Servlet response ETag Cache - http

Question

I want to use ETag to cache the version of a request and return 304 not modifed response to the client so the client can use last cached page.
So my url is like this which returns a json response
"http://server/WEB_GWT/prmCall?prmName=PRM_SIS_PROG_REG_STATUS"
In my servlet handling this request, I am always putting ETag information to store its value to be the version of url param PRM_SIS_PROG_REG_STATUS.
So response header returning to client is
HTTP/1.1 200 OK
Date: Sat, 07 Dec 2013 16:07:49 GMT
Server: IBM_HTTP_Server
ETag: "5"
Last-Modified: Sat, 07 Dec 2013 16:07:49 GMT
Content-Length: 356
Keep-Alive: timeout=10, max=99
Connection: Keep-Alive
Content-Type: application/json
Content-Language: tr-TR
In my next request, I am expecting this request header to include "If-None-Match" header to return the version of the request but I cannot get this header param. Any idea why I cannot get my ETag back.
My next request header is
GET /OZU_GWT/prmCall?prmName=PRM_SIS_PROG_REG_STATUS HTTP/1.1
Host: 10.100.199.103
Connection: keep-alive
Cache-Control: max-age=0
If-Modified-Since: Thu, 01 Jan 1970 00:00:00 GMT
User-Agent: Mozilla/5.0 (Windows NT 6.2; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/31.0.1650.57 Safari/537.36
Accept: */*
Referer: http://10.100.199.103/OZU_GWT/
Accept-Encoding: gzip,deflate,sdch
Accept-Language: tr-TR,tr;q=0.8,en-US;q=0.6,en;q=0.4
Cookie: JSESSIONID=0000kvocMBmjoWPbpKt_VAsDUMv:-1

Answer 1

Inoder to cache your request you need to include "Cache-Control" directive and specify the way in which the response should be cached and for what period.
HTTP/1.1 200 OK
Date: Sat, 07 Dec 2013 16:07:49 GMT
Server: IBM_HTTP_Server
ETag: "5"
Cache-Control : public, max-age=86400
Content-Length: 356
Keep-Alive: timeout=10, max=99
Connection: Keep-Alive
Content-Type: application/json
Content-Language: tr-TR
Here the Cache-Control header says that the content can be stored by "public" caching servers and the duration after which it needs to revalidate the content is 86400 seconds. And so when you refresh the page again "If-None-Match" and "If-Modified-Since" conditional headers will kick in and use the cached data.

Answer 2

After some investigation,I found out SmartGWT framework requests are sent to server with bypassCache:true flag which was sending my xhr request without any cache header. I managed to fixed it by overriding following method in DataSource class.
#Override
protected Object transformRequest(DSRequest dsRequest) {
dsRequest.setBypassCache(false);