A Survey of Web Caching Schemes for the Internet

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A Survey of Web Caching Schemes for the Internet

• Jia Wang

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Agenda

• The World Wide Web
• Problem and solution (caching)
• Proxy servers
• Advantages of web caching
• Disadvantages of web caching
• Elements of A WWW caching system
• Desirable properties of WWW caching system
• Problems in designing caching systems for the WWW
• Caching architecture

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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.

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SIZE OF DISTINCT STATIC WEB PAGES
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The World Wide Web

• Usage is relatively inexpensive
• Accessing information is very fast
• Documents appeal to a wide range of interests
• But..

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The World Wide Web

• Network congestion
• Server overloading

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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.

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Solution

• Caching
• Placing popular objects at locations close to
  the clients.

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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.

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proxy servers

• Belonging to same organization, clients share
  common interests.
• They probably access the same set of documents.

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thus

• On the proxy server, a previously requested and
  cached documents would likely result in future
  hits.

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proxy severs

• Caching most popular web pages on the proxy
  server can
• Save network bandwidth
• Lower access latency for the client

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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

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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.

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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.

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Elements of A WWW caching system

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

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Elements of a WWW caching system
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Desirable properties of WWW caching system

• fast access
• robustness
• transparency
• scalability
• efficiency
• adaptivity

• stability
• load balance
• ability to deal with heterogeneity
• simplicity

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Fast access

• Reduce web access latency to a minimum.
• Especially comparing to other servers not using
  caching techniques.

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Robustness

• Robustness Availability to user
• eliminate single point failure
• in case of failure fall down gracefully
• easy to recover from failure

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Transparency

• Transparent to the user
• The user should only notice
• Faster response
• Higher availability

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Scalability

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

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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

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Adaptivity

• adapt to dynamic changing in the user demand and
  network environment
• achieve optimal performance

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Stability

• Do not introduce instabilities into the network

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Load balancing

• distribute load evenly through the entire network
• no bottlenecks / hot-spots

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Ability to deal with heterogeneity

• Adapt to a range of network architecture
  (hardware software)

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Simplicity

• Mechanism simple to deploy
• simpler schemes are easier to implement and
  likely to be accepted as international standards

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

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Problems in designing caching systems for the WWW

• Caching system architecture
• how cache proxies are organized hierarchically,
  distributed or hybrid.

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Problems in designing caching systems for the WWW

• Proxy placement
• were to place a cache proxy in order to optimize
  performance

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Problems in designing caching systems for the WWW

• Caching contents
• What can be cached in the caching system

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Problems in designing caching systems for the WWW

• Proxy cooperation
• How do proxies cooperate with each other

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Problems in designing caching systems for the WWW

• Data sharing
• what kind of data/information can be shared among
  among cooperative proxies

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Problems in designing caching systems for the WWW

• Cache resolution/routing
• how does a proxy decide where to fetch a page
  requested by a client.

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Problems in designing caching systems for the WWW

• Prefetching
• How does a proxy decide what and when to prefetch
  from webservers or other proxies to reduce access
  latency.

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Problems in designing caching systems for the WWW

• Cache placement/ replacement
• how the proxy decides which page to be stored in
  its cache and which page to be removed from it.

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Problems in designing caching systems for the WWW

• Cache coherency
• how does a proxy maintain data consistency

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Problems in designing caching systems for the WWW

• Control information distribution
• how is the control information (e.g URL)
  distributed among proxies.

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Problems in designing caching systems for the WWW

• Dynamic data caching
• how to deal with data that is not cachable

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Caching architecture

• Hierarchical
• Caches are placed at multiple levels of the
  network.

national
regional
institutional
bottom
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Hierarchicalarchitecture

• Bottom clients/browsers caches.

web page not found
national
regional
web page not found
institutional
web page not found
bottom
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Hierarchicalarchitecture

• after web page is found

forward page, leave copy
national
regional
forward page, leave copy
institutional
forward page, leave copy
bottom
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Hierarchical architecture

• Advantages
• Bandwidth efficient especially when cache
  servers are slow.
• Allows to efficiently diffuse popular web pages
  towards the demand.

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Hierarchical architecture

• Disadvantages
• Cache 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.

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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.

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Distributed architecture

• 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.

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Distributed architecture

• Disadvantages
• High connection times
• Higher bandwidth usage
• Administrative issues.

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Distributed architecture

• Examples
• ICP Internet Cache Protocol (Harvest group)
• Retrieve data from neighboring caches parent
  caches
• CARP Cache Array Routing Protocol
• URL space divided to an array of caches.
• Each cache stores only documents whose URL are
  hashed to it.

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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.

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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.