✅ 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.

SQL vs NoSQL (Simple Comparison)

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

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

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

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.

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

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.

✅ 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.

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.

✅ 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

✅ 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.

✅ 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

✅ 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

✅ 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

🙌 Subscribe / Follow for More

If you are also learning DevOps + System Design and want beginner-friendly explanations with real examples, please subscribe and follow my blog 🚀

✅ I’ll be posting regularly as I learn and build.

Prerequisites

You must have an Amazon S3 source bucket (for example, ip-tutorial-bucket-1) with videos to be transcoded already stored in it. And Output Bucket (for example,video-op-bucket-2)

Depending on the environment, customer requirements, and marketplace we will use AWS Serverless to deploy Lambda Functions. Currently, MediaConvert has implemented a mechanism that will be invoked when a user uploads a video in the Input Bucket. The Lambda function will then query the MediaConvert service to produce the required video formats and distribution configurations.

• An Apple HLS adaptive bitrate stream for playout on multiple-sized devices and varying bandwidths.
• An MP4 stream
• Thumbnails for use in websites to show a video preview at a point in time.

Step 1: Create an S3 bucket for the output media file

In this step, you'll create an S3 bucket to store the transcoded media files so they can be viewed on any device, then configure CORS rules to allow devices that do not meet the CORS settings to access the files.

1. Create a bucket for the output media files

2. Sign in to the AWS Management Console and open the Amazon S3 console at https://console.aws.amazon.com/s3/.

3. In the left navigation pane, choose Buckets.

4. Choose Create bucket.

5. For Bucket name, enter a name for your bucket (for example, video-op-bucket-2).

6. For Region, choose the AWS Region where you want the bucket to reside.

7. To ensure public access to your output media files, in Block Public Access settings for this bucket, clear Block all public access.

8. Select the check box next to I acknowledge that the current settings might result in this bucket and the objects within becoming public.

9. Keep the remaining settings set to the defaults.

10. Choose Create bucket.

Add a CORS configuration to the S3 output bucket

1. In the Buckets list, choose the name of the bucket that you created earlier (for example,video-op-bucket-2).

2. Choose the Permissions tab.

3. In the Cross-origin resource sharing (CORS) section, choose Edit.

4. In the CORS configuration text box, copy and paste the following CORS configuration.

5. The CORS configuration must be in JSON format. In this example, the AllowedOrigins attribute uses the wildcard character (*) to specify all origins.

    {
        "AllowedOrigins": [
            "*"
         ],
         "AllowedMethods": [
             "GET"
         ],
         "AllowedHeaders": [
             "*"
         ],
         "ExposeHeaders": []

     }
 ]

Choose Save changes.

Step 2: Create an IAM role for MediaConvert

In order for you to use the MediaConvert console to transcode videos in your S3 bucket, you need IAM service roles to grant MediaConvert the ability to read and write video files from and to your source and destination buckets. Related entities, including metadata objects, automatically use these roles, allowing MediaConvert to automatically conform to your S3 access control rules.

To create an IAM role for MediaConvert

Sign in to the AWS Management Console and open the IAM console at https://console.aws.amazon.com/iam/.

In the navigation pane of the IAM console, choose Roles, and then choose Create role.

Choose the AWS service role type, and then choose the MediaConvert service.

Choose the MediaConvert use case for your service. Then choose Next: Permissions. The service has already defined the permissions used by the role. These permissions grant MediaConvert the following permissions:

• Full access to your Amazon S3 resources

• API Gateway invoke full access

Choose Next: Review.

For the Role name, enter a name that describes the purpose of the role. If you use the name MediaConvert_Default_Role, then the MediaConvert console will use this role by default when you run jobs.

On the Summary page, copy the Role ARN (which starts with arn:aws:iam::), and save the ARN for use later.

Review the role, and then choose to Create a role.

Step 3: Create an IAM role for your Lambda function

To batch-transcode videos with MediaConvertand S3, you must create an IAM role in order to connect the roles to MediaConvert and S3 Batch. The correct step is to practice IAM role that grants the Lambda function permissions to access MediaConvert and S3 Batch Operations

1. Create an IAM role for your Lambda function

2. Sign in to the AWS Management Console and open the IAM console at https://console.aws.amazon.com/iam/.

3. In the left navigation pane, choose Roles, and then choose Create role.

4. Choose the AWS service role type, and then under Common use cases, choose Lambda.

5. Choose Next: Permissions.

6. On the Attach permissions policies page, enter AWSLambdaBasicExecutionRole in the Filter policies box. To attach the managed policy AWSLambdaBasicExecutionRole to this role to grant write permissions to Amazon CloudWatch Logs, select the check box next to AWSLambdaBasicExecutionRole.

