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Multi-Stage Dockerfile for Clojureλ︎

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Building a Clojure project from source to create a docker image provides an effective workflow for local testing and system integration.

A multi-stage Dockerfile is recommended to ensure build tools and artefacts are not included in the final Docker image, helping minimise the size and resources used by that image.

Practicalli Project Templates provide Multi-stage Dockerfile

🌐 Practialli Project Templates provides Dockerfile, .dockerignore and compose.yaml configurations to optimise the build and run of Clojure projects.

clojure -T:project/create :template practicalli/service :name practicalli/gameboard
Practicalli Project Templates are included in the 🌐 Practicalli Clojure CLI Config which provides aliases for running community tools to support a wide range of development tasks.

Updating to Java 21 Long Term Support release

This guide will be updated to use images with Java 21, the latest Long Term Support release (LTS) as of October 2023.

Examples will be provided using Alpine Linux and Debian Linux operating system.

Builder stageλ︎

Practicalli uses the latest Clojure CLI release and the latest Long Term Support (LTS) version of Eclipse Temurin (OpenJDK).

Alpine Linux image variants are used to keep the image file size as small as possible, reducing local resource requirements (and image download time).

Set builder stage image using specific Java version

FROM clojure:temurin-21-tools-deps-alpine AS builder

clojure:temurin-21-tools-deps-alpine image details

CLOJURE_VERSION will over-ride the version of Clojure CLI in the Clojure image (which defaults to latest Clojure CLI release). Or choose an image that has a specific Clojure CLI version, e.g. temurin-17-tools-deps-

Set builder stage image with specific Clojure version

FROM clojure:temurin-17-alpine AS builder

Create directory for building the project code and set it as the working directory within the Docker container to give RUN commands a path to execute from.

Create working directory for build

RUN mkdir -p /build
WORKDIR /build

Clojure CLI and

Practicalli Clojure - Tools.Build is the recommended approach to creating an Uberjar from a Clojure CLI project.

As long as the required files are copied, any build tool and task can be used to create the Uberjar file that packages the Clojure service for deployment.

Cache Dependenciesλ︎

Clojure CLI is used to download dependencies for the project and any other tooling used during the build stage, e.g. test runners, packaging tools to create an uberjar. Dependency download should only occur once, unless the deps.edn file changes.

Copy the deps.edn file to the build stage and use the clojure -P prepare (dry run) command to download the dependencies without running any Clojure code or tools.

Create dependency cache overlay

COPY deps.edn build.clj Makefile /build/
RUN make deps

The dependencies are cached in the Docker overlay (layer) and this cache will be used on successive docker builds unless the deps.edn file or Makefile is change.

Copy the deps.edn file to the build stage and use the clojure -P prepare (dry run) command to download the dependencies without running any Clojure code or tools.

Create dependency cache overlay

COPY deps.edn build.clj /build/
RUN clojure -P -X:build

The dependencies are cached in the Docker overlay (layer) and this cache will be used on successive docker builds unless the deps.edn file is change.

Pull request welcome

deps.edn in this example contains the project dependencies and :build alias used build the Uberjar.

Build Uberjarλ︎

Copy all the project files to the docker builder working directory, creating another overlay.

Copying the src and other files in a separate overlay to the deps.edn file ensures that changes to the Clojure code or configuration files does not trigger downloading of the dependencies again.

Run the dist task to generate an Uberjar for distribution.

COPY ./ /build
RUN make dist

Run the command to generate an Uberjar.

COPY ./ /build
RUN clojure -T:build uberjar
:build is an alias to include Clojure dependencies which is used to build the Clojure project into an Uberjar.

Pull request welcome

Docker Ignore patternsλ︎

.dockerignore file in the root of the project defines file and directory patterns that Docker will ignore with the COPY command. Use .dockerignore to avoid copying files that are not required for the build

Keep the .dockerignore file simple by excluding all files with * pattern and then use the ! character to explicitly add files and directories that should be copied

Docker ignore - only include specific patterns

# Ignore all files

# Include Clojure code and config

Docker ignore - only include specific patterns

# Ignore all files

# Include Clojure code and config

Pull request welcome

test-data directory is commonly used by Practicalli to include scripts and data for testing the system once running.

