DockerとK8sによるコンテナ化

Welcome to this lesson on Containerization with Docker & Kubernetes! In modern cloud environments, packaging and managing applications efficiently is key.

You'll learn how Docker containers bundle your apps and how Kubernetes orchestrates them at scale, making your systems robust and flexible.

Imagine a tiny, self-contained box for your application. That's a container!

• Isolated: Each container runs its own app and dependencies, separate from others.
• Portable: Works the same way on any machine with a container runtime.
• Lightweight: Shares the host OS kernel, unlike heavier Virtual Machines (VMs).

This consistency solves the "it works on my machine" problem.

Docker is the most popular platform for building, sharing, and running containers. It provides the tools to create these isolated environments.

• Docker Engine: The core software that runs and manages containers.
• Docker Image: A read-only template with instructions for creating a container.
• Docker Container: A runnable instance of a Docker image.

A Dockerfile is a plain text file that contains all the commands a user could call on the command line to assemble an image.

It's like a recipe for your container. Here are some common instructions:

• FROM: Base image (e.g., Python, Node.js).
• WORKDIR: Sets the working directory.
• COPY: Copies files from your project into the image.
• RUN: Executes commands (e.g., install dependencies).
• CMD: Default command to run when the container starts.

Let's look at a very simple Python application that we could containerize. This is the code that would live inside your Docker container.

Try running this example:

Once you have a Dockerfile and your application code (like the Python script), you can build an image and run a container.

• Build Image: docker build -t my-python-app . (-t tags it, . uses current directory for Dockerfile)
• Run Container: docker run my-python-app

This creates and starts a container based on your image, executing the app inside.

Running one container is easy. Running hundreds or thousands across many servers is hard! This is where Kubernetes (K8s) comes in.

K8s is an open-source system for automating deployment, scaling, and management of containerized applications. It acts as an "orchestrator" for your containers.

In Kubernetes, the smallest deployable unit is a Pod. A Pod is an abstraction over containers.

• A Pod represents a single instance of an application.
• It typically contains one (or a few tightly coupled) containers.
• Pods share network and storage resources.
• They are ephemeral: if a Pod dies, K8s creates a new one.

Manually managing Pods is cumbersome. Deployments are a higher-level object in K8s that manage the lifecycle of Pods.

A Deployment allows you to:

• Declare the desired number of replica Pods.
• Perform rolling updates to new versions without downtime.
• Roll back to previous versions if issues arise.
• Ensure that a specified number of Pods are always running.

Which of the following statements correctly describe the benefits of using containers and container orchestration?

You've taken a big step into cloud-native development!

• Containers package apps and dependencies into isolated, portable units.
• Docker is the popular tool to build and run these containers.
• Kubernetes orchestrates containers at scale, automating deployment, scaling, and management.

Together, they form the backbone for building resilient, scalable modern applications.