A Survey of Web Caching Schemes for the Internet

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 WebThe 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.


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600

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

SIZE OF DISTINCT STATIC WEB PAGES


The World Wide WebUsage is relatively inexpensiveAccessing information is very fastDocuments appeal to a wide range of interests

But…..


The World Wide WebNetwork congestion

Server overloading


ProblemInternet 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.


SolutionCaching:

Placing popular objects at locations close to the clients.


proxy serversHTTP 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 seversCaching 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 nearbyless traffic shorter delay for docs not cached


Advantages of web caching (cont.)

Reduces workload of remote serverData 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 automaticallyCache miss can cause increase in latency (extra proxy processing).Bottleneck effect – limit # of clients per proxy.A single proxy is a single point of failureInformation 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 accessReduce web access latency to a minimum.

Especially comparing to other servers not using caching techniques.


RobustnessRobustness = Availability to usereliminate single point failurein case of failure – fall down gracefullyeasy to recover from failure


TransparencyTransparent to the user

The user should only notice:Faster response Higher availability


ScalabilityScale well along the increasing size and density of the network.All protocols should be as lightweight as possible.


Efficiencyimpose minimal additional burden on the network (in control & data packets)do not adopt any scheme which leads to under-utilization of the network


Adaptivityadapt to dynamic changing in the user demand and network environment

achieve optimal performance


StabilityDo not introduce

instabilities into

the network


Load balancingdistribute load evenly through the entire network

no bottlenecks / hot-spots


Ability to deal with heterogeneity

Adapt to a range

of network

architecture

(hardware &

software)


SimplicityMechanism simple to deploysimpler 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

WWWCaching system architecturehow cache proxies

are organized – hierarchically, distributed or hybrid.



WWWProxy placement were to place a

cache proxy in order to optimize performance



WWWCaching contentsWhat can be

cached in the caching system



WWWProxy cooperationHow do proxies

cooperate with each other



WWWData sharingwhat kind of

data/information can be shared among among cooperative proxies



WWWCache resolution/routinghow does a proxy

decide where to fetch a page requested by a client.



WWWPrefetchingHow does a proxy

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



WWWCache placement/ replacementhow the proxy decides

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



WWWCache coherencyhow does a proxy

maintain data consistency



WWWControl information distributionhow is the control

information (e.g URL) distributed among proxies.



WWWDynamic data cachinghow to deal with

data that is not cachable


Caching architectureHierarchicalCaches are placed at multiple levels of the

network.

national

regionalinstitutional

bottom


Hierarchicalarchitecture

Bottom – clients/browsers caches.

national


bottom web page not found

web page not found

web page not found


Hierarchicalarchitecture

after web page is found

national


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 architectureDisadvantagesCache 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 architectureCaches 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.

Distributed architecture


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 architectureCaches 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 architecturesHierarchical 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.