Web caching replication on the world wide web
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Web Caching: Replication on the World Wide Web. Jonathan Bulava CSC8530 – Distributed Systems Dr. Paul Schragger. Types of Web Caches. User Agent Cache (Browser Cache) Single user Shared Cache (Proxy Cache) Multi-user Gateway Cache Multi-user. Deployment. Browser Cache.

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Web Caching: Replication on the World Wide Web

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Web Caching:Replication on the World Wide Web

Jonathan Bulava

CSC8530 – Distributed Systems

Dr. Paul Schragger

Types of Web Caches

  • User Agent Cache (Browser Cache)

    • Single user

  • Shared Cache (Proxy Cache)

    • Multi-user

  • Gateway Cache

    • Multi-user


Browser Cache

  • Implemented on a single user’s machine.

  • Local hard drive space stores representation of viewed content.

  • Simple rules for control.

  • Usefulness

    • Recently viewed pages (Back/Forward button)

    • Commonly used images

Proxy Cache

  • Similar concept, but for multiple users.

  • Usually implemented on a firewall or separate device known as intermediaries.

  • Request routing can be manually or through an interception proxy.

  • Usefulness

    • Latency and network traffic are reduced

Gateway Cache

  • Also known as reverse proxy caches.

  • Implemented on separate devices by a website administrator.

  • Distributed gateway caches are used in Content Delivery Networks (CDN).

  • Usefulness

    • Increased scalability, reliability, and performance

4a.Get embedded documents from local cache or server (if not already cached)

3.Get embedded documents

5.Embedded documents

4b.Embedded documents

1. Get base documents

2.Document with refs to embedded documents

Content Delivery Network (CDN)







Cache producer within Akamai CDN

Controlling Cache

  • HTML Meta Tags

    • Written in <head> section of an HTML page.

    • Can mark expiration date or as uncacheable.

    • Only used by some browser caches and not seen by proxy caches.

  • HTTP Headers

    • Automatically created by Web server.

    • Sent before HTML.

    • Seen by browser and proxy caches.

HTTP Defining Mechanisms

  • Freshness – Content is able to be loaded from cache without having to check with the original server.

  • Validation – The process of confirming with the original server whether or not cached content is still valid to load.



HTTP Response Headers Example

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

Expires Header and Freshness

Expires: Fri, 30 Oct 1998 14:19:41 GMT

  • Indicates how long the representation is fresh. After this time passes, the cache will communicate with the original server to see if there have been any changes.

  • Beneficial for static page images as well as continually changing content.

  • Web server and cache must be synchronized.

Cache-Control Header and Freshness

Cache-Control: max-age=3600, must-revalidate

  • Gives control beyond Expires header.

Cache control response-headers:








Last-Modified and Validation

Last-Modified: Mon, 29 Jun 1998 02:28:12 GMT

  • Validate cache by looking at the last time the document was altered (Last-Modified).

  • If-Modified-Since request is sent to the original server. If changes have been made since the date given, the entire document is returned. Otherwise, the cached document can be loaded.

ETag and Validation

  • Unique identifiers created by server.

  • Changed each time the representation is altered on the original server.

  • If-None-Match request is sent to server and a simple comparison is used to validate the content.

ETag: "3e86-410-3596fbbc"


  • Nottingham, Mark. “Caching Tutorial: for Web Authors and Webmasters.” July 2006. <http://www.mnot.net/cache_docs/>.

  • Tanenbaum, Andrew S. Distributed Systems: Principles and Paradigms. New Jersey: Prentice-Hall, 2002.

  • “Web cache.” Wikipedia. <http://en.wikipedia.org/wiki/Web_cache>.

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