Pod vs Container vs Deployment

Pod:

A Pod is the smallest deployable unit in Kubernetes. It represents a single instance of a running process in a cluster.

Example Code:

apiVersion: v1
kind: Pod
metadata:
  name: my-pod
spec:
  containers:
    - name: main-app
      image: nginx:latest
    - name: sidecar
      image: busybox:latest

Diagram:

  +----------------+
  |     my-pod     |
  |                |
  | +------------+ |     +-------------+
  | | main-app   | |     |   sidecar   |
  | | Container  | |     |  Container  |
  | |            | |     |             |
  | +------------+ |     +-------------+
  +----------------+

In this example, we have a Pod named my-pod that contains two containers: main-app and sidecar. These containers share the same network namespace and can communicate with each other using localhost. They are tightly coupled within the same Pod.

Container:

A Container is a lightweight, standalone executable package that includes everything needed to run a piece of software.

Example Code:

Dockerfile for a simple Node.js application:

# Use an official Node.js runtime as the base image
FROM node:14

# Set the working directory in the container
WORKDIR /app

# Copy package.json and package-lock.json to the container
COPY package*.json ./

# Install application dependencies
RUN npm install

# Copy the rest of the application code to the container
COPY . .

# Expose a port for the application to listen on
EXPOSE 8080

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

Deployment:

A Deployment is a higher-level Kubernetes resource used for managing and scaling applications. It provides declarative updates to applications and manages the rollout of changes.

Example Code:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: my-deployment
spec:
  replicas: 3
  selector:
    matchLabels:
      app: my-app
  template:
    metadata:
      labels:
        app: my-app
    spec:
      containers:
        - name: my-app-container
          image: my-app:latest

Diagram:

  +------------------------+
  |      my-deployment     |
  |                        |
  | +------- Pod 1 -------+ |      +------- Pod 2 --------+      +------- Pod 3 -------+
  | |    my-app-container  | |     |    my-app-container  |     |    my-app-container  |
  | |     Container        | |     |     Container        |     |     Container        |
  | |                      | |     |                      |     |                      |
  | +----------------------+ |     +----------------------+     +----------------------+
  +------------------------+

In this example, we have a Deployment named my-deployment that manages three replica Pods. Each Pod contains a container running the my-app application. The Deployment ensures that the desired number of replicas (in this case, 3) is running, and it can automatically replace failed Pods or scale the application horizontally.