- Resource is not cacheable: If it made a request for resource and got the response, but request does not have either Expires or cache-control header or you explicitly set the resource as non-cachable by setting Expires equal to 0 or in the past date, or set Cache-control: no-cache.
- Cached resource is expired: A resource is cachable and lets say it was cacheable till 1 PM on 1st of August 2010, then if you query for the resource at 1.30 pm, in that case browser/ cache will make a request
When a browser makes conditional request either of two things will happen
- Client copy is fresh: That means the copy that browser has is same as that of the copy of the server or the resource is not modified. In that case the server sends HTTP status code 304 (Not modified) with only headers without body. The server can send new expires date for the resource so that the resource can get cached in the browser
- Client copy is stale: Means the server has new copy of the resource, or the resource has changed since the last time user requested for it. In that case the server will return HTTP status code 200 OK, with full resource in the body.
Conditional requests are implemented by conditional headers that start with
If. The conditional header allows method to execute only if particular conditional is met
GET /perf/images/cachesample.gif HTTP/1.0
If-Modified-Since Thu, 22 Jul 2010 23:41:23 GMT
/perf/images/cachesample.gif, only if it was modified since 22nd of July
There are two different attributes that you can use for testing creating conditional request
- Last Modified date: Means check if the document has changed since the last modified date
- ETag: Used to check if the entity tag(ETag) of the document has changed