The caching behavior of any Web content is dictated by its caching policy, which is articulated through certain HTTP (Hypertext Transfer Protocol) headers. Expires, Etag, Last-Modified, Cache-Control, Content-Length and Vary are some of the most commonly used caching headers.

1. The Most Frequently Used
Caching Headers

2. Table of Contents
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Websites and Web Hosting
Caching and Web Caching
Advantages of Web Caching
Readiness of Content for
Caching
Caching Headers
Expires
Etag
Cache-Control
Last-Modified
Vary
Content-Length

3.  We visit numerous websites daily for various purposes but not many of us know how these are
made accessible over the Internet. So, let us delve into that first.
 A website consists of a collection of related web pages and content that exists under a single
domain name. Websites are made up of files that are stored on the servers of web hosting
service providers and delivered from there as per users’ requests. These web hosting
companies make websites accessible by providing the services and the technologies that are
needed for this purpose. Web hosting can be of many types, such as shared, dedicated, reseller,
cloud,WordPress,VPS etc.
 Usually the best web hosts in the field of hosting are referred to as the “Best Windows Hosting
Company” or as the “Linux Dedicated Hosting” or as the “Top Cloud Hosting Company”
depending on the type or types of hosting services that these excel in providing.
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Websites and Web Hosting

4.  Caching
When copies of files are stored in a temporary storage location (cache), this is known as caching.
This makes it faster to access files. A web browser caches a website’s images, HTML files and
JavaScript, which helps to load a website more speedily.
 Web Caching
Web caching entails storing temporarily Web documents (Web pages and various other types of
Web multimedia). This serves to decrease server lag. A web cache system can be a computer
program or an appliance. It stores copies of documents that pass through it. The cache fulfils
subsequent requests when certain conditions are met and ensures that the origin server doesn’t get
overloaded.
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Caching andWeb Caching

5. There are numerous benefits of web caching.These are mentioned below in no particular order.
 Enhanced page delivery speed
 Better responsiveness
 Reduction in the workload of backend server
 Protection against total outages (cached content can be delivered when servers are down)
 Availability of content regardless of network interruptions
 Better performance of the same hardware
 Reduction in network costs
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Advantages ofWeb Caching

6. 6
1-800-123 -8156
Whoa!That’s a big number, aren’t
you proud?

7. Despite it being possible for every type of content to be cached, not every content should be
cached. Based on its readiness for caching, web content can be classified into three categories. These
are mentioned below.
 Cache-friendly Content- This type of content doesn’t change frequently and is capable of
being cached for longer periods of time. Its examples are style sheets, media content, icons,
logos, images and JavaScript libraries.
 Moderately Cache-friendly Content- Such content changes regularly. It includes HTML
pages, JS and CSS that get modified frequently and all those content requests that need
authentication cookies.
 Content that shouldn’t be Cached- Due to security concerns certain types of contents
should not be cached at all. This category of content includes highly sensitive and confidential
content as well as content which is specific to users.
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Readiness of Content for Caching

8.  The caching policy that is set by the owner of a particular content dictates its caching behavior.
Specific HTTP (Hypertext Transfer Protocol) headers are used for the purpose of articulating
these caching policies. The most frequently used caching headers for cache policy specification
are mentioned in the next few slides.
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Caching Headers

9.  The “Expires” header is used to set a time in the future for a specific content with regard to its
expiration. When that time arrives and the content expires, any requests for that content will
be sent to the origin server. This header is limited in its scope and should be used as a fallback
option.
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Expires

10.  The “Etag” header is made use of with cache validation. A unique Etag for an item can be
provided by the origin at the time of serving the content initially. A cache can send a content’s
Etag when there is a requirement to validate the content. The origin will either inform the
cache regarding the content being the same or it will send the updated content along with the
new Etag.
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Etag

11.  “Cache-Control” is a modern replacement for the “Expires” header that implements a design
that is more flexible. It is well supported; hence, preferable to Expires. This header helps to set
various cache policy instructions. Multiple instructions are separated by commas in it.
 Some of the “Cache-Control” options that can be used to specify the caching policy for content
are, no-cache, public, private, no-store, max-age, s-maxage, no-transform, proxy-revalidate, must-
revalidate.
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Cache-Control

12.  The “Last-Modified” header specifies the last time that a specific item had been modified. This
can be employed as part of the validation strategy for ensuring fresh content.
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Last-Modified

13.  The “Vary” header is used to communicate to caches that there is a need to pay attention to
another header when deciding if a request is for the same item. The requested host as well as
the path to the resource are used by a cache as the key that aids in storing a cache item. The
Vary header is usually used to communicate to caches to key using the Accept-Encoding header.
This helps the cache to differentiate between uncompressed and compressed content.
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Vary

14.  Despite it not being specifically involved in caching, it is important to set the “Content-Length”
header while defining caching policies. That is due to the fact that certain software won’t cache
content unless they are aware of the size of the content, in advance, for which they will require
to reserve space.
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Content-Length

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