If-Modified-Since vs. If-None-Match
Timestamp value used in Last-Modified/If-Modified-Since has limited precision - one second and that is simply not enough for fast changing content like, for instance, Web-chat application where more than one message could be posted at any given second. ETag/If-None-Match can help to solve that problem.
If-Modified-Since
is compared to the Last-Modified
whereas If-None-Match
is compared to ETag
. Both Modified-Since
and ETag
can be used to identify a specific variant of a resource.
But the comparison of If-Modified-Since
to Last-Modified
gives you the information whether the cached variant is older or newer whereas the comparison of If-None-Match
to ETag
just gives you the information whether both are identical or not. Furthermore do most of the ETag
generators include the information of the system specific inode so moving a file to a different drive may change the ETag
as well.
Regarding the differences between Last-Modified/If-Modified-Since
and ETag/If-None-Match
:
Both can be used interchangeably. However depending on the type of resource, and how it is generated on the server, one or the other question ("has this been modified since ...?" / "does this still match this ETag?") may be easier to answer.
Examples:
- If you're serving files, using the file's
mtime
as theLast-Modified
date is the simplest solution. - If you're serving a dynamic web page built from a number of SQL queries, checking whether the data returned by any of those queries has changed may be impractical (unless all of them have some sort of "last modified" column). In this case, using e.g. an md5 hash of the page content as the
ETag
will be a lot easier.
OTOH, this means that you still have to generate the whole page on the server, even for a conditional GET. Figuring out what exactly has to go into the ETag (primary keys, revision numbers, ... etc.) can save you a lot of time here.
See these links for more details on the topic:
- http://www.tbray.org/ongoing/When/200x/2008/08/14/Rails-ETags
- http://bitworking.org/news/150/REST-Tip-Deep-etags-give-you-more-benefits