It's not uncommon for a popular web site to run on a single machine. It's also common for a web site to run on multiple machines, with one machine dedicated to serving static objects (such as images and static HTML files), another serving dynamically generated responses, and perhaps even a third machine that acts as a dedicated database server.

Therefore, the situation that must be addressed is where just one machine runs the service or where the service is spread over a few machines, with each performing a unique task, such that no machine can be shut down even for a single minute, and leaving the service unavailable for more than five seconds is unacceptable. In this case, two different tasks may be required: upgrading the software on the server (including the Apache server), and upgrading the code of the service itself (i.e., custom modules and scripts).