Skip to content

Instantly share code, notes, and snippets.

@rorcraft

rorcraft/HTTP_Caching.md

Last active December 18, 2015 09:19
Show Gist options
  • Star 0 You must be signed in to star a gist
  • Fork 0 You must be signed in to fork a gist
  • Save rorcraft/5760936 to your computer and use it in GitHub Desktop.
Save rorcraft/5760936 to your computer and use it in GitHub Desktop.
Download ZIP
Raw
HTTP_Caching.md

HTTP Caching

  1. Cache by expiration time.
  2. Conditional Get by versioning.

Cache by expiration time.

Tells the client and intermediary cache what to cache.

HTTP 1.0:

Expires: Thu, 01 Dec 1994 16:00:00 GMT

Gives the date/time after which the response is considered stale. Always returns an absolute date.

HTTP 1.1:

Cache-Control: max-age=3600

Uses relative times in seconds, cf (s)max-age

    Cache-Control   = "Cache-Control" ":" 1#cache-directive
    cache-directive = cache-request-directive
         | cache-response-directive
    cache-request-directive =
           "no-cache"                          ; Section 14.9.1
         | "no-store"                          ; Section 14.9.2
         | "max-age" "=" delta-seconds         ; Section 14.9.3, 14.9.4
         | "max-stale" [ "=" delta-seconds ]   ; Section 14.9.3
         | "min-fresh" "=" delta-seconds       ; Section 14.9.3
         | "no-transform"                      ; Section 14.9.5
         | "only-if-cached"                    ; Section 14.9.4
         | cache-extension                     ; Section 14.9.6
     cache-response-directive =
           "public"                               ; Section 14.9.1
         | "private" [ "=" <"> 1#field-name <"> ] ; Section 14.9.1
         | "no-cache" [ "=" <"> 1#field-name <"> ]; Section 14.9.1
         | "no-store"                             ; Section 14.9.2
         | "no-transform"                         ; Section 14.9.5
         | "must-revalidate"                      ; Section 14.9.4
         | "proxy-revalidate"                     ; Section 14.9.4
         | "max-age" "=" delta-seconds            ; Section 14.9.3
         | "s-maxage" "=" delta-seconds           ; Section 14.9.3
         | cache-extension                        ; Section 14.9.6
    cache-extension = token [ "=" ( token | quoted-string )

Request a fresh copy.

Cache-Control: no-cache

Pragma: no-cache The Pragma request header is a legacy header and should no longer be used.

max-age: Specifies the period in seconds during which the cache will be considered fresh. On a request - how long has it already been cached. On a response, how long it can be cached for in the future.

no-cache VS max-age=0

  • no-cache: force reload
  • max-age=0: force revalidation. Caches can avoid redownloading data, but they’re not sure if it’s still fresh. See Conditional Get.

Expires VS Cache-Control

When both Cache-Control and Expires are present, Cache-Control takes precedence. If a response includes both an Expires header and a max-age directive, the max-age directive overrides the Expires header, even if the Expires header is more restrictive

Response specific:

  • public, private whether it is cacheable on intermediary proxies
  • must-revalidate, proxy-revalidate Tells client to revalidate even though they can cache.
  • no-store Not cacheable.

Conditional Get by versioning.

  • timestamp Last-Modified (HTTP 1.0)
  • checksum ETag (HTTP 1.1)

Usually use both Last-Modified and ETag together.

If-Modified-Since -> Last-Modified

  • Client stores the last modified at timestamp.
# Request
If-Modified-Since: Wed, 01 Sep 2004 13:24:52 GMT

# Response
# If not modified since the time specified, no data is returned.
HTTP/1.1 304 Not Modified
Last-Modified: Wed, 01 Sep 2004 13:24:52 GMT

# If modified since the time specified, no data is returned.
HTTP/1.1 200 OK
Last-Modified: Wed, 01 Sep 2004 13:24:52 GMT
(data)

Caveats with only using Last-Modified.

ETag -> If-None-Match

  • Clients stores the ETag
  • ETag usually hash of important information on data that might represent change.
# Request
If-None-Match: "1edec-3e3073913b100"

# Response
# If not modified since the time specified, no data is returned.
HTTP/1.1 304 Not Modified
ETag: "1edec-3e3073913b100"

# If modified since the time specified, no data is returned.
HTTP/1.1 200 OK
ETag: "abcdef-3e3073913b100"
(data)

Example

Time@0s - Request from Client with empty cache

GET / HTTP/1.1
Host: localhost

# Server Response
HTTP/1.1 200 OK
Cache-Control: max-age=10
ETag: "1edec-3e3073913b100"
Last-Modified: Wed, 01 Sep 2004 13:24:52 GMT
(other headers .e.g content-length, content-type)
(data)
  • Client caches - content, etag, last-modified - expires in: 10 seconds.

Time@3s - Client can use stored cache, makes no request.

Time@10s - Client sends ETag and Last-Modified to revalidate.

GET / HTTP/1.1
Host: localhost
Cache-Control: max-age=0
If-None-Match: "1edec-3e3073913b100"
Last-Modified-Since: Wed, 01 Sep 2004 13:24:52 GMT
  • Server load data, if ETag == "1edec-3e3073913b100" && updated_at <= Wed, 01 Sep 2004 13:24:52 GMT
HTTP/1.1 304 Not Modified
Cache-Control: max-age=10
ETag: "1edec-3e3073913b100"
Last-Modified: Wed, 01 Sep 2004 13:24:52 GMT

Reference:

Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment