Building a Docker and Kubernetes-like System from First Principles

Containers and orchestration are at the heart of modern software development. Tools like Docker and Kubernetes help developers deploy, manage, and scale their applications. But what exactly do these tools do? Let’s build a simple version of them from first principles in a way anyone can understand!

1. What Problem Are We Solving?

Imagine you have a game you made, but your friend’s computer has a different setup. Maybe it’s missing some libraries or runs a different operating system. You could package everything your game needs, including its environment, into a "box" so it runs the same way anywhere. That "box" is called a container.

Now imagine you’re a game studio with 100 games. You need a system to manage all these games running on different computers, ensuring they don’t crash or overload the machines. That’s where orchestration comes in.

2. What Is a Container?

A container is like a mini-computer inside your computer. It has:

• Its own filesystem (all the files it needs).


• Its own memory (independent of other apps).


• Its own CPU usage (won’t hog resources).

But it’s not a real computer—it uses your computer’s hardware in a smart way, saving space and speed.

Building a Basic Container System

Let’s break it down into three steps:

Step 1: Isolate the Application

To create a container, you need to isolate your application from the rest of the system. Imagine you want to run a game in a "protected room" where it can’t mess up the house.

Here’s pseudocode for isolating an app:

function create_container(app):

    room = new_virtual_room()  # Create a protected space

    copy_app_files(app, room)  # Put the app and its files in the room

    set_limited_resources(room, cpu=1_core, memory=512_MB)  # Control usage

    return room

Here:

• 
  new_virtual_room simulates creating a small, isolated environment.


• 
  copy_app_files ensures the app has all its dependencies.


• 
  set_limited_resources prevents one app from using all your computer's power.

Step 2: Run the Application in Isolation

Once isolated, you need to run the app in its own "room."

function run_in_container(room, app_command):

    enter_virtual_room(room)  # Enter the isolated space

    execute(app_command)      # Run the app

Step 3: Simulate Filesystem and Networking

Inside the container, it should feel like its own mini-world. You can simulate this by giving each container its own "fake filesystem" and "fake network."

function setup_fake_filesystem(room):

    filesystem = create_virtual_filesystem()

    attach_to_room(room, filesystem)



function setup_fake_network(room):

    network = create_virtual_network()

    attach_to_room(room, network)

Now, when your app runs, it thinks it’s in a real computer but is actually using virtualized resources.

3. What Is Orchestration?

If containers are like mini-computers, orchestration is like being the manager of a data center full of these mini-computers.

Basic Orchestration Goals

1. 
   Start/Stop Apps Automatically: If a container crashes, restart it.


2. 
   Distribute Work: Spread apps across multiple computers.


3. 
   Scale Up/Down: Add or remove containers based on need.

Building a Simple Orchestrator

Here’s how to make an orchestrator step-by-step:

Step 1: Keep Track of Containers

An orchestrator needs a list of all containers and their status.

containers = []  # A list to store all running containers



function add_container(container):

    containers.append(container)



function get_status(container):

    return check_health(container)  # Is it running fine?

Step 2: Restart Crashed Containers

Let’s make sure a crashed container is restarted:

function monitor_containers():

    for container in containers:

        if get_status(container) == "crashed":

            restart(container)



function restart(container):

    stop(container)

    start(container)

Step 3: Distribute Work

If you have multiple computers, you can spread containers across them:

function distribute_containers(apps, computers):

    for app in apps:

        best_computer = find_least_busy_computer(computers)

        container = create_container(app)

        run_on_computer(best_computer, container)

Step 4: Scale Up or Down

Imagine a game gets super popular and needs more containers to handle traffic. You can scale it automatically:

function scale_containers(app, desired_count):

    current_count = count_running_containers(app)



    if current_count < desired_count:

        for i in range(desired_count - current_count):

            container = create_container(app)

            run(container)



    elif current_count > desired_count:

        for i in range(current_count - desired_count):

            stop_one_container(app)

4. Tying It All Together

Let’s see how everything works together:

1. 
   Containers let you run apps in isolated environments.


2. An Orchestrator ensures apps are running smoothly, distributes workloads, and adjusts capacity.

Real-Life Examples

• 
  Docker is a popular tool for creating containers.


• 
  Kubernetes is a powerful orchestrator used by companies like Google, Netflix, and Facebook.

Why Are These Tools Important?

Containers and orchestrators are like Lego bricks for developers. They make it easier to:

• Build applications that work everywhere.


• Handle traffic spikes (like when a game goes viral).


• Keep apps running 24/7 without manual intervention.

Key Features

Conclusion

Now you know how Docker and Kubernetes-like systems work from first principles! You’ve seen how to isolate applications, simulate resources, and manage containers at scale. With these concepts, you’re ready to explore the exciting world of cloud computing and containerization.

Happy coding! 🚀