Deploying a Kotlin Application on Kubernetes: A Step-by-Step Guide

Deploying Kotlin on Kubernetes: A Practical Basic Guide

Modern development increasingly requires not only writing quality code but also its competent deployment. Kubernetes has become the de facto standard for container orchestration. In this guide, we will walk through how to package a Kotlin application into a Docker container and deploy it in a Kubernetes cluster. You will get ready-made manifests and practical examples in Kotlin (using the Ktor framework).

1. Preparing the Kotlin Application

For demonstration, we'll take a simple REST API on Ktor. Make sure you have Gradle (or Maven) configured with the application plugin.

Example build.gradle.kts:

plugins {\n    kotlin("jvm") version "1.9.22"\n    application\n    id("com.github.johnrengelman.shadow") version "8.1.1"\n}\n\napplication {\n    mainClass.set("com.example.ApplicationKt")\n}\n\ndependencies {\n    implementation("io.ktor:ktor-server-netty:2.3.7")\n    implementation("io.ktor:ktor-server-content-negotiation:2.3.7")\n    implementation("io.ktor:ktor-serialization-kotlinx-json:2.3.7")\n}

The simplest endpoint (src/main/kotlin/com/example/Application.kt):

package com.example\n\nimport io.ktor.server.engine.*\nimport io.ktor.server.netty.*\nimport io.ktor.server.routing.*\nimport io.ktor.server.response.*\nimport io.ktor.serialization.kotlinx.json.*\nimport io.ktor.server.plugins.contentnegotiation.*\n\nfun main() {\n    embeddedServer(Netty, port = 8080) {\n        install(ContentNegotiation) {\n            json()\n        }\n        routing {\n            get("/") {\n                call.respond(mapOf("message" to "Hello from Kotlin on K8s!"))\n            }\n            get("/health") {\n                call.respond(mapOf("status" to "UP"))\n            }\n        }\n    }.start(wait = true)\n}

Build the fat JAR with the command:

./gradlew shadowJar

The ready JAR will appear in build/libs/.

2. Creating the Docker Image

Create a Dockerfile in the project root:

FROM openjdk:17-jdk-slim\n\nWORKDIR /app\n\nCOPY build/libs/*-all.jar app.jar\n\nEXPOSE 8080\n\nENTRYPOINT ["java", "-jar", "app.jar"]

Build the image and push it to a registry (e.g., Docker Hub):

docker build -t yourusername/kotlin-k8s-demo:v1 .\ndocker push yourusername/kotlin-k8s-demo:v1

Be sure to replace yourusername with your Docker Hub login or specify the address of a private registry.

3. Kubernetes Manifests

For deployment, we will need two YAML files: Deployment (manages pods) and Service (provides access).

3.1. Deployment (deployment.yaml)

apiVersion: apps/v1\nkind: Deployment\nmetadata:\n  name: kotlin-app\nspec:\n  replicas: 2\n  selector:\n    matchLabels:\n      app: kotlin-app\n  template:\n    metadata:\n      labels:\n        app: kotlin-app\n    spec:\n      containers:\n      - name: kotlin-app\n        image: yourusername/kotlin-k8s-demo:v1\n        ports:\n        - containerPort: 8080\n        livenessProbe:\n          httpGet:\n            path: /health\n            port: 8080\n          initialDelaySeconds: 5\n          periodSeconds: 10\n        readinessProbe:\n          httpGet:\n            path: /health\n            port: 8080\n          initialDelaySeconds: 3\n          periodSeconds: 5

3.2. Service (service.yaml)

apiVersion: v1\nkind: Service\nmetadata:\n  name: kotlin-app-service\nspec:\n  selector:\n    app: kotlin-app\n  ports:\n  - port: 80\n    targetPort: 8080\n  type: LoadBalancer

Apply the manifests:

kubectl apply -f deployment.yaml\nkubectl apply -f service.yaml

Check the status:

kubectl get pods\nkubectl