Page Comparison

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In typical usage, when a URL is retrieved the web server will return the resource along with its corresponding ETag value, which is placed in an HTTP "ETag" field:

Code Block
ETag: "686897696a7c876b7e"

The client may then decide to cache the resource, along with its ETag. Later, if the client wants to retrieve the same URL again, it will send its previously saved copy of the ETag along with the request in a "If-None-Match" field.

Code Block
If-None-Match: "686897696a7c876b7e"

On this subsequent request, the server may now compare the client's ETag with the ETag for the current version of the resource. If the ETag values match, meaning that the resource has not changed, then the server may send back a very short response with an HTTP 304 Not Modified status. The 304 status tells the client that its cached version is still good and that it should use that.
However, if the ETag values do not match, meaning the resource has likely changed, then a full response including the resource's content is returned, just as if ETags were not being used. In this case the client may decide to replace its previously cached version with the newly returned resource and the new ETag.

Example using curl:

$ curl -i -u admin:test http://127.0.0.1:8080/openmrs/ws/rest/patient?q=Dar

RESPONSE HEADER IS:

HTTP/1.1 200 OK
Expires: Thu, 01 Jan 1970 00:00:00 GMT
Set-Cookie: JSESSIONID=1jw2itu9oyt5v;Path=/openmrs
Content-Type: application/json;charset=UTF-8
*ETag: "078c5b8fe25b332a40b4174bd38f5ee90"*
Content-Length: 399
Server: Jetty(6.1.10)

$ curl -i -u admin:test http://127.0.0.1:8080/openmrs/ws/rest/patient?q=Darius

RESPONSE HEADER IS:

HTTP/1.1 200 OK
Expires: Thu, 01 Jan 1970 00:00:00 GMT
Set-Cookie: JSESSIONID=1jw2itu9oyt5v;Path=/openmrs
Content-Type: application/json;charset=UTF-8
*ETag: "078c5b8fe25b332a40b4174bd38f5ee90"*
Content-Length: 399
Server: Jetty(6.1.10)

We see that both the ETag are same and hence the client knows that the response would be the same and can save bandwidth.

After some days, if Darius's records have not been updated then, the client can send the ETag and check for modifications:

$ curl -i -H 'If-None-Match:"078c5b8fe25b332a40b4174bd38f5ee90"' -u admin:test http://127.0.0.1:8080/openmrs/ws/rest/patient?q=Darius

RESPONSE IS:

HTTP/1.1 304 Not Modified
Expires: Thu, 01 Jan 1970 00:00:00 GMT
Set-Cookie: JSESSIONID=dlcztsmushgm;Path=/openmrs
Content-Type: application/json;charset=UTF-8
ETag: "078c5b8fe25b332a40b4174bd38f5ee90"
Content-Length: 0
Server: Jetty(6.1.10)

From the 304 Not Modified, we know that the records are the same and the client doesn't have to get the data again.

Downloads

View Source: http://source.openmrs.org/browse/Modules/webservices.rest/trunk/
Checkout Source: http://svn.openmrs.org/openmrs-modules/webservices.rest/trunk/
Download: http://modules.openmrs.org/modules/view.jsp?module= (To be updated once module is formally released)

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