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Docker

Docker is a platform for containerizing applications. It allows an application to be packaged into an isolated "container" that contains all necessary libraries, configurations, and runtime. Thanks to this, the application works the same on every server, regardless of where it's run.

What Is Docker Used For?

  • creating isolated environments for applications
  • quickly launching services without complex installation
  • software development (every developer has the same environment)
  • running databases, web servers, APIs, or microservices
  • testing different application versions
  • CI/CD pipeline automation (GitLab CI, GitHub Actions)
  • easy replication and scaling of services

Docker significantly reduces the risk of library conflicts and allows running many different applications on one server without them affecting each other.

Docker Advantages

  • extremely fast start (milliseconds to seconds)
  • low resource consumption (shared OS kernel)
  • simple portability – works the same everywhere
  • excellent for DevOps and CI/CD
  • ideal for microservices
  • easy testing without modifying the host system
  • ability to have multiple applications on one server without conflicts

Docker Disadvantages

  • lower isolation than a full virtual machine
  • not suitable for desktop environments or full OS
  • shares OS kernel, so not all applications are compatible
  • doesn't provide automatic scaling or self-healing
  • containers are short-lived if not managed by an orchestrator

Docker is excellent for applications — not for "entire operating systems."

Kubernetes

Kubernetes (K8s) is a container orchestrator. It doesn't create containers like Docker but manages their lifecycle, scaling, availability, and communication.

Simplified:

Docker = packaging and running applications Kubernetes = managing hundreds to thousands of Docker containers

Kubernetes solves problems that Docker alone cannot:

  • automatic restarts on failure
  • application scaling (more containers when load increases)
  • balancing requests between containers
  • self-healing (if a container crashes, the system replaces it)
  • roll-out / roll-back of application versions
  • configuration and secret management
  • networking between services
  • working in a multi-server cluster

Kubernetes is ideal for large, cloud, modern applications — microservices, API systems, scalable services, and CI/CD environments.

How Do Docker and Kubernetes Relate?

Docker enables creating a container. Kubernetes runs, monitors, moves, replaces, scales, and keeps these containers running.

Docker without orchestration wouldn't be suitable for large applications. Kubernetes without Docker (or another runtime) would have nothing to run.

Their connection works like this:

  • Docker creates an image
  • Kubernetes downloads this image
  • Kubernetes runs containers according to definition (Deployment, StatefulSet…)
  • Kubernetes monitors container health
  • When a container crashes, Kubernetes automatically replaces it
  • When load increases, Kubernetes adds more copies

Differences Between Docker and Kubernetes

Area Docker Kubernetes
Primary purpose Application containerization Container orchestration
Execution Individual containers Thousands of containers at once
Control Manual / Compose Automatic, intelligent
Recovery on crash None Self-healing
Scaling Basic Automatic (HPA)
Updates Manual Rolling updates + rollback
Networking Simple Extensive virtual network
Use development, testing, smaller services large systems, production, microservices

When to Use Docker / Kubernetes?

Use Docker if:

  • you want to quickly run an application
  • testing development or libraries
  • running a smaller project
  • need to isolate environment
  • want simple configuration on one server

Use Kubernetes if:

  • need to scale applications
  • have microservices or large APIs
  • want highly available systems
  • need automation, monitoring, self-healing
  • application must run without downtime