List of Kubernetes Controllers with Business Use Cases
A Kubernetes Controller is a control loop that watches the state of your cluster and makes changes to achieve the desired state. Controllers automatically create, update, delete, or scale resources to maintain system health.
1. Deployment Controller
- Purpose: Manages stateless applications and handles rolling updates, rollbacks, and replica scaling.
- When to Use: For web apps, APIs, and microservices with no persistent state.
- Business Use Case:
- Running a shopping cart API that needs to be available across 3 replicas with zero-downtime deployments.
2. StatefulSet Controller
- Purpose: Manages stateful applications where each pod requires a stable identity and persistent storage.
- When to Use: For apps like databases or distributed systems needing stable hostnames and volumes.
- Business Use Case:
- Deploying a Kafka or Cassandra cluster where each node must retain its data and be addressable by name.
3. DaemonSet Controller
- Purpose: Ensures that a copy of a pod runs on all (or some) nodes.
- When to Use: For logging, monitoring, or security tools that must run on every node.
- Business Use Case:
- Running FluentBit or Prometheus node exporters across all Kubernetes worker nodes.
4. ReplicaSet Controller
- Purpose: Ensures a specified number of pod replicas are running at all times.
- When to Use: Typically used under the hood by Deployments.
- Business Use Case:
- Maintaining exactly 4 backend API pods at all times during traffic surges.
5. Job Controller
- Purpose: Runs pods that complete a specific task (once or retry until successful).
- When to Use: For one-time jobs like batch processing or database migrations.
- Business Use Case:
- Executing a one-time job to calculate quarterly analytics for a finance app.
6. CronJob Controller
- Purpose: Schedules Jobs to run periodically at fixed times, dates, or intervals.
- When to Use: For recurring tasks like backups or scheduled notifications.
- Business Use Case:
- Sending daily emails at 9 AM or backing up databases every night at midnight.
7. HorizontalPodAutoscaler (HPA) Controller
- Purpose: Automatically scales the number of pods based on CPU, memory, or custom metrics.
- When to Use: For applications with fluctuating traffic.
- Business Use Case:
- Scaling a video streaming service during peak hours and scaling down at night.
8. Node Controller
- Purpose: Monitors node health and updates the cluster when a node fails or becomes unresponsive.
- When to Use: Always enabled — part of the core control plane.
- Business Use Case:
- Ensuring your cluster re-balances workloads if a node crashes or is taken down for maintenance.
9. ServiceAccount & Token Controller
- Purpose: Automates token creation for pods to access the Kubernetes API securely.
- When to Use: Automatically managed; required for RBAC, security, and monitoring.
- Business Use Case:
- Granting read-only access to monitoring services like Prometheus.
10. ReplicaSet + HPA + Deployment Combined
- Purpose: This trio provides self-healing, auto-scaling, and rolling updates.
- When to Use: For production-grade, highly available web services.
- Business Use Case:
- An online ticket booking system scaling during high-demand events like concerts.
Summary Table
| Controller | When to Use | Business Use Case Example |
|---|---|---|
| Deployment Controller | Stateless apps needing rollout | Web frontend/API for online food delivery |
| StatefulSet Controller | Stateful apps, stable identity | MongoDB, Kafka, ElasticSearch |
| DaemonSet Controller | Run pod on every node | Logging, monitoring agents |
| ReplicaSet Controller | Maintain fixed pod count | Backend replicas for consistent uptime |
| Job Controller | One-time jobs | DB migration or image processing |
| CronJob Controller | Scheduled tasks | Daily backup, cron emails |
| HPA Controller | Auto-scale pods on demand | Scaling retail APIs during festive sales |
| Node Controller | Handle node failures | Failover workloads to healthy nodes |
| ServiceAccount Controller | Manage pod access securely | Secure API access from inside pods |