Sharing knowledge is fun: March 2018

How Docker can make your coding life easier

Have you ever had a situation where you have to use different versions of the same database? Probably because some legacy apps can only work with an old version of the database, while other apps work with a more recent one. If you have, and you had headache, then Docker probably is the answer for you.

So…, what is it?

I won’t tell you more about the architecture in details. TL;DR; Docker is like VM (Virtualbox or VMWare) in a way that both allow you to install many applications a long with their configuration, but different in a way that, Docker is much lighter because all instances of it share the same Linux kernel. It is a way to encapsulate an application and its configuration within a small image. You can install Docker on many popular OSes, including Ubuntu Linux and Windows.

Running an image

“Running an image” also means “creating a container” as container is a running instance of an image.

docker run --rm <account>/<image name>:<tag> command on the 
container

For e.g.

docker run --rm openjdk:8-jre java -version

is only required if your image is not published in Docker official? Option --rm is to automatically remove the container when it exits.

Running a Java app locally using a JRE Docker image

Supposed that you want to run this JAR app using Java 8 image. In a normal environment, you’d run it with:

java -jar h2-1.4.196.jar

To use JRE inside Docker image, you have to map local directory containing the JAR file into a directory inside the Docker image. The directory will be created automatically if it doesn’t exist.

docker run --rm -v c:\Downloads:/app openjdk:8-jre java -jar /app/h2-1.4.196.jar

Creating an image

You can use either Dockerfile or Packer. Packer is more generic, not just for building Docker images. But, IMHO, the parameters used in Packer for Docker build script are also specific. It’s much worth to use Dockerfile instead :)

So, I’m going to tell you how to create image using Dockerfile which is a Docker script file containing instructions how to build an image. Here, we’re going to create a Java image based on openjdk 8, and have a H2 database app installed inside.

FROM openjdk:8-jre

ADD http://central.maven.org/maven2/com/h2database/h2/1.4.196/h2-1.4.196.jar /app.jar

# The final blocking command to run the app
CMD ["java", "-jar", "/app.jar"]

To build it, run:

cd /path/to/dir/containing/docker/script/
docker build -t sancho21/h2:1.4.196 .

To run the image, run:

docker run --rm sancho21/h2:1.4.196

Note about -t:

sancho21 => your Docker online account
h2 => Name of this image
1.4.196 => Tag

Notice that, port 8082 is not accessible by host machine (your notebook), hence you have to map that docker port into a host port. To do it, do:

Update the script to expose that port by adding EXPOSE 8082 right before CMD ... part. Then rebuild with the same command above.

Map that port into host machine when running it.

docker run --rm -p 8989:8082 sancho21/h2:1.4.196

Open http://localhost:8989 in your browser.

Some useful script commands

# ADD: To add files from local or from URL
ADD http://hello.com/file.sh /opt/install

# RUN: Run anything in the OS of the container
RUN touch /tmp/combination

# Exposing port 3306
EXPOSE 3306

# Defining a variable
ENV OTRS_VER 5.0.15

Storage

Persistency are achievable by either 2 ways

Volume is where the exact location in the host system will be determined by Docker. No non-Docker app may have access to it as the location is managed and protected by Docker. It is the most recommended persistency model, as it’s easier to backup.

This is to specify that this path should persistent directory of the container, even after shutdown: --mount source=myvol,target=/var/lib/mysql which is equivalent to VOLUME ["/var/lib/mysql"]. FYI, source is an optional volume name (recommended to use!).
Bind mount is a mapping of your local machine directory into a directory in the container. Non-Docker apps may have access to the directory. This is how to di it: --mount type=bind,source=/home/ichsan/mysql/conf,target=/etc/mysql.conf.d which is equivalent to VOLUME ...?

An example how to start a mysql service using the volume

docker run \
  --mount source=otrs-ssh-volume,target=/etc/mysql/conf.d,readonly \
  --name some-mariadb -e MYSQL_ROOT_PASSWORD=secret mariadb

Read all possible options at Docker website.

Creating a volume

docker volume create --driver vieux/sshfs \
   -o sshcmd=ichs002@10.16.30.11:/tmp/otrs \
   -o password=secret123 otrs-ssh-volume

Important options to run an image

To let host access an exposed port: -p your_machine_port:internal_docker_port e.g. docker run -p 8081:8080 id.web.michsan/stemmer-spring-boot:1.0
Then you can access it using localhost:8081
To run in background: -d e.g. docker run -d -p 8081:8080 id.web.michsan/stemmer-spring-boot:1.0
To attach to the daemon, use docker attach TheHashEG12912

But, when the docker service is restarted (the /etc/init.d/docker),
it won’t be restarted unless --restart with a certain value is used (TODO).
If you want to set your working dir: -w /path/on/internal_docker
e.g. docker run --rm -v $PWD:/app -w /app sancho21/oracle-java:6 java sancho21.Application

Important commands

To list running containers: docker ps (To list stopped ones, use docker ps -f "status=exited")
To stop a running container: docker stop TheHashEG12912
To start a container: docker start TheHashEG12912
To remove a container: docker rm TheHashForEG12912
Access its shell (bash): docker exec -i -t TheHashForEG12912 /bin/bash

If you hate using hash, you can use its name.

Docker compose

It is a tool to build an application consisting of multiple containers working together. The configuration file is named docker-compose.yml and you start the app by running docker-compose up. If you use other name like otrs.yml, then you must run it this way docker-compose -f otrs.yml up

The following example will start a wordpress application consisting of a wordpress apache image and a database image.

version: '3.4'

services:

  wordpress:
    image: wordpress
    restart: always
    ports:
    - 8080:80
    environment:
      WORDPRESS_DB_PASSWORD: secret
    links:
    - wordpress_db:mysql

  wordpress_db:
    image: mariadb
    restart: always
    environment:
    MYSQL_ROOT_PASSWORD: secret

A popular alternative to Docker compose is Kubernetes
Ref: https://blog.codeship.com/docker-machine-compose-and-swarm-how-they-work-together/

Friday, March 2, 2018