7. Choose Next: Tags.

8. (Optional) Add tags to the managed policy.

9. Choose Next: Review.

10. For Role name, enter tutorial-lambda-transcode-role.

11. Choose Create role.

12. Type the role name into the filter box on the Roles page which you created

On the Permissions tab, click on the Add Inline Policy link and choose the JSON tab. Copy and paste the following JSON in the Policy Document Box. You will need to edit this policy in the next step to fill in the resources for your application.

    "Version": "2012-10-17",
    "Statement": [
        {
            "Action": [
                "logs:CreateLogGroup",
                "logs:CreateLogStream",
                "logs:PutLogEvents"
            ],
            "Resource": "*",
            "Effect": "Allow",
            "Sid": "Logging"
        },
        {
            "Action": [
                "iam:PassRole"
            ],
            "Resource": [
                "<ARN for MediaConvert_Default_Role>"
            ],
            "Effect": "Allow",
            "Sid": "PassRole"
        },
        {
            "Action": [
                "mediaconvert:*"
            ],
            "Resource": [
                "*"
            ],
            "Effect": "Allow",
            "Sid": "MediaConvertService"
        }
    ]
}

• Choose Review Policy.

• For Name, enter tutorial-lambda-policy.

• Choose Create Policy.

Step 4: Create a Lambda function for converting videos

AWS Lambda will run your code in response to events such as a putObject into S3 or an HTTP request. In this step you'll build the core function that will process videos using the MediaConvert python SDK. The lambda function will respond to putObject events in your S3 source bucket. Whenever a video file is added, the lambda will start a MediaConvert job.

Make sure to configure your function to use the tutorial-lambda-transcode-role IAM role you created in the previous section.

Step-by-step instructions

1. Choose Services then select Lambda in the Compute section.

2. Choose Create function.

3. Choose the Author from scratch button.

4. On the Author from Scratch panel, enter tutorial-lambda-convert in the Function name field.

5. Select Python 3.8 for the Runtime.

6. Expand the Choose or create an execution role.

7. Select Use an existing role.

8. Select tutorial-lambda-transcode-role from the Existing Role dropdown.

9. click on Create function.

1. On the Configuration tab of the tutorial-lambda-convert page, in the function code panel:

2. Select Upload a file from Amazon S3 for the Code entry type Enter the following for the URL: https://rodeolabz-us-west-2.s3-us-west-2.amazonaws.com/vodconsole/lambda.zip.

3. Note that this zip file is simply the convert.py script and the job JSON file provided in this repo that you could zip up yourself if desired.

4. Enter convert.handler for the Handler field.

• On the Environment Variables panel of the tutorial-lambda-convert page, enter the following keys and values:

DestinationBucket = video-op-bucket-2 (or whatever you named your bucket )

MediaConvertRole = arn:aws:iam::ACCOUNT NUMBER:role/MediaConvert_Default_Role

Application = Batch-Transcoding

• On the Basic Settings panel, set Timeout to 2 minutes.

• Scroll back to the top of the page and click on the Save button.

Step 5: Create a S3 Put Event Trigger for your Convert lambda

Step-by-step instructions

1. In the Configuration->Designer panel of the tutorial-lambda-convert function, click on Add trigger button.

2. Select S3 from the Trigger configuration dropdown.

3. Select ip-tutorial-bucket-1 or the name you used for the Source bucket you created earlier.

4. Select PUT for the Event type.

5. Leave the rest of the settings as the default and click the Add button.

Final Step: Test the MediaConvert(ip-tutorial-bucket-1) automation

You can use your own video or use the test.mp4 video included in this folder to test the workflow.

1. Open the S3 console overview page for the ip-tutorial-bucket-1 S3 bucket you created earlier.

2. Select Upload and then choose the file test.mp4 from your computer.

3. Note the time that the upload was completed.

4. Open the MediaConvert jobs page and find a job for the input 'test.mp4' that was started near the time your upload was completed.

Navigate to the MP4 or HLS output and play the test video by clicking on the test.mp4 output http resource.

Summary

In this post, I have shown you how to convert any video file to HLS or MP4 format using MediaConvert and Lambda.