1. Traditional Deployment Issues

• Physical Servers:
  • No resource boundaries for applications.
  • Multiple apps on one server could lead to resource hogging, causing underperformance.
  • Solution: Run each app on a separate server.
  • Problem: Underutilized resources, expensive to maintain multiple servers.

2. Virtualization

• Definition: Creation of virtual versions of OS, servers, storage, or network resources using software to simulate hardware.
• Benefits:
  • Run multiple OS and apps on a single server.
• Types:
  • Network Virtualization: Divides bandwidth into secure, assignable channels for improved speed, availability, and security.
  • Storage Virtualization: Pools multiple storage devices into a single, centrally managed device.
    • Benefits: Remote storage appears local, combines smaller volumes, improves reliability/performance, high availability, disaster recovery.
  • Data Virtualization: Aggregates data from various sources into a single, logical view.
    • Benefits: Abstracts technical details (location, structure), connects multiple sources, federates data, delivers as requested.
  • Desktop Virtualization: Isolates desktop OS from endpoint, enables remote access, supports shared hosted desktops (e.g., Microsoft Remote Desktop Services).
  • Application Virtualization: Runs apps separately from the accessing device.
  • Others: Hardware, OS, Containerization.

3. Docker Containers

• Definition: A running instance of a Docker image, containing one or more processes in a self-contained, isolated environment.
• Key Points:
  • Wraps application, dependencies, networking, and volumes.
  • Cannot modify the original image.
  • Stored in /var/lib/docker/containers.
• Basic Operations:
  • Create, start, run, list (running/all), inspect, stop, delete containers.
• Attaching to Containers:
  • Attach: Connects to a container with one input/output stream.
  • Exec: Runs a command inside a container (e.g., docker exec <container> <command>).
• Hostname and Naming:
  • Hostname: First 12 characters of container ID (check with hostname inside container).
  • Name: Randomly generated by Docker.
• Port Publishing:
  • Required for external access to container apps.
  • Set at container creation (e.g., docker run -p 8080:80 httpd).
  • Cannot update ports on a running container.

4. Docker Images

• Definition: Read-only instructions to run containers, composed of layers.
• Storage: Held in repositories within a Docker registry (e.g., Docker Hub).
• Layered File System:
  • Uses UnionFS; updates add new layers.
  • Changes in a running container are written to a writable layer.
  • Example: Base (Ubuntu) → Update apt cache → Apache (httpd).
• Creating Custom Images:
  • Commit: Save changes from a running container (e.g., create directory/file, then commit).
  • Dockerfile: Define image with instructions.
• Dockerfile Instructions:
  • FROM, ENV, RUN, CMD, EXPOSE, WORKDIR, ADD, COPY, LABEL, MAINTAINER, ENTRYPOINT.
  • Each instruction adds a layer, processed top-to-bottom.

5. Container Orchestration