# Amazon EC2 instance mutability in AMS
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You and AMS can maintain the Amazon Elastic Compute Cloud (Amazon EC2) instances in your infrastructure in one of two manners:
+ Immutable: This model uses Amazon Machine Images (AMIs) *baked* (created) with the necessary features. When deploying an update, the existing instances are torn down and completely replaced with new ones created from an updated AMI. To minimize down-time, this rolling process leaves some instances not updated and accessible while others are being updated until, eventually, the new change is completely deployed.
+ Mutable: In this model, the infrastructure is updated with new code being deployed on existing systems in the Cloud. This model is a mix of manually pushing updates and using infrastructure-as-code to deploy updates and does not rely on new AMIs.

 These maintenance models are discussed in more detail in later sections of this guide.