Chapter 6

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Web Server Performance Where to Cache

‘‘Tired of having to make coffee while you wait for ahome page to download?’’.... how the proper integration of several newtechnologies can make page downloads 20%-400%faster and reduce Web-generated Internet traffic by asmuch as 50%. Soon, you may be down to one cup a day!

W3C Recommendations Reduce ‘‘World Wide Wait’’

n this chapter, we cover web caching.

Table Of Contents

Proxy Caching Introduction Where to Cache Controlling the Cache Cache Replacement Algorithms Cache Hierarchies

Proxy Caching

Battling the "World Wide Wait"

Introduction

Definition of Proxy Caching

"Internet Object Caching is a way to store requested Internet Objects (i.e., data availablethrough http, ftp and gopher protocols) on a system closer to the requesting site than thesource. Web browsers can then use the local cache as a proxy HTTP server, reducingaccess time as well as reducing bandwidth consumption." http://squid-cache.org/Doc/FAQ/FAQ-1.html

Proxy servers work by intercepting requests for documents or files and then seeing ifthey have a local copy of that particular object. If a current one exists, then thatdocument is returned to the client. If that document does not exist in the cache, or it isdeemed that the document held in the cache is no longer current, then a new copy of thefile is obtained via the web. This object is then forwarded onto the client, and a copy iskept locally so that the next computer to request the same object can obtain thedocument more swiftly. Caching can take place at the browser, or at the server.

The diagram above shows the relationship between a client (ie a web browser), and a cache. The cacheintercepts the web requests and either returns something it already has stored, or passes the request ontothe Location in the original request.

The technology used within a cache is very similar to that used in any web server,however there are some subtle differences. Proxy Caches do not work with the sameefficiency as a Web Server, and all requests to a cache must be made with a completeURL (rather than a relative path, which is fine on a web browser).

Why?

Security

Beyond the obvious savings in download time and bandwidth costs, Proxy Servers canalso provide a valuable service as part of the security policy of a company. For instance,networks can be configured so that the only device that can make HTTP requests is theproxy server. All computers must make their http requests through the proxy server.This would serve to

1. Reduce the risk of attack to individual PC’s and the network as a whole becauseonly one single machine is making external requests, and this is far simpler andmore robust to administer

2. Allow filtering of sites that users of the proxy can access - for example requestsfor documents that are not cached might be rejected.

The clients (browsers) in the above diagram are behind a firewall - only the cache can make requeststhrough the firewall, the individual clients do not have this privilege.

Speed

There are a number of factors at play in reducing the efficiency at which your browser isable to retrieve a web page. Factors such as DNS lookups for URLs, slow responsetimes of Web servers, the size of the object desired for retrieval, and general networkcongestion all lead to the network leg of any data transfer being the slowest of all. Also,Web Servers that can only run on HTTP/1.0 will always behave more slowly than thosethat run on HTTP/1.1. The notion of sidestepping all of these issues should then haveimmediate appeal. Keeping a copy of the objects that you want closer to the client willavoid many of these pitfalls.

Bandwidth

As well as decreasing response times, many organizations pay for their Internetconnection based on data volume rather than length of connection. Objects retrievedfrom within the organization, rather than the wild, will ultimately save the organizationmoney, and increase the efficiency with which the organizations conduct their onlineaffairs.

Checkpoint Questions

1. What are the three main benefits of using a cache?

Web Server Performance Where to Cache

C0SC1300 - Lecture NotesWeb Servers and Web Technology

Copyright © 2000 RMIT Computer ScienceAll Rights Reserved

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Introduction Controlling How a web page is cached

n this section, we cover a complete study ofcaching and related issues.

Table Of Contents


Where to cache?

There are two approaches to caching - browser caching and server caching (or proxycaching). Both use the same approach of intercepting requests for Internet objects, andchecking to see if they have a valid local copy already stored.

Browser Caching

Browsers can be configured to keep local copies of the files you browse on your ownhard disk. They use simple algorithms and allow minor configuration options.

Internet Explorer allows you to specify when the caching is done, but provides nocontrol over whether caching is done using Disk Cache or Memory Cache.

IE Settings

IE Settings

Cache configuration panel from Internet Explorer 5

Internet Explorer allows the users to choose the cache level, among the followingavailable options.

Every Time You Start IE - If you log on to the Internet and access a page you’vepreviously visited, your browser will check only once during that session to see if thatpage has been updated. At all other times, it will take the page from cache. For mosttypes of Web activity, this setting should be sufficient, and this setting is recommended.

