Caching in HTTP
A detailed description of the caching options available through the HTTP protocol
Caching is a technique that can help network connections be faster, because the less things need to be transferred, the better.
Many resources can be very large, and be very expensive in terms of time and also actual cost (on mobile, for example) to retrieve.
There are different caching strategies that are made available by HTTP and used by browsers.
No caching
First, the Cache-Control
header can tell the browser to never use a cached version of a resource without first checking the ETag value (more on this later), by using the no-cache
value:
Cache-Control: no-cache
A more restrictive no-store
option tells the browser (and all the intermediary network devices) the not even store the resource in its cache:
Cache-Control: no-store
If Cache-Control
has the max-age
value, that’s used to determine the number of seconds this resource is valid as a cache:
Cache-Control: max-age=3600
The Expires
header
When an HTTP request is sent, the browser checks if it has a copy of that page in the cache, based on the URL you required.
If there is, it checks the page for freshness.
A page is fresh if the HTTP response Expires
header value is less than the current datetime.
The Expires header takes this form:
Expires: Sat, 01 Dec 2018 16:00:00 GMT
Conditional GET
There are different ways to perform a conditional get. All are based on using the If-*
request headers:
- using
If-Modified-Since
andLast-Modified
- using
If-None-Match
andETag
Using If-Modified-Since
and Last-Modified
The browser can send a request to the server and instead of just asking for the page, it adds an If-Modified-Since
header, based on the Last-Modified
header value it got from the currently cached page.
This tells the server to only return a response body (the page content) if the resource has been updated since that date.
Otherwise the server returns a 304 Not Modified
response.
Using If-None-Match
and ETag
The Web Server (depending on the setup, how page are served, etc) can send an ETag header.
That is the identifier of a resource. Every time the resource changes, for example it’s updated, the ETag should change as well.
It’s like a checksum.
The browser sends an If-None-Match
header that contains one (or more) ETag value.
If none match, the server returns the fresh version of the resource, otherwise a 304 Not Modified
response.
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