🟢 BEGINNER LEVEL
1. What is a Terraform module?
Answer:
A Terraform module is a reusable set of Terraform configuration files that defines infrastructure resources. It allows us to write code once and reuse it across multiple environments or teams.
2. What problem do modules solve?
Answer:
Modules solve:
- code duplication
- lack of standardization
- maintenance complexity
Without modules, teams copy-paste the same resources. With modules, we follow the DRY principle (Don’t Repeat Yourself).
3. What is the DRY principle?
Answer:
DRY means:
Do not repeat the same logic multiple times.
In Terraform, modules implement DRY by allowing reusable infrastructure code.
4. What is a root module?
Answer:
The root module is the main Terraform configuration where we run commands like:
terraform init
terraform apply
It calls other modules.
5. What is a child module?
Answer:
A child module is any module that is called from another module (usually the root module).
6. How do you call a module?
Answer:
module "ec2" {
source = "../modules/ec2"
}
7. What is source in module?
Answer:
source defines where the module code is located:
- local path
- Git repository
- Terraform Registry
8. What files are inside a module?
Answer:
Typically:
-
main.tf→ resources -
variables.tf→ inputs -
outputs.tf→ outputs
🟡 INTERMEDIATE LEVEL
9. How do modules communicate?
Answer:
Modules communicate using:
- outputs (from one module)
- variables (input to another module)
Example:
subnet_id = module.vpc.subnet_id
10. What are module inputs?
Answer:
Inputs are variables passed into a module.
Defined in:
variable "instance_type" {}
11. What are module outputs?
Answer:
Outputs are values returned by a module.
Example:
output "vpc_id" {
value = aws_vpc.main.id
}
12. Can a module call another module?
Answer:
Yes. This is called nested modules.
13. What is a local module?
Answer:
A module stored locally:
source = "../modules/ec2"
14. What is a remote module?
Answer:
A module stored remotely:
- GitHub
- Terraform Registry
Example:
source = "terraform-aws-modules/vpc/aws"
15. Where are modules stored after terraform init?
Answer:
Inside:
.terraform/modules/
16. What happens during terraform plan with modules?
Answer:
Terraform:
- reads module code
- expands it into resources
- shows what will be created
Even if resources are not written in root module.
17. Why use modules in real-world projects?
Answer:
- reuse code
- enforce standards
- reduce errors
- simplify onboarding
- faster infrastructure deployment
🔵 ADVANCED LEVEL
18. How do you version Terraform modules?
Answer:
Using Git tags:
source = "git::https://github.com/org/module.git?ref=v1.0.0"
19. Why is versioning important?
Answer:
- prevents breaking changes
- ensures stable deployments
- allows controlled upgrades
20. What is the difference between module and resource?
Answer:
| Module | Resource |
|---|---|
| reusable | single component |
| multiple resources | one resource |
| abstraction | implementation |
21. When should you NOT use modules?
Answer:
- very small one-time config
- simple demo scripts
22. What is a public module?
Answer:
A module from Terraform Registry:
registry.terraform.io
23. How do you choose a good module?
Answer (IMPORTANT):
- high downloads
- active GitHub repo
- many contributors
- good documentation
- version history
24. Why avoid random modules?
Answer:
- security risks
- unmaintained code
- breaking changes
25. What do companies do with public modules?
Answer:
- fork them
- customize
- maintain private versions
🔴 PRODUCTION / REAL-WORLD QUESTIONS
26. How do you structure Terraform in big companies?
Answer:
infra-modules/
infra-live/
-
infra-modules→ reusable code -
infra-live→ environment configs
27. Why separate modules and environments?
Answer:
- avoids duplication
- easier updates
- version control
- scalability
28. What is a typical module design in production?
Answer:
Small, focused modules:
- VPC module
- EC2 module
- ALB module
- RDS module
29. What is a bad module design?
Answer:
One big module:
- VPC + EC2 + RDS + ALB together
Problems:
- not reusable
- hard to maintain
- hard to debug
30. How do teams use modules?
Answer:
Platform team:
- builds modules
Application teams:
- consume modules
31. How do you pass environment-specific values?
Answer:
Using:
terraform.tfvars- variables
32. How do you avoid hardcoding?
Answer:
Use:
- variables
- data sources
- SSM / secrets
33. How do you handle AMI in production?
Answer:
- data source
- SSM parameter
- golden AMI
34. How do you enforce standards using modules?
Answer:
- restrict inputs
- define defaults
- enforce tagging
- enforce security rules
35. What is module composition?
Answer:
Combining multiple modules together:
module "vpc"
module "ec2"
module "alb"
36. How do you scale modules?
Answer:
- use
count/for_each - parameterize everything
- avoid hardcoding
37. How do you test modules?
Answer:
terraform validateterraform plan- test in dev
- CI pipelines
38. What is the biggest mistake engineers make with modules?
Answer:
- hardcoding values
- making modules too large
- not using outputs
- no versioning
🟣 SCENARIO QUESTIONS (VERY IMPORTANT)
39. You have 50 teams creating EC2 instances differently. What do you do?
Answer:
Create a centralized EC2 module and enforce its usage to standardize infrastructure.
40. You need to update instance type across all environments. How?
Answer:
Update module once → all environments inherit change.
41. Your module breaks after update. Why?
Answer:
No version pinning → module changed unexpectedly.
42. Multiple environments need different configs. Solution?
Answer:
Use:
- separate env folders
- tfvars files
43. How do you securely manage modules?
Answer:
- private Git repos
- code reviews
- restrict access
- scan code
🧠 PERFECT INTERVIEW ANSWER (USE THIS)
If they ask:
“Explain Terraform modules in real-world”
Answer:
Terraform modules are reusable infrastructure components that allow us to standardize and scale infrastructure across teams and environments. In production, we separate modules into a reusable repository and use environment-specific configurations to consume them. Modules help enforce best practices, reduce duplication, and simplify infrastructure management.
🔥 WHAT YOU MUST KNOW TO PASS
You MUST understand:
- module structure
- variables vs outputs
- root vs child module
- module communication
- local vs remote modules
- versioning
- infra-modules vs infra-live
- DRY principle
- real-world usage
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