The classic approach for Docker ignoer patters is to specify all files and directories to exclude in a Clojure project, although this can lead to more maintenance as the project grows.

Run-time stageλ︎

The Alpine Linux version of the Eclipse Temurin image is used as it is around 5Mb in size, compared to 60Mb or more of other operating system images.

Image for run-time stage

FROM eclipse-temurin:17-alpine AS final

Run-time containers are often cached in a repository, e.g. AWS Container Repository (ECR). LABEL adds metadata to the container helping it to be identified in a repository or in a local development environment.

LABEL org.opencontainers.image.authors=""
LABEL io.github.practicalli.service="Gameboard API Service"
LABEL"Practicalli Engineering Team"
LABEL version="1.0"
LABEL description="Gameboard API service"

Use docker inspect to view the metadata

Optionally, add packages to support running the service or helping to debug issue in the container when it is running. For example, add dumb-init to manage processes, curl and jq binaries for manual running of system integration testing scripts for API testing.

apk is the package tool for Alpine Linux and --no-cache option ensures the install file is not part of the resulting image, saving resources. Alpine Linux recommends setting versions to use any point release with the ~= approximately equal version, so any same major.minor version of the package can be used.

Add Alpine packages

RUN apk add --no-cache \
    dumb-init~=1.2.5 \
    curl~=8.0.1 \

Check Alpine packages if new major versions are no longer available (low frequency)

Non-root group and userλ︎

Docker runs as root user by default and if a container is compromised the root permissions and could lead to a compromised host system. Docker recommends creating a user and group in the run-time image to run the service

Create a non-privileged user account

ARG UID=10001
RUN adduser \
    --disabled-password \
    --gecos "" \
    --home "/nonexistent" \
    --shell "/sbin/nologin" \
    --no-create-home \
    --uid "${UID}" \

Create Non-root group and user

RUN addgroup -S practicalli && adduser -S practicalli -G practicalli

Create directory to contain service

Create directory to contain service archive, owned by non-root user

RUN mkdir -p /service && chown -R clojure. /service

Set user

Instruct docker that all future commands should run as the appuser user

USER clojure

Copy Uberjar to run-time stageλ︎

Create a directory to run the service or use a known existing path that will not clash with any existing files from the image.

Set the working directory and copy the uberjar archive file from Builder image

Copy Uberjar from build stage

WORKDIR /service
COPY --from=builder --chown=clojure:clojure /build/practicalli-service.jar /service/practicalli-service.jar

Optionally, add system integration testing scripts to the run-time stage for testing from within the docker container.

RUN mkdir -p /service/test-scripts
COPY --from=builder --chown=clojure:clojure /build/test-scripts/curl--* /service/test-scripts/

Service Environment variablesλ︎

Define values for environment variables should they be required (usually for debugging), ensuring no sensitive values are used. Environment variables are typically set by the service provisioning the containers (AWS ECS / Kubernettes) or on the local OS host during development (Docker Desktop).

# optional over-rides for Integrant configuration

Java Virtual Machine Optimisationλ︎

Clojure Uberjar runs on the Java Virtual Machine which is a highly optimised environment that rarely needs adjusting, unless there are noticeable performance or resource issue use.

Minimum and maximum heap sizes, i.e. -XX:MinRAMPercentage and -XX:MaxRAMPercentage are typically the only optimisations required.

java -XshowSettings -version displays VM settings (vm), Property settings (property), Locale settings (locale), Operating System Metrics (system) and the version of the JVM used. Add the category name to show only a specific group of settings, e.g. java -XshowSettings:system -version.

JDK_JAVA_OPTIONS can be used to tailor the operation of the Java Virtual Machine, although the benefits and constraints of options should be well understood before using them (especially in production).

Show settings on JVM startup

Show system settings on startup, force container mode and set memory heap maximum to 85% of host memory size.

ENV JDK_JAVA_OPTIONS "-XshowSettings:system -XX:+UseContainerSupport -XX:MaxRAMPercentage=85"

Use relative heap memory settings, not fixed sizes

Relative heap memory settings (-XX:MaxRAMPercentage) should be used for containers rather than the fixed value options (-Xmx) as the provisioning service for the container may control and change the resources available to a container on deployment (especially a Kubernettes system).

