User-server interaction: conditional GET

• Goal: don’t send object if client has up-to-date stored (cached) version

• client: specify date of cached copy in http request

  If-modified-since: <date>
  

• server: response contains no object if cached copy up-to-date:

  HTTP/1.0 304 Not Modified
  

client

server

http request msg

If-modified-since: <date>

object

not

modified

http request msg

If-modified-since: <date>

http response

HTTP/1.1 200 OK

…

<data>

object

modified

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