Run Dropwizard application in Docker with templated configuration using environment variables

Last Updated on by

Post summary: How to run Dropwizard application inside a Docker container with configuration template file which later gets replaced by environment variables.

In Build a RESTful stub server with Dropwizard post I have described how to build a Dropwizard application and run it. In the current post, I will show how this application can be inserted into Docker container and run with different configurations, based on different environment variables. Code sample here can be found as a buildable and runnable project in GitHub sample-dropwizard-rest-stub repository.

Dropwizard templated config

Dropwizard supports replacing variables from config file with environment variables if such is defined. The first step is to make the config file with a special notation.

version: ${ENV_VARIABLE_VERSION:- 0.0.2}

Dropwizard substitution is based on Apache’s StrSubstitutor library. ${ENV_VARIABLE_VERSION:- 0.0.2} means that Dropwizard will search for environment variable with name ENV_VARIABLE_VERSION and replace its value with given variable. If no variable is configured then it will replace with a default value of 0.0.2. If default is not needed then just ${ENV_VARIABLE_VERSION} can be used.

Next step is to make Dropwizard substitute config file with environment variables on its startup before the file is being read. This is done with following code:

@Override
public void initialize(Bootstrap<RestStubConfig> bootstrap) {
	bootstrap.setConfigurationSourceProvider(new SubstitutingSourceProvider(
			bootstrap.getConfigurationSourceProvider(), 
			new EnvironmentVariableSubstitutor(false)));
}

EnvironmentVariableSubstitutor is used with false in order to suppress throwing of UndefinedEnvironmentVariableException in case environment variable is not defined. There is an approach to pack config.yml file into JAR file and use it from there, then bootstrap.getConfigurationSourceProvider() should be changed with path -> Thread.currentThread().getContextClassLoader().getResourceAsStream(path).

Docker

Docker is a platform for building software inside containers which then can be deployed in different environments. A container is like a virtual machine with the significant difference that it does not build full operating system. In this way host’s resources are optimized, container consumes as much memory as needed by the application. Virtual machine itself consumes memory to run the operating system as well. Containers have resource (CPU, RAM, HDD) isolation so that application sees the container as a separate operating system.

Dockerfile

A Dockerfile is a text document that contains all the commands a user could call on the command line to assemble an image. Using docker build users can create an automated build that executes several command-line instructions in succession. In given example Dockerfile looks like this:

FROM openjdk:8u121-jre-alpine
MAINTAINER Automation Rhapsody https://automationrhapsody.com/

WORKDIR /var/dropwizard-rest-stub

ADD target/sample-dropwizard-rest-stub-1.0-SNAPSHOT.jar /var/dropwizard-rest-stub/dropwizard-rest-stub.jar
ADD config-docker.yml /var/dropwizard-rest-stub/config.yml

EXPOSE 9000 9001

ENTRYPOINT ["java", "-jar", "dropwizard-rest-stub.jar", "server", "config.yml"]

Dockerfile starts with FROM openjdk:8u121-jre-alpine which instructs docker to use this already prepared image as a base image for the container. At https://hub.docker.com/ there are numerous of images maintained by different organizations or individuals that provide different frameworks and tools. They can save you a lot of work allowing you to skip configuration and installation of general things. In given case openjdk:8u121-jre-alpine has OpenJDK JRE 1.8.121 already installed with minimalist Linux libraries. MAINTAINER documents who is the author of this file. WORKDIR sets working directory of the container. Then ADD copies JAR file and config.yml to the container. ENTRYPOINT configures the container to be run as executable. This instruction translates to java -jar dropwizard-rest-stub.jar server config.yml command. More info can be found at Dockerfile reference link.

Build Docker container

Docker container can be built with the following command:

docker build -t dropwizard-rest-stub .

dropwizard-rest-stub is the name of the container, it is later used to run container with this name. Note that dot, in the end, is mandatory.

Run Docker container

docker run -it -p 9000:9000 -p 9001:9001 -e ENV_VARIABLE_VERSION=1.1.1 dropwizard-rest-stub

Name of the container is dropwizard-rest-stub and is put in the end. With -p 9000:9000 port 9000 form guest machine is exposed to host machine, otherwise container will not be accessible. With -e ENV_VARIABLE_VERSION=1.1.1 environment variable is being set. If not passed it will be substituted with 0.0.2 in config.yml file as described above. In case of many environment variables, it is more comfortable to put them in a file with content KEY=VALUE. Then use this file with –env-file= instead of specifying each individual variable.

Conclusion

Dropwizard has a very easy mechanism for making configuration dependant on environment variables. This makes Dropwizard applications extremely suitable to be built into Docker containers and run in different environments just by changing environment variables being passed to the container.

Related Posts

Category: Java, Tutorials | Tags: ,