1️⃣ *Why Terraform? *

Terraform is used to automate cloud infrastructure so humans don’t manually create:

• VPCs
• Subnets
• Security Groups
• ECS clusters
• Task definitions
• Load balancers
• IAM roles
• RDS
• S3
• DynamoDB
• Secrets
• ECR registries
• Route53
• CloudWatch alarms

In big companies:

Without Terraform

• Engineers click around the console
• No history
• No review/tracking
• Accidental misconfiguration
• Hard to reproduce environments
• Hard to recover after failure
• Hard to scale
• Impossible to maintain dozens of environments

With Terraform

• Everything is code
• The entire infrastructure is repeatable
• You can recreate the entire system from scratch
• Teams use Git history, PR reviews, CI/CD pipelines
• One command updates all cloud resources correctly

Your project has:

• 7 microservices (Python)
• ECS cluster
• ALB
• Couchbase
• Confluent Cloud
• PostgreSQL/RDS
• VPN / VPC
• Kafka Brokers
• Lambda (optional)
• S3 state backend

This is exactly what Terraform is built for.

2️⃣ Terraform Core Concepts EVERY Senior DevOps Must Know

(A) Terraform is:

✔️ Declarative

You describe WHAT you want, Terraform decides HOW to build it.

(B) Terraform Files

Your project has these:

In an interview you MUST be able to explain why each file exists.

3️⃣ Terraform State — What Companies Expect You to Know

Terraform keeps a “memory” of everything it created in:

terraform.tfstate

This file contains:

• All AWS resource IDs
• Dependencies between resources
• Current configuration
• Attributes of each resource

🚨 This file is critical — losing it is catastrophic.

That’s why no one stores tfstate locally in enterprises.

Instead, you stored it in:

✔️ S3 → stores the .tfstate

✔️ DynamoDB → manages locks

4️⃣ Why S3 Backend? (Your Project Example)

Your project has:

• GitHub Actions
• Local development
• Different machines
• Multiple terraform apply executions

If each machine stored local tfstate, then:

• One apply would overwrite another
• GitHub cannot see local changes
• Terraform gets confused
• Many resources would duplicate or destroy incorrectly

S3 fixes all of that:

✔️ One central state for all Terraform runs

✔️ GitHub Actions + your laptop read the same state

✔️ Safe to delete local terraform.tfstate

✔️ State is versioned → rollback possible

5️⃣ Why DynamoDB Lock? (Your Exact Issue Today)

Terraform must prevent two applies at the same time.

So when you run:

terraform plan
terraform apply

Terraform writes a lock into DynamoDB:

LockID = kafka-enterprise-orders-tfstate/terraform.tfstate

If GitHub Actions tries to run while your local run is running → lock prevents corruption.

If the previous run crashed → the lock stays forever → you must delete it manually.

This happened to you today.

6️⃣ Terraform Apply Flow (Your Project Real Flow)

Your CI/CD pipeline does:

STEP 1 — terraform init

• Downloads AWS provider
• Reads backend config
• Connects to S3
• Connects to DynamoDB

STEP 2 — terraform plan

• Compares desired infrastructure (code)
• With actual infrastructure (AWS)
• Shows changes

STEP 3 — terraform apply

• Creates ECS task definitions
• Updates ECS services
• Updates ALB target groups
• Updates security groups
• Updates subnet associations
• Updates IAM roles
• Updates ECS service connections
• Creates RDS if needed
• Produces output (ALB DNS, SG IDs, etc.)

7️⃣ Terraform Drift Detection (VERY important for senior roles)

Terraform ALWAYS detects drift:

Examples:

Case 1 — A subnet was deleted

You saw:

InvalidSubnetID.NotFound

Terraform sees that AWS changed behind its back → tries to fix it.

8️⃣ Terraform Variables & Secrets (Your Project)

You pass these from GitHub → terraform:

TF_VAR_container_image_producer
TF_VAR_container_image_payment
TF_VAR_container_image_fraud
TF_VAR_container_image_analytics
TF_VAR_web_backend_image
TF_VAR_web_frontend_image
TF_VAR_confluent_bootstrap_servers
TF_VAR_confluent_api_key
TF_VAR_confluent_api_secret
TF_VAR_rds_password
TF_VAR_existing_vpc_id
TF_VAR_existing_public_subnet_ids
TF_VAR_existing_private_subnet_ids
TF_VAR_existing_ecs_tasks_sg_id
TF_VAR_existing_alb_sg_id
TF_VAR_existing_rds_sg_id

This is how code dynamically reads images for each deploy.

When you push code → CI/CD builds images → pushes to GHCR → injects into Terraform → updates ECS.

9️⃣ Terraform in Senior DevOps Interviews — What They Expect

Must Know Topics

✔ Terraform state
✔ Backends (S3, Azure Blob, GCS)
✔ Locking (DynamoDB)
✔ Modules
✔ Workspaces
✔ Providers
✔ Data sources
✔ Variables & outputs
✔ Dependency graph
✔ Lifecycle rules
✔ terraform import
✔ terraform taint
✔ terraform graph
✔ CI/CD pipelines
✔ Secrets management

Explain real examples

Use your project:

“Our Terraform manages ECS, ALB, VPC, RDS, SGs, and Kafka resources.
State is centralized in S3 with DynamoDB locking to avoid concurrency issues.
GitHub Actions injects images built in CI into ECS task definitions via TF_VAR_ variables.”

This is exactly what senior engineers say.

🔟 Why Terraform Is Required in *Your Project*

Because you have 17+ dependent AWS resources, and these must update together:

• Change a container image → ECS tasks update
• Change port → ALB target group updates
• Change AWS region → VPC + subnets + RDS must recreate
• Change Kafka clusters → environment variables update
• Change security groups → ECS & ALB update

Terraform guarantees everything deploys in the right order.

1️⃣1️⃣ How Terraform Works Internally (“Graph Theory”)

Terraform builds a dependency graph:

aws_vpc -> subnets -> route tables -> SG -> ALB -> ECS cluster -> ECS services -> tasks

Terraform then executes:

• parallel where possible
• sequential where required

1️⃣2️⃣ Your ECS Errors Today — Terraform Detecting Infra Issues

⛔ Invalid security group

You passed "***" in secrets
Terraform rejected it
Corrected.

⛔ Subnet deleted manually

Terraform warned you
You fixed by updating correct subnet IDs.

1️⃣3️⃣ Terraform CI/CD — Your Pipeline

Your GitHub Actions does:

1️⃣ Build Docker images
2️⃣ Push to GitHub Container Registry
3️⃣ Run Terraform
4️⃣ Update ECS
5️⃣ New containers deploy instantly

This is enterprise-grade.