The Most Frequently Used Caching Headers

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The Most Frequently Used Caching Headers
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Table of Contents

• 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

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Websites and Web Hosting

• 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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Caching and Web Caching

• 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 websites 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 doesnt
  get overloaded.

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Advantages of Web Caching

• 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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1-800-123 -8156

• Whoa! Thats a big number, arent you
  proud?

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Readiness of Content for Caching

• 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
  doesnt 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 shouldnt 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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Caching Headers

• 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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Expires

• 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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Etag

• 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 contents
  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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Cache-Control

• 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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Last-Modified

• 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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Vary

• 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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Content-Length

• 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 wont 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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