Immutable means, during application upgrades, you will not only replace software, but hardware too. Organizations make a significant capital investment in hardware and develop the practice of updating them with a new version of the application and configuration.
To create replaceable servers, you need to make your application stateless and avoid the hardcoding of any server IP or database DNS name. Basically, you need to apply the idea of treating your infrastructure as software instead of hardware, and not apply updates to the live system. You should always spin up new server instances from the golden machine image, which has all necessary security and software in place.
Creating immutable infrastructure becomes comfortable with the use of a virtual machine, where you can create a golden image of your virtual machine and deploy it with the new version, rather than trying to update an existing version. This deployment strategy is also beneficial for troubleshooting...
