Stateful application — State management in Kubernetes in real-time.
Scenario: Ideally, in the modern era, applications (services/microservices) should be stateless. But, there are few scenarios that arise which force the applications to become stateful. Typical example is, you do have a tiny data set (going through less updates), you would like to hold onto a memory (like embedded cache) and you want to avoid a separate cache server to manage complexity of your infrastructure.
Challenge: In the Kubernetes world, if you keep on changing the number of pods (stateful application pods), managing the state data/information across different pods in real-time.
Solution: Creating a headless service.
A headless service is a service with a service IP but instead of load-balancing it will return the IPs of the associated Pods. This allows us to interact directly with the Pods instead of a proxy. It’s as simple as specifying None for .spec.clusterIP and can be utilized with or without selectors — you’ll see an example with selectors in a moment.
Below are Kubernetes artifacts
- Application deployment descriptor file
2. Application service descriptor file
3. Headless service descriptor file
Usage: Generally embedded cache solutions like Infinispan, Hazelcast, Redis can be managed through this way.
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References:
1. https://dev.to/kaoskater08/building-a-headless-service-in-kubernetes-3bk8
2. https://kubernetes.io/docs/tutorials/stateful-application/cassandra/
