Welcome to Metric Tree Labs blog series and in today’s blog, we will be walking you through the basics of docker and how to deploy a node.js app on docker.

WHAT is Docker.

Docker is a service that uses OS-level virtualization to deliver software in packages called containers. Compared to virtual machines that run host operating systems, docker doesn’t need a hypervisor, all it needs is an OS that supports programs, libraries and system resources to run a specific program. And it can run a lot more applications at the same time than what would be possible with a virtual machine.

Installation

• https://docs.docker.com/install/ link to download and install docker.
• Docker –version and docker info to get version and detailed information about your docker installation.
• Run docker run hello-world to test the installation.

Node App

• We have a simple node.js server (app.js) running on port 3000.
• The server exposes a route “/” that responds with a message saying whether the server is running inside a container or not by looking up the environment variable IN_CONTAINER.
• You can start the server by running npm start.
• Visit localhost:3000 on your browser after starting the server.

Dockerfile

• To deploy our server on docker we start by creating a file named Dockerfile.
• Contents of Dockerfile
  • FROM: The FROM instruction initializes a new build stage and sets the Base Image for subsequent instructions.
  • WORKDIR: Sets the current working directory for subsequent commands and instructions
  • COPY: copies new files or directories from <src> and adds them to the filesystem of the container at the path
  • RUN: To run commands
  • ENV: Sets the environment variable
  • CMD: Default command to execute. A Dockerfile can have only one CMD instruction
• We also create a dockerignore file that ignores specified files and folders when copying from our directory to the container.

Creating a Docker image

• To build the docker image run docker build –tag=<name>.
• We can list the images using docker image ls command

Running Containers

• Use the command docker run -p 3000:3000 <image-name> ( -p flag exposes and connects the port 3000 on the container to the system’s port )
• Visit localhost:3000 in your browser and It should now display the message server running inside a container.
• To stop the container press Ctrl+c. Containers can be run in the background using the detached mode by adding the flag -d.
• To view, a list of running containers run docker container ls
• Stop a container by running docker container stop <container-id>

Deleting Images

• Run docker image rm <image-id>
• Run docker image rm <image-id>