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My problem is: sometimes browser over-cached some resources even if i've already modified them. But After F5, everything is fine.
I studied this case whole afternoon. Now i completely understood the point of "Last-Modified" or "Cache-Control". And i know how to solve my issue (just .js?version or explicit max-age=xxxx). But the problem is still unsolved: how does browser handle the response header without "Cache-Control" like this:
Content-Length: 49675 Content-Type: text/html Last-Modified: Thu, 27 Dec 2012 03:03:50 GMT Accept-Ranges: bytes Etag: "0af7fcbdee3cd1:972" Server: Microsoft-IIS/6.0 X-Powered-By: ASP.NET Date: Thu, 24 Jan 2013 07:46:16 GMT They clearly cache them when "Enter in the bar"
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The max-age directive on a response implies that the response is cacheable (i.e., "public") unless some other, more restrictive cache directive is also present.
To use Cache-Control headers, choose Content Management | Cache Control Directives in the administration server. Then, using the Resource Picker, choose the directory where you want to set the headers. After setting the headers, click 'OK'.
Cache-control is an HTTP header used to specify browser caching policies in both client requests and server responses. Policies include how a resource is cached, where it's cached and its maximum age before expiring (i.e., time to live).
Cache-control: max-age It is the maximum amount of time specified in the number of seconds. For example, max-age=90 means that a HTTP response remains in the browser as a cached copy for the next 90 seconds before it can be available for reuse.
RFC 7234 details what browsers and proxies should do by default:
Although caching is an entirely OPTIONAL feature of HTTP, it can be assumed that reusing a cached response is desirable and that such reuse is the default behavior when no requirement or local configuration prevents it. Therefore, HTTP cache requirements are focused on preventing a cache from either storing a non-reusable response or reusing a stored response inappropriately, rather than mandating that caches always store and reuse particular responses.
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Caching is usually enabled by default in browers, so cache-control can be used to either customise this behaviour or disable it.
Although caching is an entirely OPTIONAL feature of HTTP, it can be assumed that reusing a cached response is desirable and that such reuse is the default behavior when no requirement or local configuration prevents it. Therefore, HTTP cache requirements are focused on preventing a cache from either storing a non-reusable response or reusing a stored response inappropriately, rather than mandating that caches always store and reuse particular responses. [https://www.rfc-editor.org/rfc/rfc7234#section-2]
The time the browser considers a cached response fresh is usually relative to when it was last modified:
Since origin servers do not always provide explicit expiration times, a cache MAY assign a heuristic expiration time when an explicit time is not specified, employing algorithms that use other header field values (such as the Last-Modified time)... If the response has a Last-Modified header field (Section 2.2 of [RFC7232]), caches are encouraged to use a heuristic expiration value that is no more than some fraction of the interval since that time. A typical setting of this fraction might be 10%. [https://www.rfc-editor.org/rfc/rfc7234#section-4.2.2]
This post has details of how the different browsers calculate that value.
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