DevOps Configuration & Deployment Basics: YAML, DNS, SSL, HTTP and Automation

YAML (YAML Ain't Markup Language) is a human-readable serialisation format used almost universally for DevOps configuration: Kubernetes manifests, Docker Compose files, GitHub Actions workflows, GitLab CI pipelines, Ansible playbooks, Helm charts, and more. YAML's indentation-based structure is readable — but also its primary source of bugs. See how YAML compares to JSON →

services:
  web:
    image: nginx:alpine
    ports:
      - "80:80"
    environment:
      - NODE_ENV=production

Key rules: Indentation uses spaces only (never tabs). Each level adds two or four spaces consistently. Colons followed by a value require a space after the colon. Strings containing special characters (:, #, {}, []) must be quoted.

Validate with: YAML Validator — reports syntax errors with line and column numbers, and normalises indentation.

DNS (Domain Name System) translates domain names into IP addresses and other routing information. Every domain has a set of records stored at authoritative nameservers. When you deploy a new service or change hosting, you update DNS records — but they propagate gradually because resolvers cache results for the record's TTL (time-to-live).

Check live DNS with: DNS Record Validator — queries Cloudflare's DNS-over-HTTPS (1.1.1.1) and returns all record types for any domain.

An SSL/TLS certificate is a cryptographic document that proves a server's identity and enables encrypted connections. Every HTTPS connection starts with a TLS handshake in which the server presents its certificate; the browser checks that it is valid, unexpired, signed by a trusted CA, and covers the requested hostname via a Subject Alternative Name (SAN).

Certificates have a finite validity window — Let's Encrypt issues certificates valid for 90 days; commercial CAs issue 1-year certificates. An expired certificate causes immediate browser errors and broken API calls. A certificate that does not cover the correct hostname (missing SAN) causes the same error even if it is otherwise valid.

Inspect live certificates with: SSL Certificate Validator (check hostname, expiry, SANs, chain). Decode a raw PEM with: SSL Certificate Decoder.

HTTP headers are key-value pairs sent with every request and response. They carry critical metadata: the Content-Type of the payload, Authorization tokens, Cache-Control directives, Access-Control-Allow-Origin for CORS, and security headers like Strict-Transport-Security.

Cookies are sent as Cookie and Set-Cookie headers. Each cookie has a name, value, and optional attributes: Domain, Path, Expires, Secure, HttpOnly, and SameSite. Misconfigured cookie attributes are a common source of session bugs and security vulnerabilities.

Parse in your browser: Query String Parser · Cookie Parser · API Response Formatter

Cron is a Unix scheduler that runs commands at specified times. A cron expression is a string of five space-separated fields: minute (0–59), hour (0–23), day of month (1–31), month (1–12), and day of week (0–7, where 0 and 7 both mean Sunday).

* * * * *         # Every minute
0 9 * * MON-FRI   # 9 AM on weekdays
*/15 * * * *      # Every 15 minutes
0 0 1 * *         # Midnight on the 1st of each month
0 2 * * 0         # 2 AM every Sunday

Validate and preview with: Cron Expression Validator — explains the schedule in plain English and shows the next five run times.

The robots.txt file at a domain's root tells web crawlers which paths they are permitted or forbidden to index. It uses a simple directive syntax: User-agent, Allow, Disallow, and optionally Sitemap. robots.txt is advisory — it relies on crawlers following the standard voluntarily.

An XML sitemap is a machine-readable index of URLs you want search engines to discover. It lists page URLs with optional metadata (last modified date, priority, change frequency). Sitemaps are submitted to Google Search Console and Bing Webmaster Tools and referenced from robots.txt.