Github user afs commented on the pull request:
https://github.com/apache/jena/pull/95#issuecomment-149257400
`ETags` or `If-Modified-Since`+`Last-Modified` facilitate client-side
caching. The client already has a copy and wants to know if it is still
current and the server send either "your copy is good" or a new copy or whether
the server resource has changed. The client had to knowits the "same" query.
Having Fuseki support those would be good but not an alternative to a
server-side cache shared across clients. `ETags` also enable optimistic
concurrency control.
It would be possible to use have an intermediary (client to Fuseki, server
to the application) that managed a cache with Fuseki backend and, handling the
updates, could invalidate correctly. If those servlets (for each SPARQL
protocol and operation) were servlet filters, they could reside in a separate
machine/container/webserver or the Fuseki server itself. Code like this PR
could be the cache engine part of that.
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