Caching Server

In computing, a cache is a high-speed data storage layer which stores a subset of data.

Caching, or temporarily storing content from previous requests, is part of the core content delivery strategy implemented within the HTTP protocol.

Locations Where Web Content Is Cached:

  • Browser cache

    Web browsers themselves maintain a small cache. This may be user-specific content or content deemed expensive to download.

  • Intermediary caching proxies

    Any server in between the client and your infrastructure can be used to cache.

  • Reverse Cache

    Your server infrastructure can implement its own cache for backend services. This way, content won’t be retrieved by the backend servers on each request.

Caching Headers

The majority of caching behavior is determined by the caching policy, which is set by the content owner. These policies are mainly articulated through the use of specific HTTP headers.

  • Expires

    It sets a time in the future when the content will expire.

    Syntax:

    Expires: <http-date>
// Example
Expires: Wed, 21 Oct 2015 07:28:00 GMT

  • Cache-Control

    It’s a modern replacement for the Expires header. It is used for modern cache policy specification.

    Syntax:

    Cache-Control: <option>, <option>, ...
// Examples
Cache-Control: no-store
Cache-Control: public, max-age=604800

    Some of the options you can use to dictate your content’s caching policy are:

    • no-cache
    • no-store
    • public
    • private
    • max-age
    • s-maxage
    • must-revalidate
    • proxy-revalidate
    • no-transform

  • Etag

    It lets caches be more efficient and save bandwidth.

    Syntax:

    ETag: W/"<etag_value>"
ETag: "<etag_value>"
// Examples
ETag: W/"0815"
ETag: "33a64df551425fcc55e4d42a148795d9f25f89d4"

    W/: (case-sensitive, optional) indicates that a weak validator is used. Weak etags are easy to generate, but are far less useful for comparisons.

    "<etag_value>": Entity tag uniquely representing the requested resource. They are a string of ASCII characters placed between double quotes.

  • Last-Modified

    It specifies the last time that the item was modified.

    Syntax:

    Last-Modified: <day-name>, <day> <month> <year> <hour>:<minute>:<second> GMT
// Example
Last-Modified: Wed, 21 Oct 2015 07:28:00 GMT

    <day-name>: One of Mon, Tue, Wed, Thu, Fri, Sat, or Sun (case-sensitive).

    <day>: 2 digit day number, e.g. 04 or 23.

    <month>: One of Jan, Feb, Mar, Apr, May, Jun, Jul, Aug, Sep, Oct, Nov, Dec (case sensitive).

    <year>: 4 digit year number, e.g. 1990 or 2016.

    <hour>: 2 digit hour number, from 00 or 23.

    <minute>: 2 digit minute number, from 00 to 59.

    <second>: 2 digit second number, from 00 to 59.

    GMT: Greenwich Mean Time. HTTP dates are always expressed in GMT, never in local time.

  • Content-Length

    It will refuse to cache content if it does not know the size of the content it will need to reserve space for.

    Syntax:

    Content-Length: <length>

    <length>: The length in decimal number of octets.

  • Accept-Encoding

    It advertises which content encoding, usually a compression algorithm, the client is able to understand.

    Syntax:

    Accept-Encoding: <algorithm>;q=<value>
// Examples
Accept-Encoding: gzip
Accept-Encoding: gzip, compress, br
Accept-Encoding: br;q=1.0, gzip;q=0.8, *;q=0.1

    Possibles algorithms:

    • gzip: A compression format using the Lempel-Ziv coding (LZ77), with a 32-bit CRC.
    • compress: A compression format using the Lempel-Ziv-Welch (LZW) algorithm.
    • deflate: A compression format using the zlib structure, with the deflate compression algorithm.
    • br: A compression format using the Brotli algorithm.
    • identity: Indicates the identity function (i.e. no compression, nor modification). This value is always considered as acceptable, even if not present.
    • *: Matches any content encoding not already listed in the header. This is the default value if the header is not present.

    ;q=<value>: (optional) Any value is placed in an order of preference expressed using a relative quality value called weight.

  • Vary

    It can be used to tell caches to pay attention to an additional header. It’s is most commonly used to tell caches to key by the Accept-Encoding header.

    Syntax:

    Vary: *
Vary: <header-name>, <header-name>, ...
// Example
Vary: User-Agent

    *: Each request for a URL is supposed to be treated as a unique and uncacheable request.

    <header-name>: A comma-separated list of header names to take into account when deciding whether or not a cached response can be used.

Resources