Options that are most relevant to running Clojure & Java Virtual Machine in a container include:

  • -XshowSettings:system display (container) system resources on JVM startup
  • -XX:InitialRAMPercentage Percentage of real memory used for initial heap size
  • -XX:MaxRAMPercentage Maximum percentage of real memory used for maximum heap size
  • -XX:MinRAMPercentage Minimum percentage of real memory used for maximum heap size on systems with small physical memory
  • -XX:ActiveProcessorCount specifies the number of CPU cores the JVM should use regardless of container detection heuristics
  • -XX:±UseContainerSupport force JVM to run in container mode, disabling container detection (only useful if JVM not detecting container environment)
  • -XX:+UseZGC low latency Z Garbage collector (read the Z Garbage collector documentation and understand the trade-offs before use) - the default Hotspot garbage collector is the most effective choice for most services
Only optimise if performance test data shows issues

Without performance testing of a specific Clojure service and analysis of the results, let the JVM use its own heuristics to determine the most optimum strategies it should use

Expose Clojure serviceλ︎

If Clojure service listens to network requests when running, then the port it is listening on should be exposed so the world outside the container can communicate to the Clojure service.

e.g. expose port of HTTP Server that runs the Clojure service

Expose service port



Finally define how to run the Clojure service in the container. The java command is used with the -jar option to run the Clojure service from the Uberjar archive.

The java command will use arguments defined in JDK_JAVA_OPTIONS.

ENTRYPOINT directive defines the command to run the service

ENTRYPOINT ["java", "-jar", "/service/practicalli-service.jar"]
Docker ENTRYPOINT directive

ENTRYPOINT is the recommended way to run a service in Docker. CMD can be used to pass additional arguments to the ENTRYPOINT command, or used instead of ENTRYPOINT.

jshellis the default ENTRYPOINT for the Eclipse Temurin image. jshell will run if a CMD directive is not included in the run-time stage of the Dockerfile.

The ENTRYPOINT command runs as process id 1 (PID 1) inside the docker container. In a Linux system PID 1 should respond to all TERM and SIGTERM signals.

dump-init provides a simple process supervisor and init system, designed to run as PID 1 and manage all signals and child processes effectively.

Use dumb-init as the ENTRYPOINT command and CMD to pass the java command to start the Clojure service as an argument. dumb-init ensures TERM signals are sent to the Java process and all child processes are cleaned up on shutdown.

ENTRYPOINT to cleanly manage service

ENTRYPOINT ["/usr/bin/dumb-init", "--"]
CMD ["java", "-jar", "/service/practicalli-service.jar"]

Alternatively, run dumb-jump and java within the ENTRYPOINT directive, ENTRYPOINT ["/usr/bin/dumb-init", "--", "java", "-jar", "/service/practicalli-service.jar"]. This approach cannot be overridden correctly with an additional CMD directive on the command line when using docker run.

Build and Run with dockerλ︎

Ensure docker services are running, i.e. start docker desktop.

Build the service and create an image to run the Clojure service in a container with docker build. Use a --tag to help identify the image and specify the context (in this example the root directory of the current project, .)

docker build --tag practicalli/service-name:1.1 .

After the first time building the docker image, any parts of the build that havent changed will use their respective cached layers in the builder stage. This can lead to very fast, even zero time builds.

Docker build image optomised to use docker layer cache for build stage

Maximise use of Docker cache

Maximising the docker cache by careful consideration of command order and design in a Dockerfile can have a significant positive affect on build speed.

Each command is an overlay (layer) in the Docker image and if its respective files have not changed, then the cached version of the command will be used.

Run the built image in a docker container using docker run, publishing the port number so it can be used from the host (developer environment or deployed environment). Use the name of the image created by the tag in the docker build command.

docker run --publish 8080:8080 practicalli/service-name

Orchestrate multiple services with Compose

Create a compose.yml file to defines all services to run to support local integration testing, optionally adding health checks and conditional starts.

Run docker compose up to start all the services.