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doc/chapters/sessions.inc

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+This chapter discusses how one should manage sessions, that is, share state between multiple
+HTTP requests from the same user.  We use a simple example where the user submits multiple
+forms and the server is supposed to accumulate state from all of these forms.  Naturally, as
+this is a network protocol, our session mechanism must support having many users with
+many concurrent sessions at the same time.
+
+In order to track users, we use a simple session cookie.  A session cookie expires when the
+user closes the browser.  Changing from session cookies to persistent cookies only requires
+adding an expiration time to the cookie.  The server creates a fresh session cookie whenever
+a request without a cookie is received, or if the supplied session cookie is not known to
+the server.
+
+@heading Looking up the cookie
+
+Since MHD parses the HTTP cookie header for us, looking up an existing cookie
+is straightforward:
+
+@verbatim
+FIXME.
+@end verbatim
+
+Here, FIXME is the name we chose for our session cookie.
+
+
+@heading Setting the cookie header
+
+MHD requires the user to provide the full cookie format string in order to set
+cookies.  In order to generate a unique cookie, our example creates a random
+64-character text string to be used as the value of the cookie:
+
+@verbatim
+FIXME.
+@end verbatim
+
+Given this cookie value, we can then set the cookie header in our HTTP response 
+as follows:
+
+@verbatim
+FIXME.
+@end verbatim
+
+
+@heading Remark: Session expiration
+
+It is of course possible that clients stop their interaction with the
+server at any time.  In order to avoid using too much storage, the
+server must thus discard inactive sessions at some point.  Our example
+implements this by discarding inactive sessions after a certain amount
+of time.  Alternatively, the implementation may limit the total number
+of active sessions.  Which bounds are used for idle sessions or the
+total number of sessions obviously depends largely on the type of
+the application and available server resources.
+
+@heading Example code
+
+A sample application implementing a website with multiple
+forms (which are dynamically created using values from previous
+POST requests from the same session) is available
+as the example @code{sessions.c}.
+
+Note that the example uses a simple, $O(n)$ linked list traversal to
+look up sessions and to expire old sessions.  Using a hash table and a
+heap would be more appropriate if a large number of concurrent
+sessions is expected.
+
+@heading Remarks
+
+Naturally, it is quite conceivable to store session data in a database
+instead of in memory.  Still, having mechanisms to expire data
+associated with long-time idle sessions (where the business process
+has still not finished) is likely a good idea.