Docker: What Are Containers and Their Day-to-Day Functionalities
Tags: docker, beginners, tutorial, devops
Published: 2024-02-26 18:07:36

In the tech world, efficiency and agility in the development, deployment, and management of applications are crucial. Docker has revolutionized how developers, operators, and companies approach software development, understanding that software delivery is part of a whole ecosystem. Launched in 2013 by Solomon Hykes and the DotCloud team (now Docker Inc.), Docker quickly became the preferred open-source containerization platform. Maintained by Docker Inc. and an active contributing community, this article examines Docker containers and how their features streamline the software development lifecycle.

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**What are Containers?**
Containers are a form of lightweight virtualization that allows applications and their dependencies to run in isolated processes. Unlike traditional virtual machines that virtualize an entire operating system, containers share the host operating system's kernel but operate in isolated spaces. This results in faster startups and reduced resource use, making containers an efficient choice for deploying and scaling applications.

_Docker's Day-to-Day Functionalities_

Docker simplifies and automates the process of packaging, distributing, and managing containerized applications. 

**Here are some of its most impactful daily functionalities**:

- **Portability**
A major advantage of Docker containers is portability. An application packaged in a Docker container can run on any system that supports Docker, regardless of the development environment, eliminating the "it works on my machine" problem.

- **Consistency and Isolation**
Docker ensures that the application runs in a consistent and isolated environment, regardless of where the container is deployed, allowing developers to focus on application logic without concern for the execution environment's specifics.

- **Rapid Development and Deployment**
With Docker, containers can be created in seconds, significantly accelerating the development and deployment cycle. Additionally, the ability to mirror production environments locally allows for more accurate and efficient testing.

- **Scalability and Management**
Docker facilitates application scalability, enabling containers to be quickly replicated or scaled down as demand varies. Container orchestration tools like Kubernetes enhance Docker by providing powerful solutions for managing containers at scale.

- **Ecosystem and Community**
Docker has an extensive ecosystem and an active community. Docker Hub is a repository offering a wide variety of ready-to-use container images, which can further speed up application development. Moreover, the Docker community is an excellent knowledge resource, sharing best practices and solutions to common challenges.

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> To better understand the topic discussed in the article, we will look at a more practical part of how to prepare an environment to use Docker and how to apply it practically.

_Step-by-Step for Docker Installation_

Before diving into creating a Docker image, it's essential to have Docker installed on your system. Here's a simplified guide to installing Docker on different operating systems:

**For Windows users**:
Visit the official Docker website and download Docker Desktop for Windows.
Run the downloaded installer and follow the on-screen instructions.
After installation, open Docker Desktop to start the Docker service.
Open a terminal and type `docker --version` to check if Docker was installed correctly.

**For Mac users**:
Download Docker Desktop for Mac from the official Docker website.
Open the downloaded .dmg file and drag Docker into the Applications folder.
Start Docker from the Applications folder.
Open a terminal and type `docker --version` to confirm the installation.

**For Linux users**:
Installation on Linux varies by distribution. Here are the commands for Ubuntu:

Open a terminal and update the package index with `sudo apt-get update` and install the necessary packages to allow the use of a repository over HTTPS.

```
sudo apt-get install \
 apt-transport-https \
 ca-certificates \
 curl \
 software-properties-common
```

Add the official Docker repository key to ensure the authenticity of the software being installed.
`curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo apt-key add -
`

> This step is crucial for security and integrity, allowing your system to verify that the packages you're installing are the ones officially published by Docker. This process typically involves retrieving the key from a Docker server and adding it to your system's list of trusted keys. Each Linux distribution has its specific command to accomplish this, so it's important to follow the instructions tailored to your particular system.


After updating the package index, you can proceed to install Docker:

`sudo apt-get update
sudo apt-get install docker-ce`

To verify the installation of Docker, open a terminal and type `docker --version`.


#Creating and Distributing a Docker Image for a Fictional Application "**XPTO**";

Now that Docker is installed, let's create a Docker image for a fictional application named "**XPTO**".

**Step 1**: Prepare the Dockerfile

1. Create a directory for your **XPTO **application.
2. Within this directory, create a file named Dockerfile. This file will describe the steps to create your application's image.
3. Add the following content to the Dockerfile (adjust as needed for your application):

> Note: Specific Dockerfile content was not provided, so this is a general guideline on creating and setting up a Dockerfile for a Docker image.

```
# Use an official base image, for example, Node for a Node.js application
FROM node:14

# Set the working directory in the container
WORKDIR /app

# Copy the package.json file and install dependencies
COPY package.json ./
RUN npm install

# Copy the rest of the application files
COPY . .

# Expose the port that your application will listen on
EXPOSE 3000

# Command to run the application
CMD ["node", "app.js"]

```

**Step 2**: Build the Docker Image

Navigate to your **XPTO **application directory in the terminal and execute the following command to build the Docker image:

`docker build -t xpto-app . `

**Step 3**: Run the Image as a Container
 
After building the image, you can start a container using your image with the command:

`docker run -p 3000:3000 xpto-app`

**Step 4**: Distribute the Image

Tag your image to prepare it for upload. Replace `youruser` with your Docker Hub username and `xpto-app` with the name of the image you wish to upload:

`docker tag xpto-app youruser/xpto-app:latest`

Push the image to Docker Hub:

`docker push seuusuario/xpto-app:latest` 

After this command, your image will be available on Docker Hub and can be downloaded and run anywhere with Docker installed, using the command:

`docker pull seuusuario/xpto-app:latest`

These steps outline the process of preparing a Docker image of a fictional "**XPTO**" application for use, from building the image to running it as a container, and finally distributing it through Docker Hub.

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Docker and container technology have revolutionized how applications are developed, deployed, and managed. By offering portability, consistency, efficiency, and ease of use, Docker has become an essential tool for developers and businesses alike. Whether you're a beginner or an experienced professional, integrating Docker into your workflow can provide significant benefits, making the software development process faster, more reliable, and scalable.

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## Bonus

Rollback Strategies Using Tags in Docker Images
In software development, having an effective rollback plan is crucial for version management and production environment stability. In Docker, image tags function similarly to version control system commits, providing a means to version and revert to previous application states. To utilize rollback strategies with Docker, it's good practice to tag every Docker image build specifically, in addition to the 'latest' tag. This approach helps in identifying and reverting to specific image versions if needed.

When building your image, use the `docker build` command with the `-t` flag followed by the image name, a colon, and the desired tag. For example, to version your **XPTO** application, you might use:

`docker build -t xpto-app:v1.0.0 .`

> Note: The specific command for tagging an image was not provided, indicating a placeholder for where users should input their versioning scheme.

To distribute versioned images after building them with a specific tag, you push them to Docker Hub or another container registry using the command:

`docker push xpto-app:v1.0.0`

---

Enhancing the material with Docker's origins and including rollback strategies through tagging Docker images, akin to version control commits, enriches the narrative. This updated introduction and the bonus on rollback strategies emphasize Docker's significance in software development for managing versions and ensuring production stability. Docker tags not only facilitate versioning but also offer a streamlined approach to reverting applications to previous states, underscoring Docker's utility in continuous integration and deployment pipelines.