Idempotency

The same as with a single-server configuration, reapplying an infrastructure template should not do the job twice. If you have a template defining 50 different resources, from EC2 instances to S3 buckets, then you do not want to duplicate or recreate all of them every time you apply the template. You want only missing parts to be created, existing ones to be in the desired state, and the ones which have become obsolete to be destroyed.