Kubernetes Operators: Simplifying the Stateful Symphony
Tags: kubernetes, devops, k8s, cloud
Published: 2023-12-17 11:37:00

Imagine you’re a conductor, leading an orchestra. Each musician plays their part, but it’s your job to ensure they all work together harmoniously. In the world of Kubernetes, an Operator plays a similar role. It’s a software extension that manages applications and their components, ensuring they all work together in harmony.

The Operator tunes the complexities of deployment and management, ensuring each containerized instrument hits the right note at the right time. It’s a harmonious blend of technology and expertise, conducting a seamless production in the ever-evolving concert hall of Kubernetes.

**What is a Kubernetes Operator?**

A Kubernetes Operator is essentially an application-specific controller that helps manage a Kubernetes application.

It’s a way to package, deploy, and maintain a Kubernetes application, particularly useful for stateful applications, which include persistent storage and other elements external to the application that may require extra work to manage and maintain.

Operators are built for each application by those that are experts in the business logic of installing, running, and updating that specific application.

For example, if you want to create a cluster of MySQL replicas and deploy and run them in Kubernetes, a team that has domain-specific knowledge about the MySQL application creates an Operator that contains all this knowledge.

**Stateless vs Stateful Applications**

To understand the importance of Operators, let’s first compare how Kubernetes manages stateless and stateful applications.

**Stateless Applications**

Consider a simple web application deployed in a Kubernetes cluster. You create a deployment, a config map with some configuration attributes for your application, a service, and the application starts. Maybe you scale the application up to three replicas. If one replica dies, Kubernetes automatically recovers it using its built-in control loop mechanism and creates a new one in its place

All these tasks are automated by Kubernetes using this control loop mechanism. Kubernetes knows what your desired state is because you stated it using configuration files, and it knows what the actual state is. It automatically tries to match the actual state always to your desired state

**Stateful Applications**

Now, let’s consider a stateful application, like a database. For stateful applications, the process isn’t as straightforward. These applications need more hand-holding when you create them, while they’re running, and when you destroy them

Each replica of a stateful application, like a MySQL application, has its own state and identity, making things a bit more complicated. They need to be updated and destroyed in a certain order, there must be constant communication between these replicas or synchronization so that the data stays consistent, and a lot of other details need to be considered as well

**The Role of Kubernetes Operator**

This is where the Kubernetes Operator comes in. It replaces the human operator with a software operator. All the manual tasks that a DevOps team or person would do to operate a stateful application are now packed into a program that has the knowledge and intelligence about how to deploy that specific application, how to create a cluster of multiple replicas of that application, how to recover when one replica fails, etc

At its core, an Operator has the same control loop mechanism that Kubernetes has that watches for changes in the application state. Did a replica die? Then it creates a new one. Did an application configuration change? It applies the up-to-date configuration. Did the application image version get updated? It restarts it with a new image version

**Final Notes: Orchestrating Application Harmony**

In summary, Kubernetes can manage the complete lifecycle of stateless applications in a fully automated way. For stateful applications, Kubernetes uses extensions, which are the Operators, to automate the process of deploying every single stateful application

So, just like a conductor ensures every musician in an orchestra plays in harmony, a Kubernetes Operator ensures every component of an application works together seamlessly. It’s a powerful tool that simplifies the management of complex, stateful applications, making life easier for DevOps teams everywhere.

**Practical Demonstration: PostgreSQL Operator**

Here’s an example of how you might use a Kubernetes Operator to manage a PostgreSQL database within a Kubernetes cluster:

```
apiVersion: "acid.zalan.do/v1"
kind: postgresql
metadata:
  name: pg-cluster
  namespace: default
spec:
  teamId: "myteam"
  volume:
    size: 1Gi
  numberOfInstances: 2
  users:
    admin:  # Database admin user
      - superuser
      - createdb
  databases:
    mydb: admin  # Creates a database `mydb` and assigns `admin` as the owner
  postgresql:
    version: "13"
```
This snippet highlights how Operators simplify the management of stateful applications, making them as straightforward as deploying stateless ones.

Remember, “The truth you believe and cling to makes you unavailable to hear anything new.” So, be open to new ways of doing things, like using a Kubernetes Operator to manage your stateful applications. It might just make your life a whole lot easier.