Web Caching

Web Caching 1

Web Caching

By

Amisha Thakkar

Alpa Shah

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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

• Comparison of Architectures

• Cache Deployment Scheme

• Client Side Cache Cooperation

• Active Caching

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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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Comparison of Architectures

• Hierarchical : caches placed at multiple levels

• Distributed :caches only at bottom level; no intermediate caches

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• Performance parameters.

Connection time (Tc)is defined as the time since the document is requested & first data byte is received

Transmission time (Tt)is defined as the time taken to transmit the document

Total latency = Tc +Tt .

Bandwidth usage

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• Fig 3 -Connection time for different document’s popularity

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• For unpopular documents high connection time

• No of requests increases avg.. connection time decreases

• For extremely popular documents distributed has smaller connection times

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• Fig 4 Network traffic generated

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• On lower levels, distributed caching practically double the network bandwidth usage

• Around the root node in national network, the network traffic is reduced to half

• Distributed caching uses all possible network shortcuts between institutional caches, generating more traffic in the less congested low network levels

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• Fig 5 a, Not congested national network

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• The only bottleneck on the path from the client to the origin server is the international path. Hence transmission times are similar for both

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• Fig 5 b Congested National Networks

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• Both have higher transmission times compared to the previous case

• Distributed caching gives shorter transmission times than hierarchical because many requests travel through lower network levels

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• Fig 6 Average total latency

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• For large documents transmission time is more relevant than connection times

• Hierarchical caching gives lower latencies for documents smaller 200 KB due to lower connection times

• Distributed caching gives lower latencies for larger documents due to lower transmission times

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• The size- threshold depends on the degree of congestion in national network

• Higher the congestion, lower is the size- threshold

• Distributed caching has lower latencies than hierarchical

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Comparison of ArchitecturesWith Hybrid Scheme

• Fig 7 connection time

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• Fig 8.

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• In the hybrid scheme if the number of cooperating caches (kc) is very small , the connection time is high

• When number of cooperating caches increases, the connection times decreases up to a minimum

• If the number increases over the threshold , the connection time increases very fast

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• Fig 9 Transmission time

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• For un-congested n/w the no.of coop caches (kt) at every level hardly influences Tt

• If no. of coop caches is very small , high Tt & vice -versa

• If the no increases above the threshold the Tt increases

• Optimum no. of caches depends on the no of caches reachable avoiding congested links

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• Fig 10

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• Fig 11 total latency

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• The no. of coop caches(kopt) at every level depend on the document size to minimize the total latency

• For small documents the optimum no. is closer to kc

• For large documents the the optimum no. is closer to kt

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• Fig 12

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• For any document the optimum kopt that minimizes the total latency is such that kc koptkt

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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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Client Side Cache Cooperation.

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Active Caching

• Current problem unable to cache dynamic documents

• Caching Dynamic contents on the web using active web

• 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

• Other functions of the applet-

* Logging user accesses

* Checking access permissions

* Rotating advertising banners

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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

Documents

Web Caching