Java Reference
In-Depth Information
Last
-
Modified:
Tue
,
15
May
2013 09
:
56
EST
<
customer id
=
"123"
>...</
customer
>
This initial response from the server is stating that the XML returned is valid for 1,000
seconds and has a timestamp of Tuesday, May 15, 2013, 9:56 AM EST. If the client supports
revalidation, it will store this timestamp along with the cached data. After 1,000 seconds, the
client may opt to revalidate its cache of the item. To do this, it does a
conditional
GET re-
quest by passing a request header called
If-Modified-Since
with the value of the cached
Last-Modified
header. For example:
GET
/
customers
/
123
HTTP
/
1.1
If
-
Modified
-
Since:
Tue
,
15
May
2013 09
:
56
EST
When a service receives this GET request, it checks to see if its resource has been modified
since the date provided within the
If-Modified-Since
header. If it has been changed since
the timestamp provided, the server will send back a 200, “OK,” response with the new rep-
resentation of the resource. If it hasn't been changed, the server will respond with 304, “Not
Modified,” and return no representation. In both cases, the server should send an updated
Cache-Control
and
Last-Modified
header if appropriate.
ETag
The
ETag
header is a pseudounique identifier that represents the version of the data sent
back. Its value is any arbitrary quoted string and is usually an MD5 hash. Here's an example
response:
HTTP
HTTP
/
1.1 200
OOK
Content-Type
:
application/xml
Cache-Control
:
max-age=1000
ETag: "3141271342554322343200"
<customer id="123">...</customer>
Like the
Last-Modified
header, when the client caches this response, it should also cache
the
ETag
value. When the cache expires after 1,000 seconds, the client performs a revalida-
tion request with the
If-None-Match
header that contains the value of the cached
ETag
. For
example:
GET
/customers/123
HTTP
/
1.1
If-None-Match: "3141271342554322343200"
/customers/123
HTTP
When a service receives this GET request, it tries to match the current
ETag
hash of the re-
source with the one provided within the
If-None-Match
header. If the tags don't match, the
