Replication
In Kubernetes, replication ensures that a specified number of pod replicas are running at any given time. It provides reliability, load balancing, and scalability to your applications.
Why Replication Matters
- High Availability: Keep applications running even if pods fail
- Load Balancing: Distribute traffic across multiple instances
- Scaling: Easily adjust the number of instances based on demand
- Rolling Updates: Update applications without downtime
Replication Controllers
Replication Controller was the original Kubernetes replication mechanism. It ensures that a specified number of pod replicas are running at any time. If pods fail, are deleted, or are terminated, the Replication Controller replaces them to maintain the desired count.
Replication Controller is considered legacy. While still supported, new workloads should use ReplicaSets and Deployments instead.
ReplicaSets
ReplicaSet is the next-generation Replication Controller, providing the same functionality but with more flexible selector support. ReplicaSets use set-based selectors instead of equality-based selectors.
Key advantages of ReplicaSets over Replication Controllers:
- More expressive label selectors
- Better integration with Deployments
- Improved reconciliation logic
Key Differences
| Feature | Replication Controller | ReplicaSet |
|---|---|---|
| API version | v1 | apps/v1 |
| Selector support | Equality-based | Set-based (includes equality) |
| Usage | Direct (legacy) | Usually via Deployments |
| Label requirements | Must match exactly | Can use In, NotIn, Exists, etc. |
Although you can work directly with ReplicaSets, it’s generally better to use Deployments which manage ReplicaSets and provide additional features like rolling updates and rollbacks.
For the CKA exam, understand both ReplicationController and ReplicaSet, but focus more on ReplicaSets as they represent the current best practice.
See the child pages for detailed information about each replication mechanism.