The classic LAMP case
The classic solution to this, as
provided for many years by application development
environments such as mod_perl and PHP, is for the Apache server
process to hold a database connection open, saving the overhead
of opening and closing a connection for every request. With
Apache 1.x, this is essentially the best you can do, and is
the usual way of working.
However, this solution has its own problems. Although it substantially
reduces the per-hit overhead, it introduces another: namely that of
holding a large number of backend connections open. This in itself
puts a load both on the webserver itself and the backend and limits
the number of users that can be concurrently serviced. This doesn't
just affect database-driven traffic: requests for static webpages
also have to be served by an Apache process that is keeping an open
connection to the backend.