Every Visit to the Page - Every time you access a web page, the browser will check tosee if it has been changed. If the last modified date of the page is older than the date ofthe cached copy, it retrieves the page from the cache, and if it has been modifiedrecently, it retrieves the page directly from its source. This setting is harmless butunnecessary.

Never - Your browser will never check if a page has been updated and will always use acached page. This setting is not recommended.

Netscape Settings

Netscape Settings

The Cache configuration panel from Netscape 4.08

Netscape provides a further element in that it allows you to determine how much RAMand disk space are available for use as a cache. Storing objects in memory for retrievalwill enable better performance than storing them on disk. As always with these things,there is a limit to what can be achieved due to system constraints. This is as valid on aserver as it is on browser.

The main benefit of caching locally over caching on a server is that local caching willeliminate any network hops.

Server Caching / Proxy Server

Server caching follows the same scheme - saving files that have been requested and thenintercepting future web requests to see if that particular request has been stored locally.However Server Caches can provide a much higher level of service for a number ofreasons. The most relevant reason is that they are dealing with requests from many moreclients and therefore will have a much larger and greater variety of web objects tosupply to their clients. In addition, server caches have more room with which objectscan be stored, so, the chances of your request for a web page already being in existencein the cache is improved markedly. Also, server caches are likely to be installed oncomputers that are tuned for the best performance in running a cache. Hard disk spaceand the amount of information held in memory at any one time are configured tooptimise the perfomance of the cache.

A further benefit is that server caches can be part of what are known as Cache Farms -which essentially is a network of server caches working together in order to capture thegreatest variety and quantity of objects possible. Companies like Bigpond offer theservices of their Web farms at a price. Cache farms are discussed in more detail later.


1. What benefits does browser caching provide? 2. What benefits does networked caching provide?

Introduction Controlling How a web page is cached



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Where to Cache Cache Replacement Algoritms


Table Of Contents


Controlling how a Web Page is cached

As authors of web pages, we sometimes want to control how long our pages are currentfor, and sometimes whether they are cached at all.

In some cases, the HTTP header information on the web page would indicate whether itis valid or not. For instance,HTTP/1.0 and HTTP/1.1 headers contain EXPIRESinformation that caching systems will use to determine the ’freshness’ of a document(ref Web Servers Section Hypertext Transfer Protocol). The administrator of the cachewill have little control over these type of rules. By default, some pages are not cached atall. Authenticated pages are typically not cached, nor are objects requested via theSecure Sockets Layer (SSL).

Meta tags have also been used by page designers in order to control how their pages arecached, however this approach is somewhat flawed as proxy caches rarely parse theHTML contents of the document, and only look at the header information, so a pagedesigner who wants to manage when and how a page is cached should use HTTPHeaders rather than HTML meta tags.

HTTP 1.0 was somewhat limited in the amount of control it gave to control caching,however HTTP/1.1 introduced a new set of Cache Control Response Headers whichallow the page designer much greater control. Some elements of control are listedbelow.

max-age=[seconds] The amount of time an object will be considered freshPublic Forces a page to be cachable no-cache The item will not be cachedmust-revalidate Must obey any ’freshness’ directives

Your browser settings will also impact on whether the document is refreshed or not, andquite commonly you may find yourself specifically telling the browser to get a freshcopy of the document from the server.

HTTP/1.1 200 OK Date: Fri, 30 Oct 1998 13:19:41 GMT Server: Apache/1.3.3 (Unix) Cache-Control: max-age=3600, must-revalidate Expires: Fri, 30 Oct 1998 14:19:41 GMT Last-Modified: Mon, 29 Jun 1998 02:28:12 GMT ETag: "3e86-410-3596fbbc" Content-Length: 1040 Content-Type: text/html

Example of HTML Header information for a web page that contains caching instructions.

When is a page no longer valid?

In an ideal world, a cache could keep copies of every document that is likely to berequested more than once. However this is an unlikely scenario, as hard disk spaceconstraints will eventually come into play. A cache is also limited in the quantity ofinformation that it can keep; it can’t just keep on collecting local copies of pagesforever. How do we determine what to keep and how long to keep it for?

The caching system enforce one of the following policies to determine the pages to beremoved from the cache, in the event the cache is full.


1. Why do we need to control how our web pages are cached, and how do we as webdevelopers manage it?

Where to Cache Cache Replacement Algorithms



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Controlling the Cache Cache Farms and Cache Hierarchies

n this section, we cover caching replacementalgorithms.

