kube-controller-manager

Creates (initializes) and runs controllers

Core Initialization (run function)

• CreateControllerContext
  • Creates SharedInformerFactory and Client shared by all controllers
• BuildControllers
  • Creates and registers informers
  • Controllers register handlers (subscribe) to informers
• InformerFactory.Start
  • Starts the InformerFactory to begin cache synchronization
• RunControllers
  • Iterates through each controller's loop

Controller

An infinite loop that aligns the desired state with the current state

Exists only for resources that require active state management (Reconciliation)

Role

• Orchestrates multiple controllers (existing per resource)
• Manages shared dependencies (Informers, Clients)
• Handles High Availability (Leader Election)

Leader Election

Three kube-controller-manager processes (instances) are launched, one on each of 3 Master Nodes.

• Only one of these is elected as the leader (election criteria: first come, first served) and creates/runs all controllers.
• The leader renews a document called Lease every 2 seconds (for periodic liveness checks).
• The others act as standby, taking over when the leader fails.
  • Since the original source of truth for all data is stored only in etcd, no handover is necessary.

Operation

Only the key is put into the queue, and a Worker retrieves it from the queue to execute business logic (sync).

• Registers EventHandler to the informer
  • The EventHandler executes the logic to put the key into the queue.
• Uses a Rate Limiting Queue
  • workqueue.TypedRateLimitingInterface[string]
  • Retries become slower as the number of retries increases.
• Key format
  • "Namespace/Name"
• Data is always looked up just before processing.
  • Data is looked up through the lister from the informer's local cache.

Informer

"Instead of directly querying the DB, let's subscribe to changes and keep a copy in our local memory (Cache)."

Exists for almost every resource type (Kind)

Role

• API (Kubernetes API Server) Watch
  • Creates a connection with a single request and continuously receives the response body.
• Cache Construction
  • Local Cache
    • cache.cacheStorage: treadSafeMap
      • lock (sync.RWMutex): Concurrency control
      • items (map[string]any): Actual data storage
      • index (storeIndex): Auxiliary index for fast searching
• Notifies subscribed controllers of events

sharedInformerFactory

Manages informers shared by controllers in a map,
ensuring that only one informer (watch, cache) exists for the same resource.

Reference

• https://github.com/kubernetes/kubernetes