While working on real DevOps projects, I realized one thing:

✅ Deploying an app is not only about commands…
✅ It’s about understanding what happens behind the scenes

So today I revised and wrote short notes on these important concepts:

• HTTP status codes (1xx to 5xx)

• DNS record types

• 3-tier architecture (Frontend → Backend → Database)

• SQL vs NoSQL

• Polyglot persistence (multiple databases in one system)

• Queues (why they are used in system design)

This post is written in a simple beginner-friendly way, but with real-world understanding.

👋 If you found this article helpful and want to follow my DevOps + System Design learning journey*,
connect with me on LinkedIn 👉 [**Ramu Chelloju*](https://www.linkedin.com/in/ramuchelloju/)

3-Tier Architecture (Frontend → Backend → Database)

Most real applications follow 3-tier architecture:

1) Frontend (UI Layer)

• Runs in browser

• React / Angular / HTML CSS JS

• Communicates via APIs

2) Backend (Application Layer)

• Implements business logic

• REST APIs

• Authentication, validation, processing

3) Database (Data Layer)

Stores data.

Common DB options:

• MySQL/PostgreSQL (SQL)

• MongoDB/DynamoDB (NoSQL)

• Redis (Caching / sessions)

• RabbitMQ/Kafka (Queue / streaming)

This separation makes the system scalable and manageable.

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SQL vs NoSQL (Simple Comparison)

There is no “best database”.
We choose based on requirement.

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SQL Databases (Relational)

Examples:

• MySQL

• PostgreSQL

• Oracle

• MS SQL

Features:

✅ Fixed schema (tables, rows, columns)
✅ Strong consistency (ACID)
✅ Best for relational data
✅ Excellent for transactions and banking

Example use case:

• Orders

• Payments

• Billing

• User accounts

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NoSQL Databases

Examples:

• MongoDB

• DynamoDB

• Cassandra

• Redis

Features:

✅ Flexible schema (JSON documents / key-value)
✅ Easy to scale horizontally
✅ Faster for certain workloads
✅ Best for unstructured or semi-structured data

Example use case:

• product catalogs

• activity logs

• recommendations

• caching

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Polyglot Persistence (Modern System Design Concept)

In modern systems, we don’t use only one database.

Instead we use multiple databases in one application based on needs.

Example architecture:

✅ MySQL → core transactional data (users, billing)
✅ Redis → caching + sessions
✅ MongoDB/DynamoDB → product catalog or logs
✅ Elasticsearch → searching and analytics

This approach is called:

✅ Polyglot Persistence

Meaning:

Using multiple data storage technologies within a single application.

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Why Queues Matter (Queue = Decoupling)

A queue is like a buffer that holds messages until they are processed.

Without Queue (Tightly Coupled)

Service A calls Service B directly using REST.

Problem:
If Service B is down → Service A fails → user gets 500 error

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With Queue (Loosely Coupled)

Service A sends message to queue (SQS / RabbitMQ)
Service B will process later when it is available.

✅ Even if Service B is down, messages remain safe
✅ Service B can come back later and process
✅ This improves reliability and scalability

Examples of queues:

• AWS SQS

• RabbitMQ

• Kafka (event streaming)

This is one of the most important system design fundamentals.

✅ Quick DevOps Troubleshooting Commands (Backend Check)

When your backend service is not responding, these are super useful:

Check service status

systemctl status <service-name>

Check which process is running (example Node.js)

ps -ef | grep node

Check which port is listening

netstat -lntp

Check application health endpoint

curl http://localhost:8080/health

In real deployments, these 4 commands are enough to debug many issues quickly.

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✅ HTTP Status Codes (Most Important for DevOps + APIs)

When we call an API, we receive an HTTP response code.
Understanding these codes helps you debug issues faster.

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1xx — Informational

These responses mean: “Request received, continuing…”

Example:

• 100 Continue → server says “you can continue sending request body”

Not used very often in day-to-day DevOps debugging, but good to know.

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✅ 2xx — Success

These mean the request succeeded.

Common 2xx codes:

200 OK

✅ Request succeeded

Example:

• GET request returns data

• Health API returns status

201 Created

✅ Resource created successfully

Example:

• POST request created a new expense record

204 No Content

✅ Request succeeded, but server returns no content

Example:

• DELETE success but no response body returned

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✅ 3xx — Redirection

These mean the resource is available, but at a different location.

301 Moved Permanently

✅ Permanent redirection
Example: HTTP → HTTPS

302 Found (Temporary Redirect)

✅ Temporary redirection
Example: short time redirect to another URL

304 Not Modified

✅ Resource not changed (browser uses cache)

This is why sometimes changes don’t reflect immediately due to caching.

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✅ 4xx — Client Side Errors

These happen due to wrong request from client (frontend/Postman/user).

400 Bad Request

Client request is wrong
Example:

• invalid JSON

• missing required field

401 Unauthorized

Authentication failed / token missing
Example:

• no login token provided

403 Forbidden

User is authenticated but has no permission
Example:

• you are logged in but not allowed to access

404 Not Found

Resource/API endpoint not found
Example:

• wrong API path /api/transacton instead of /api/transaction

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✅ 5xx — Server Side Errors

These happen due to issues in server/backend/database.

500 Internal Server Error

Backend crashed or unhandled error occurred.

Common reasons:

• Node.js exception

• DB connection failure

• wrong environment variables

503 Service Unavailable

Service is down / overloaded.

Example:

• backend is stopped

• load balancer returns 503

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✅ DNS Record Types (Most Common and Practical)

DNS controls how domain names map to services.

NS Record — Name Server

NS tells who manages your domain DNS.

Example:

• Route53 gives NS records

• You update them at domain registrar

SOA — Start of Authority

Contains domain zone info like:

• primary nameserver

• admin email

• serial number

Mostly maintained automatically.

A Record — IPv4 address mapping

Maps domain → IPv4 address

Example:

learndev.in → 3.110.xx.xx

AAAA Record — IPv6 mapping

Same as A record but for IPv6.

MX Record — Mail exchange

Used for email routing.

Example:

• Gmail setup

• Zoho mail setup

TXT Record

Used for verification and security.

Examples:

• domain ownership verification

• SPF / DKIM records for email security

CNAME Record — Alias

Maps one name to another name.

Example:

blog.learndev.in → hashnode-host-domain

What I’m Going to Do Next (My Learning Plan)

From today onwards, I’m going to start System Design concepts from the beginning and write blogs in a simple easy-to-understand way.

Upcoming topics:

• Load balancer

• Caching

• Database indexing

• Rate limiting

• CDN

• Queue vs Stream

• Microservices basics

• Scalability concepts

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