web caching1 by amisha thakkar. web caching2 overview what is a web cache ? caching terminology why...

Web Caching 1

Web Caching

By

Amisha Thakkar

Web Caching 2

Overview

• What is a Web Cache ?

• Caching Terminology

• Why use a cache?

• Disadvantages of Web Cache

• Other Features

• Caching Rules

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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 ISP’s often set

them up on the firewalls

* They are type of shared caches

• Browser caches

* Use a section of the computer’s 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

won’t be cached

* Object’s 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

National Network National Network

Regional NetworkRegional Network

Institutional NetworkInstitutional Network Institutional Network Institutional Network

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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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• 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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• 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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More the users more disk space higher in the hierarchy

* Exponential growth of number of documents on WWW

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• 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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• 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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* 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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• 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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• 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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• Transparent Proxy caching

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• Advantages

No browser configuration

Cost of upgrading h/w & s/w is limited

No administration of intermediate systems required

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• 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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• Transparent proxy caching with web cache redirection.

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• 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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• 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 system:CacheOS

• 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

web caching1 by amisha thakkar. web caching2 overview what is a web cache ? caching terminology why...

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