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http://www.openssl.org/docs/ssl/SSL_CTX_set_session_cache_mode.html suggests that SSL session caching already occurs by default in server mode:
"SSL_SESS_CACHE_SERVER
 Server sessions are added to the session cache. When a client proposes a session to be reused, the server looks for the corresponding session in (first) the internal session cache (unless SSL_SESS_CACHE_NO_INTERNAL_LOOKUP is set), then (second) in the external cache if available. If the session is found, the server will try to reuse the session. This is the default."
A nice and easy thing to do would be to export the statistics given by http://www.openssl.org/docs/ssl/SSL_CTX_sess_number.html# .