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DevOps Foundations and Docker Notes

1. DevOps Foundations

DevOps is the practice of connecting development and operations so software can be delivered reliably, repeatedly, and with less friction.

Servers

  • A server is a machine or process that responds to requests.
  • It can host APIs, databases, static files, or full applications.
  • In deployment contexts, you usually care about CPU, memory, disk, network, and uptime.

Load Balancers

  • Load balancers distribute traffic across multiple servers.
  • They improve availability, scale, and resilience.
  • Common strategies include round robin, least connections, and IP-based routing.

DNS Records

DNS maps human-readable names to IP addresses and other routing data.

Common record types:

  • A: maps a name to an IPv4 address
  • AAAA: maps a name to an IPv6 address
  • CNAME: alias from one hostname to another
  • MX: mail exchange record
  • TXT: arbitrary text, often used for verification or security
  • NS: nameserver record

Reverse Proxy

  • A reverse proxy sits in front of backend servers.
  • It receives client requests and forwards them to the appropriate server.
  • Common uses:
    • SSL termination
    • load balancing
    • caching
    • request compression
    • hiding backend topology

2. Docker

Docker packages applications and their dependencies into portable containers.

Images

  • An image is a read-only template used to create containers.
  • Images are built from a Dockerfile.
  • Images can be shared through registries like Docker Hub.

Containers

  • A container is a running instance of an image.
  • Containers are isolated, lightweight, and easy to start or stop.
  • The same image can run many containers.

Volumes

  • Volumes persist data outside a container's writable layer.
  • They are useful for databases, uploads, and any state you do not want to lose when a container is removed.

Dockerfile

A Dockerfile describes how an image should be built.

Common instructions:

  • FROM
  • WORKDIR
  • COPY
  • RUN
  • EXPOSE
  • CMD

docker-compose

  • Docker Compose defines and runs multi-container applications.
  • It is useful for apps with an API, database, cache, and frontend.

Dockerizing a MERN App

Typical flow:

  1. Build a backend image.
  2. Build a frontend image or serve frontend through a web server.
  3. Connect services with Compose.
  4. Use environment variables for config.
  5. Mount volumes for persistent data where needed.

Docker Good Practices

  • keep images small
  • use .dockerignore
  • avoid shipping secrets in images
  • pin versions where practical
  • separate development and production config