Staging environments are the last line of test environments. They are interchangeable with production environments. You can install your new releases on the staging servers, check that everything works, and then swap out your old production servers and replace them with the staging servers, which will then become the new production servers. This is sometimes called the blue-green deployment strategy.
The exact details of how to perform this style of deployment depends on the product being deployed. Sometimes, it is not possible to have several production systems running in parallel, usually because production systems are very expensive.
At the other end of the spectrum, we may have hundreds of production systems in a pool. ...
