Kubernetes 1.7 has an alpha feature called initializers. CoreOS has the concept of an operator. Both seem to involve deploying code that watches the Kubernetes API server for changes to resources—possibly custom—in the cluster, based on annotations those resources contain and which the code understands.
What's the difference? If initializers are part of the core platform, why would I need to create something new that does what looks to my eyes like the same thing?
Operators are standalone "microservices" continuously and asynchronously reconciling the configured desired state towards the system's current state. Initializers are synchronous hooks validating or mutating runtime objects before they are created or updated. Also see admission controllers. They are usually baked into some "microservice". When you consider the lifecycle of a runtime object then initializers are first to act, like once. Then operators watching runtime objects reconcile the system upon their desired definitions.