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Docker Basics

  • Docker is a platform which packages an application and all its dependencies together in the form of containers

  • Docker works like Client-Server model

  • Docker Daemon or Docker Runtime acts as the server & it can be communicated using Docker CLI client (Command Line Interface)

  • Docker image is a template & Docker container is the running instance of the image. Every Docker image will have its own Id & Every Container instance also will have its own Id assigned. We can create any number of Container instance from single Docker image

Dockerizing application with Docker commands​

Lets assume we have a Spring Boot application which generates WAR file (say demo.war). In order to run WAR file, we need Tomcat server. So lets begin from Tomcat image

Pull Tomcat Image​

First, we need to pull the Tomcat Java image using pull command

docker pull tomcat:8.5.21-jre8-alpine

Remember

  • Specifying tomcat:8.5.21-jre8-alpine will always pull the latest image from Docker hub
  • alpine is a Linux distribution like Ubuntu but has size of just 5 MB. Docker images built on top of alpine are usally small in size & hence downloads much faster. Hence we used alpine version of tomcat in our sample

Verify if image got downloaded​

Once pull command execution is complete, run docker images command to get list of images. You should see something like below as response

docker images

REPOSITORY                      TAG                 IMAGE ID            CREATED             SIZE
tomcat:8.5.21-jre8-alpine       latest              d991231dccdf        4 weeks ago         113MB

Running Docker Image (aka creating container)​

docker run -p 8080:8080 tomcat:8.5.21-jre8-alpine

Port mapping​

In docker run command, its important to do port mapping using -p command. Reason is, Docker container runs on its own Operating system (mostly Linux flavour). So the tomcat mentioned in above run command, Tomcat will be running in 8080 port (default tomcat port) on Linux OS of the Docker container.

But when we try to hit http://localhost:8080, we are hitting only 8080 port of our host machine but not the container's 8080. So its necessary to do port mapping to reach a port from outside the container

To do port mapping, we have passed -p 8080:8080 as argument for the run command. Here the -p argument represents the port to be mapped from Docker container to the Host machine. In -p 8080:8080, 8080 before the colon (:) represents the port number of host system & 8080 after the colon (:)" represents the port in container

Verify Container creation​

Once we execute run command, Docker creates an instance for the Docker image called as the Docker container. Every container instance will have its own id assigned called CONTAINER_ID. The CONTAINER_ID can be verified using the docker ps command

docker ps

CONTAINER ID        IMAGE                       COMMAND             CREATED             STATUS              PORTS                    NAMES
b414970f84c1        tomcat:8.5.21-jre8-alpine   "catalina.sh run"   10 seconds ago      Up 9 seconds        0.0.0.0:8080->8080/tcp   nifty_aryabhata

Adding war file to Tomcat container​

Now Tomcat container is up & running. Its time to copy our WAR file inside Tomcat container.

docker cp demo.war <CONTAINER_ID>:/usr/local/tomcat/webapps

For the docker cp command, we need the <CONATINER_ID> which we can use from output of docker ps command

docker cp demo.war b414970f84c1:/usr/local/tomcat/webapps

Once the cp command is succesful, you can notice in logs that Tomcat container delploying demo.war automatically & start the application

If you want to see the logs separately 

docker logs -f <CONATAINER_ID>

So thats all about running WAR file with Tomcat Docker Insance using Docker commands.

But do we need to remember all these commands & execute them one by one to get the job done ? Is there any simple alternative to create docker images ? Yes, Fortunately, we have Dockerfile to automate these steps