Docker

In this section, we'll see how to deploy a Ktor application to a Docker container, which can then be run either locally or on your cloud provider of choice.

Docker is a container system that allows for packaging software in a format that can then be run on any platform that supports Docker, such as Linux, macOS, and Windows. Conceptually Docker is an operating system with layers providing multiple services. While the basics of Docker will be covered, if you're not familiar with it, check out the Getting Started documentation.

Clone a sample application

In this tutorial, we'll be using a project created in Creating a new Ktor project: ktor-get-started-sample.

Get the application ready

In order to run on Docker, the application needs to have all the required files deployed to the container. As a first step, you need to create a fat JAR file containing the application and its dependencies. Depending on the build system you're using, there are different ways to accomplish this:

To prepare a Gradle project, follow the steps from the Configure the Shadow plugin section.
To prepare a Maven project, follow the steps from Configure the Assembly plugin.

Prepare Docker image

To dockerize the application, we'll use multi-stage builds:

First, we'll use the gradle/maven image to generate a distribution of the application.
Then, the generated distribution will be run in the environment created based on the openjdk image.

In the root folder of the project, create a file named Dockerfile with the following contents:

FROM gradle:7-jdk11 AS build
COPY --chown=gradle:gradle . /home/gradle/src
WORKDIR /home/gradle/src
RUN gradle shadowJar --no-daemon

FROM openjdk:11
EXPOSE 8080:8080
RUN mkdir /app
COPY --from=build /home/gradle/src/build/libs/*.jar /app/ktor-docker-sample.jar
ENTRYPOINT ["java","-jar","/app/ktor-docker-sample.jar"]

FROM maven:3-openjdk-11 AS build
COPY . /home/maven/src
WORKDIR /home/maven/src
RUN mvn package

FROM openjdk:11
EXPOSE 8080:8080
RUN mkdir /app
COPY --from=build /home/maven/src/target/*-with-dependencies.jar /app/ktor-docker-sample.jar
ENTRYPOINT ["java","-jar","/app/ktor-docker-sample.jar"]

The second stage of the build works in the following way:

Indicates what image is going to be used (openjdk in this case).
Specifies the exposed port (this does not automatically expose the port, which is done when running the container).
Copies the contents from the build output to the folder.
Runs the application (ENTRYPOINT).

Build and run the Docker image

The next step is to build and tag the Docker image:

docker build -t my-application .

Finally, start the image:

docker run -p 8080:8080 my-application

If using IntelliJ IDEA, you can click Run in the Dockerfile to perform these steps:

Learn more from the Docker topic.

Last modified: 28 June 2022