without solution WWW will become too congested and its entire appeal would be lost.


Solution

Caching:

Placing popular objects at

locations close to the clients.


proxy servers

HTTP servers handled by companies

for security reasons.

The bottleneck of the connection

between the client and the internet.

Shared by all clients inside the firewall.


Clients within the firewall

Proxy on firewall machine

HTTP server

HTTP server

HTTP server


proxy servers

Belonging to same organization, clients

share common interests.

They probably access the same set of

documents.


thus

On the proxy server, a previously

requested and cached documents

would likely result in future hits.


proxy severs

Caching most popular web pages on

the proxy server can:

Save network bandwidth

Lower access latency for the client


Advantages of web caching Reduces bandwidth consumption

Decreases network traffic

Lessens network congestion

Access latency:

frequently used docs are cached nearby

less traffic shorter delay for docs not cached


Advantages of web caching (cont.)

Reduces workload of remote server

Data can be accessed when remote server is down (enhanced robustness).

Allows analysis of organization usage patterns

cooperation between caches increases efficiency.


Disadvantages of web caching

Data not updated automatically

Cache miss can cause increase in latency (extra proxy processing).

Bottleneck effect – limit # of clients per proxy.

A single proxy is a single point of failure

Information providers can not monitor # of visits per site.


Elements of A WWW caching system

Documents can be cached at the clients, the proxies and the servers.

Web server Web server

Proxy server cooperation

clients

clients

clients


Elements of a WWW caching system

request

client

Does proxy have requested page yes

no

Does cooperative proxies have web page

yes no

Find web page on server


Desirable properties of WWW caching system

fast access

robustness

transparency

scalability

efficiency

adaptivity

stability

load balance

ability to deal with

heterogeneity

simplicity


Fast access

Reduce web access

latency to a

minimum.

Especially

comparing to other

servers not using

caching techniques.


Robustness

Robustness = Availability to user

eliminate single point failure

in case of failure – fall down gracefully

easy to recover from failure


Transparency

Transparent to

the user

The user should

only notice:

Faster response

Higher availability


Scalability

Scale well along the increasing size and density of the network.

All protocols should be as lightweight as possible.


Efficiency

impose minimal additional burden on the network (in control & data packets)

do not adopt any scheme which leads to under-utilization of the network


Adaptivity

adapt to dynamic

changing in the user

demand and network

environment

achieve optimal

performance


Stability

Do not introduce

instabilities into

the network


Load balancing

distribute load

evenly through

the entire

network

no bottlenecks /

hot-spots


Ability to deal with heterogeneity

Adapt to a range

of network

architecture

(hardware &

software)


Simplicity

Mechanism simple to deploy

simpler schemes are easier to implement and likely to be accepted as international standards


What Problems do we face in designing caching systems for the WWW ???


Problems in designing caching systems for the

WWW

Caching system architecturehow cache proxies

are organized – hierarchically, distributed or hybrid.



WWW

Proxy placement were to place a

cache proxy in order to optimize performance



WWW

Caching contentsWhat can be

cached in the

caching system



WWWProxy

cooperationHow do proxies

cooperate with

each other



WWW

Data sharingwhat kind of

data/information can be shared among among cooperative proxies



WWW

Cache resolution/routinghow does a proxy

decide where to fetch a page requested by a client.



WWW

PrefetchingHow does a proxy

decide what and when to prefetch from webservers or other proxies to reduce access latency.



WWW

Cache placement/ replacementhow the proxy decides

which page to be stored in its cache and which page to be removed from it.



WWW

Cache coherencyhow does a proxy

maintain data consistency



WWW

Control information distributionhow is the control

information (e.g URL) distributed among proxies.



WWW

Dynamic data cachinghow to deal with

data that is not cachable


Caching architecture

HierarchicalCaches are placed at multiple levels of the

network.

national

regional

institutional

bottom


Hierarchicalarchitecture

Bottom – clients/browsers caches.

national

regional

institutional

bottom web page not found

web page not found

web page not found


Hierarchicalarchitecture

after web page is found

national

regional

institutional

bottom

forward page, leave copy




Hierarchical architecture

Advantages:Bandwidth efficient – especially when

cache servers are slow.

Allows to efficiently diffuse popular

web pages towards the demand.


Hierarchical architecture

DisadvantagesCache server needs to be placed at key

access points of the network requires coordination among caches.

Each level adds a delay.High levels are bottlenecks.multiple copies at different cache levels.


Distributed architecture

Caches at the bottom level only.

No other intermediate caching levels.

Each cache server contains meta-data on the data stored on other servers.

Hierarchy used only for distributing information about location of the copy.

No copying of actual documents.


Advantages:Traffic flows through low network

levels which are less congested.No additional disk space required for

intermediate network levels.Better load sharing.More fault tolerant.



Disadvantages:High connection timesHigher bandwidth usageAdministrative issues.



ExamplesICP – Internet Cache Protocol (Harvest group)

Retrieve data from neighboring caches +

parent caches

CARP – Cache Array Routing ProtocolURL space divided to an array of caches.

Each cache stores only documents whose

URL are hashed to it.



Hybrid architecture

Caches may cooperate with other caches at

the same level or at a higher level using

distributed caching.

ICP is an example:

the document is fetched from a parent/neighbor

cache that has the lowest RTT.


Performance of architectures

Hierarchical caching has shorter connection

times than distributed caching.

Additional copies at intermediate level reduces

retrieval latency for small documents.

Distributed caching has shorter transmission

times & higher bandwidth usage.

“Well configured” hybrid scheme can reduce

both connection time and transmission time.

Web Caching Schemes1 A Survey of Web Caching Schemes for the Internet Jia Wang.

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