An immutable system is a system that does not change after it is deployed. Any changes or updates are done by creating a new system from a common image or template and replacing the old one. An immutable system meets the requirements of storing data outside of the VMs, preventing unauthorized modifications to the VMs, and deploying a new VM if a change needs to be done. An immutable system can improve the security, reliability, and consistency of the VMs by avoiding configuration drift, human errors, or malicious tampering. An immutable system can also simplify the deployment process and enable faster recovery from failures. Verified Reference:
https://cloudinfrastructureservices.co.uk/vm-types-for-devops-pets-vs-cattle-vs-immutable/
https://www.digitalocean.com/community/tutorials/what-is-immutable-infrastructure