Table Of Contents


Not Recently Used Caching

A very simple caching algorithm is to mark data in four classes as:

(1) Not read, not written(2) Not read, written(3) Read, not written(4) Read and written

Periodically, the ‘‘read’’ mark is reset (say, every few clock cycles).

When the cache is full, data in the lowest-numbered, non-empty class is evicted.

The premise is that modified, unread data should be removed in preference to read,unmodified data. This works on the assumption that data is read more often than it iswritten, which is certainly true in web caching.

The attraction is simplicity, but the disadvantage is that it is certainly not optimal.

NRU caching is infrequently used in practice.

First-In First-Out (FIFO)

The idea is to remove the oldest data, that is, the first item added to the cache is the firstremoved.

Does not typically work well, since age has little to do with frequency of use; the firstitem added to the cache is frequently the most popular. The is often true with web pages

and web traffic.

FIFO is not used in an unmodified form in practical caching.

Second-Chance

Second-chance is just like FIFO, but data is ‘‘marked’’ or flagged each time it is used,and the data is managed by a queue.

When evicting, we inspect the oldest data that was added to the cache:

if the data is unmarked, we evict it if it marked, we make it ‘‘newest data in’’ and ‘‘unmark it’’, and move on to thenext-oldest item

The basic principle is to look for old data that has not been referenced for a period oftime.

Second-chance is inefficient for the reason that we need to maintain queues of data byage, and reorganise this queue frequently. Another approach is to arrange data in acircular queue, and to move a ‘‘hand’’ around the queue. The hand points to the oldestpage.

If the hand points to an unmarked item, it is evicted. If it points to a marked item, it isunmarked and the hand moves on.

The only difference to second-chance is the implementation.

Least-Recently Used (LRU)

A good observation about web data is that data that is accessed frequently is likely to beaccessed again in the near future. It is also true that data that has not been used for along period is unlikely to be used in the near future.

The idea of LRU is to evict the data that has been unused for the longest time. This has acost: we must maintain information about all data stored in the cache.

One approach is to maintain a central counter that is incremented each time data isaccessed. As the data is accessed, the current value is stored with the data. The data withthe lowest counter is the oldest and will evicted.


1. Use with the FIFO page, and determine the state of the cache after the followingvalues are accessed 0 1 2 3 4 5 6 3 4 5 6 2 3 4.

2. Use with the LRU page, and determine the state of the cache after the followingvalues are accessed 0 1 2 3 4 5 6 3 4 5 6 2 3 4.

3. Use with the 2nd page, and determine the state of the cache after the followingvalues are accessed 0 1 2 3 4 5 6 3 4 5 6 2 3 4.

Links

1. FIFO page 2. LRU page 3. Second Chance page

Controlling the Cache Cache Farms and Cache Hierarchies



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Cache Replacement Algorithms


Table Of Contents


Cache Farms and Cache Hierarchies

As mentioned briefly earlier, one of the great benefits of server caching is that it can becombined with other caches to provide a much wider variety and much larger quantityof cached objects available for faster retrieval through increasing the user basesupplying the requests to the proxy. Another benefit of this approach is that if theproxies are intelligent and capable of communicating with other proxies, then it ispossible to manage the duplication of data, or even to balance the load placed on theproxy by sharing resources between different proxies.

A cache hierarchy describes a kind of pyramid in which there may be many cacheslower down the pyramid and fewer caches near the top. The benefits of this approachmean that network traffic diminishes as you head down the hierarchy, but there are costsinvolved in terms of replication of disk storage, as each successive pass through eachlevel of cache will be cached there as well.

This situation can be improved if we increase the intelligence and cooperation of ournetwork of caches - in this case, the paradigm is moves away from a pyramid where theinformation flows one way, to a more parallel scheme, with many caches on similarlevels and with a greater intelligence available in the extent of communication betweenthem. Harvard University developed a communications protocol called ICP (InternetCache Protocol) which uses UDP to communicate between caches. Requests forinformation are passed between caches using UDP, and when a positive result isreceived a complete request is made using HTTP. The net result of this more informeddiscussion is that redundancy of stored objects is decreased, and load sharing betweendifferent components of the cache farm is much more viable. When the requesting cachereceives a positive response via ICP, it makes a formal request for the document fromthe cache that made the positive reponse, otherwise, it goes out to the Internet to requestthe document from the original location.


1. What benefits does a cache farm have over a cache hierarchy?

Cache Replacement Algorithms



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Chapter 6