Web Caching

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Web Caching

• By
• Amisha Thakkar

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Overview

• What is a Web Cache ?
• Caching Terminology
• Why use a cache?
• Disadvantages of Web Cache
• Other Features
• Caching Rules

3
Overview

• Caching Architectures
• Cache Deployment Scheme
• Active Caching
• Real World Solution
• Research Areas

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What is a Web Cache ?

• Cache is a place where temporary copies of
  objects are stored
• Cached information is generally closer to the
  requester than the permanent information is
• Objects -HTML pages, images, files

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What is a Web Cache?
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Caching Terminology

• Client - An application program that establishes
  connections for sending requests
• Server- An application program that accepts
  connection to service requests by sending back
  responses
• Origin Server-The server on which the given
  resource resides or is to be created

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Caching Terminology

• Proxy- An intermediary program which acts both as
  a server and a client which requests on behalf
  of the other clients
• Proxy is not necessarily a cache
• Proxy does not always cache the replies
  passing through it
• It may be used on a firewall to monitor
  accesses

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Why use a cache ?

• To reduce latency
• To reduce network traffic
• Load on origin servers will be reduced
• Can isolate end users from network failures

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Disadvantages of Web cache

• With cached data there is always a chance of
  receiving stale information
• Content providers lose access counts when cache
  hits are served
• Manual configuration is often required
• Operation of cache requires additional resources
• In some situations the cache can be a single
  point of failure

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Other Features

• Depending on the perspective the following may be
  good or bad
• Cache requests on behalf of clients the
  servers never see the clients IP addresses
• Cache provides an easy opportunity to
  monitor and analyze browsing activities
• Cache can be used to block certain requests

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Types of Web Caches

• Proxy caches
• Serve a large number of users
• Large corporations and ISPs often set
  
• them up on the firewalls
• They are type of shared caches
• Browser caches
• Use a section of the computers hard disk
• to store objects that you have seen

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Caching Rules

• Rules on which caches work -
• Some of them set in protocols
• Some are set by cache administrator
• Most common rules
• If the object is authenticated or secure it
• wont be cached
• Objects headers indicate whether the
• object is cacheable or not

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Caching Rules

• Object is considered fresh when -
• ? It has an expiry time or other age
• controlling directive set is still
  
• within the fresh period
• ? If the browser cache has already seen
• the object has been set to check
• once a session

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Caching Rules

• ? If a proxy cache has seen the object
• recently it was modified relatively
• long ago
• Fresh documents are served directly from the
• cache without checking with the origin server

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Caching Rules

• For a stale object , the origin server will
• be asked to validate the object , or tell
  the
• cache whether the copy is still good
• The most common validator is the time
• that the object was last changed

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Caching Architectures Hierarchical /Simple Cache

• Browser-cache interaction is same as browser
  -host interaction, i.e. a TCP connection is made
  item requested
• If not found send request to parent cache
• Hierarchy built up - each level serving
  indirectly a wider community of users

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Caching Architectures Hierarchical /Simple Cache
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Caching Architectures Distributed /Co-operating
Cache

• Decentralized(Cache Mesh)
• Multiple servers cooperate in such a way that
  they share their individual caches to create a
  large distributed one
• Simply put caching proxies communicating with
  each other to serve different users
• On a cache miss, it checks with other proxy
  caches before contacting the origin server

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Caching Architectures Distributed /Co-operating
Cache

• Caches communicate amongst themselves using a
  protocol like ICP (Internet Cache Protocol)
• Caches can be selected on the basis of
• Distances from the end user
• Specialize in particular URLs(location hint).

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Caching Architectures Distributed /Co-operating
Cache

• Why Distributed - limitations of hierarchy
• Width of cache in hierarchy caches at same
  level are inaccessible to each other
• LRU policy implies sufficient disk space
• Cost in replication of disk storage
• Amount of disk space reqd. depends on number
  of users served breadth of reading

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Caching Architectures Distributed /Co-operating
Cache

• More the users ? more disk space higher in the
  hierarchy
• Exponential growth of number of documents on
  WWW

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Caching Architectures Distributed /Co-operating
Cache

• Caching close to user - more effective, higher
  the level lower the efficiency
• Can be created for load balancing
• Most effective when serving a community of
  interests

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Caching Architectures Distributed /Co-operating
Cache

• First an UDP packet sent for cache inquiry.
• Cache selection decision is determined by RTT
• Potential problem -network congestion because of
  UDP
• In favor-
• UDP exchange 2 IP packets, TCP at least 8
  packets

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Caching Architectures Distributed /Co-operating
Cache

• UDP reply from cache can indicate
• a. Presence
• b. Speed
• c. Availability of requested documents

