Apache: The Definitive Guide, 3rd Edition

The Global Session Cache

SSL uses a session key to secure each connection. When the connection starts, certificates are checked, and a new session key is agreed between the client and server (note that because of the joys of public-key encryption, this new key is only known to the client and server). This is a time-consuming process, so Apache-SSL and the client can conspire to improve the situation by reusing session keys. Unfortunately, since Apache uses a multiprocess execution model, there’s no guarantee that the next connection from the client will use the same instance of the server. In fact, it is rather unlikely. Thus, it is necessary to store session information in a cache that is accessible to all the instances of Apache-SSL. This is the function of the gcache program. It is controlled by the SSLCacheServerPath, SSLCacheServerPort, SSLSessionCacheTimeout directives for Apache v1.3, and SSLSessionCache for Apache v2, described later in this chapter.