Note_Tech

All technological notes.

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Linux - Container: Fundamental

container
- a standard unit of software that includes everything needed to run an application: code, runtime, libraries, dependencies, and configuration.
- isolated from each other and the underlying operating system, making them a preferred method for deploying and managing applications.
Features
- Isolation:
  - Each container runs in its own environment, separate from other containers and the host system.
  - Achieved using technologies like namespaces (process, network, filesystem) and cgroups (resource control).
- Lightweight:
  - Containers share the host operating system’s kernel, avoiding the overhead of a full operating system per container (unlike virtual machines).
- Portability:
  - Containers run consistently across different environments (development, testing, production) since all dependencies are packaged with the application.
- Efficiency:
  - Containers use fewer resources than virtual machines because they don’t need a full OS for each instance.
- Scalability:
  - Containers can be created, stopped, and destroyed quickly, enabling fast scaling of applications.
An OS can run single or multiple containers at the same time.
Roles:
- developer: mostly use containers to write codes, build, and deploy applications
- system administrator: install, congigure, and manage them.

Containers rely on containerization technology, which is a form of operating system-level virtualization.
Key components include:
- Container Engine:
  - Software like Docker, Podman, or containerd that manages containers.
- Container Images:
  - A container runs from an image, which is a static, immutable template containing the application and its dependencies.
    - Example: An Nginx image might include the Nginx server, its dependencies, and configuration files.
- Container Runtime:
  - Executes and manages containers on the host system.
  - Examples: runc, CRI-O, or Docker’s built-in runtime.

Feature	Containers	Virtual Machines
Isolation	Process-level isolation	Full hardware virtualization
Startup Time	Seconds or less	Minutes
Resource Usage	Lightweight (shares host OS kernel)	Heavy (requires full OS for each VM)
Portability	Easily portable across environments	Less portable due to dependence on hypervisor
Use Case	Microservices, cloud-native applications	Legacy applications, full OS environments

Docker:
- not supported in RHEL 8.
- Most popular containerization platform.
- Includes tools for building, running, and managing containers.
Podman:
- default in Redhat
- A daemonless alternative to Docker.
- Designed to work without root privileges for better security.
Kubernetes:
- Orchestrates containers across clusters for high availability, scaling, and management.
CRI-O:
- A lightweight container runtime for Kubernetes.