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Caching Architectures Hybrid Cache

• Note ICP

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Cache Deployment Schemes

• Proxy caching

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Cache Deployment Schemes

• Advantages
• ? Clients point all web requests directly to
  cache no effect on non web traffic
• ?Cost of upgrading h/w s/w is limited
• ? Administration on caches limited to basic
  configuration

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Cache Deployment Schemes

• Disadvantages
• ?Every browser must be configured to point to
  the cache
• ?Each client can hit only one cache
• ?Single point of failure
• ? Unnecessary duplication of data
• ? Bottleneck in cases where content is otherwise
  available in LAN

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Cache Deployment Schemes

• Transparent Proxy caching

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Cache Deployment Schemes

• Advantages
• ?No browser configuration
• ?Cost of upgrading h/w s/w is limited
• ?No administration of intermediate systems
  required

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Cache Deployment Schemes

• Disadvantages
• ? Each client can hit only one cache
• ?If cache goes down internet as well as
  intranet access lost
• ? Negative impact on non web traffic
• ? Cache has to route non web traffic
• ? Routing ,packet examination n/w addr.
  translation steal CPU cycles from the main cache
  serving function

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Cache Deployment Schemes

• Transparent proxy caching with web cache
  redirection.

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Cache Deployment Schemes

• Advantages
• ?Switch/ router examines the packets
• ?Minimal impact on non-web traffic
• ?Frees up CPU cycles for the web cache
• ? Allows client load to be dynamically spread
  over multiple caches
• ? Eliminates single point of failure especially
  if redundant redirectors are used

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Cache Deployment Schemes

• Disadvantages
• ?Additional intermediate systems must be
  deployed
• ? Increases expense

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Active Caching

• Current problem unable to cache dynamic documents
• Cache applet is server supplied code that is
  attached with an URL , or collection of URLs
• Applet is written in platform independent language

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Active Caching

• On a user request the applet is invoked by the
  cache
• The applet decides what is to be sent to the user
  -
• Giving the proxy a new document to send back to
  the user
• Allowing the proxy to use the cached copy
• Instructing the proxy to send the request to
  the web server

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Active Caching

• Functions of the applet-
• Logging user accesses
• Checking access permissions
• Client-Specific Information Distribution

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Active Caching

• The proxy has the freedom to not invoke the
  applet but send the request to the server
• Proxy promises to not send back a cached copy
  without invoking the applet
• If applet too huge ,send request to server
• Proxy not obligated to cache any applet , in that
  case agrees to not service the request for that
  document

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Active Caching

• Proxy can devote resources to the applets
  associated with the hottest URLs to its user
• Proxy that receives the request is typically the
  proxy closest to the user , the scheme
  automatically migrates the server processing to
  the nodes that are close to users
• Thus increasing the scalability of web based
  services

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Real World Solution

• CacheFlow has successfully implemented caching
  solutions for e-commerce
• Provide client-side server-side solution
• On the client-side the cache is placed between
  the network the firewall i.e. in front of the
  firewall the web server
• Request for dynamic content or secure
  transactions are passed to origin servers for
  processing

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Real World Solution

• This offers several advantages-
• Offloads load from servers firewalls
• Scale the network to handle more customer
  transactions large traffic spikes
• Reduce capital operating costs
• Reduces the security risks of users accessing
  servers that are inside the firewalls

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Real World Solution

• They have developed an operating systemCacheOS
• Main features related to caching Adaptive
  Asynchronous Refresh , Object Pipelining
• Variables tracked for AAR
• Frequency of request (model of use)
• Frequency of change (model of change)
• Time cost to retrieve object

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Real World Solution

• CacheOS then automatically determines refresh
  pattern
• 90 hit rate
• Some facts As many as 90 or more web objects
  can be static
• 8 sec threshold

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Real World Solution

• Successful Implementations Proflowers.com
• Kbkids.com
• delta-air.com
• Xerox

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Research Areas

• How are the cache proxies organized,
  hierarchically, distributed, or hybrid?-cache
  architectures
• Where to place a cache proxy in order to achieve
  optimal performance?proxy placement

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Research Areas

• How do proxies cooperate with each other?-proxy
  co-operation
• What kind of data/information can be shared among
  co-operating proxies?-data sharing
• How does a proxy decide what and when to prefetch
  from Web server or other proxies to reduce access
  latency in the future?-prefetching

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Research Areas

• How does a proxy manage pages?-cache placement
  and replacement
• How does a proxy maintain data consistency?-cache
  coherency
• How is the control information distributed among
  pages?-control information Distribution
• How to deal with data which is not
  cacheable?-dynamic data caching