What They Are
Custom controllers extend Kubernetes functionality by watching cluster resources and automatically reconciling desired states with reality. Paired with Custom Resource Definitions (CRDs), they enable the Operator pattern - packaging domain-specific logic into Kubernetes-native objects without modifying core APIs.
They run as Deployments, using controller-runtime libraries to watch for changes and trigger reconciliation loops. Built primarily in Go with tools like Kubebuilder or Operator SDK, they handle tasks Kubernetes doesn't address natively: third-party integrations, stateful application management, or custom security enforcement.
The Enterprise Case
Three legitimate use cases emerge:
Workload automation: Provisioning cloud resources when custom objects appear, managing complex networking configurations declaratively.
External system lifecycle: Coordinating database instances, message queues, or storage with Kubernetes-native patterns.
Policy enforcement: Implementing validation webhooks with custom rules, using mutating webhooks for standardization.
The declarative model extends familiar patterns. Define desired state in YAML, let the controller handle implementation.
The Complexity Tax
Skepticism focuses on operational burden. Custom controllers demand:
- Robust unit and integration testing to avoid cluster instability
- Go proficiency (client-go's caching/queuing outperforms alternatives)
- Careful error handling with exponential backoff and retry logic
- Proper finalizer implementation for cleanup
- RBAC security to limit elevated privileges
Controllers consume cluster resources and introduce new failure modes. Version skew between Kubernetes releases requires maintenance.
Job configurations illustrate complexity: backoffLimit vs restartPolicy, activeDeadlineSeconds tuning, podFailurePolicy handling. Each decision cascades.
The Pattern Check
Before building a custom controller, ask:
- Does Helm or existing operators solve this?
- Can standard Jobs or CronJobs handle it?
- Do we have Go expertise and testing infrastructure?
- Is the operational overhead justified?
The CNCF certifies operators for common use cases. Check existing solutions first.
Current State
No significant developments in early February 2026. Recent guides emphasize controller-runtime patterns and testing strategies. Sample repos like kubernetes/sample-controller provide starting points.
The pattern works. The question is whether your team has capacity to maintain it properly. Most don't.