Operators

An important concept for orchestration is the use of operators. These are also known as watch-loops and controllers. They query the current state, compare it against the spec, and execute code based on how they differ. Various operators ship with Kubernetes, and you can create your own, as well. A simplified view of an operator is an agent, or Informer, and a downstream store. Using a DeltaFIFO queue, the source and downstream are compared. A loop process receives an obj or object, which is an array of deltas from the FIFO queue. As long as the delta is not of the type Deleted, the logic of the operator is used to create or modify some object until it matches the specification.

The Informer which uses the API server as a source requests the state of an object via an API call. The data is cached to minimize API server transactions. A similar agent is the SharedInformer; objects are often used by multiple other objects. It creates a shared cache of the state for multiple requests.

A Workqueue uses a key to hand out tasks to various workers. The standard Go workqueues of rate limiting, delayed, and time queue are typically used.

The endpoints, namespace, and serviceaccounts operators each manage the eponymous resources for Pods.