Monorepo vs multi-repo for Terraform - Performance Comparison

Time Complexity: Monorepo vs multi-repo for Terraform

O(n)

Understanding Time Complexity

We want to understand how the time to manage infrastructure changes grows when using one big repository versus many smaller ones in Terraform.

How does the number of repositories affect the work Terraform does?

Scenario Under Consideration

Analyze the time complexity of applying Terraform configurations in two setups.

# Monorepo example
module "network" {
  source = "./modules/network"
}
module "compute" {
  source = "./modules/compute"
}

# Multi-repo example
# Separate repos for network and compute modules

This shows a single repo with multiple modules versus separate repos each with their own module.

Identify Repeating Operations

Look at what Terraform does repeatedly when applying changes.

• Primary operation: Terraform plan and apply commands for each repo or module.
• How many times: Once per repo in multi-repo, once for all modules in monorepo.

How Execution Grows With Input

As the number of modules or repos grows, the work changes differently.

Input Size (n)	Approx. Api Calls/Operations
10	~10 applies in multi-repo, 1 apply with 10 modules in monorepo
100	~100 applies in multi-repo, 1 apply with 100 modules in monorepo
1000	~1000 applies in multi-repo, 1 apply with 1000 modules in monorepo

Multi-repo grows linearly with number of repos; monorepo runs once regardless of module count.

Final Time Complexity

Time Complexity: O(n)

This means the time grows directly with the number of repositories in multi-repo, but stays mostly constant in monorepo.

Common Mistake

[X] Wrong: "More repos always mean faster Terraform runs because each is smaller."

[OK] Correct: Running Terraform separately for many repos adds overhead each time, making total time grow with repo count.

Interview Connect

Understanding how repo structure affects Terraform run time helps you design infrastructure code that scales well and stays manageable.

Self-Check

"What if we split a monorepo into fewer but larger repos? How would the time complexity change?"

Practice

1. What is a key advantage of using a monorepo for Terraform code?

easy

A. All Terraform code is stored in one place, making sharing easier

B. Each team works completely independently without any shared code

C. It forces teams to use different Terraform versions per project

D. It automatically splits infrastructure into multiple cloud accounts

Solution

1. Step 1: Understand monorepo concept

   A monorepo stores all Terraform code in a single repository, allowing easy sharing and reuse.

2. Step 2: Compare options

   All Terraform code is stored in one place, making sharing easier correctly describes this advantage. Other options describe multi-repo or unrelated features.

3. Final Answer:

   All Terraform code is stored in one place, making sharing easier -> Option A

4. Quick Check:

   Monorepo = single repo for all code [OK]

Hint: Monorepo means one repo for all Terraform code [OK]

Common Mistakes:

• Confusing monorepo with multi-repo
• Thinking monorepo isolates teams completely
• Assuming monorepo enforces different Terraform versions

2. Which of the following is a correct way to organize Terraform code in a multi-repo setup?

easy

A. Store all Terraform modules and environments in one repository

B. Split Terraform code into separate repositories per environment or team

C. Use a single Terraform state file for all repositories

D. Combine Terraform and application code in the same repository

Solution

1. Step 1: Understand multi-repo structure

   Multi-repo means splitting Terraform code into multiple repositories, often by environment or team.

2. Step 2: Evaluate options

   Split Terraform code into separate repositories per environment or team correctly describes this. Store all Terraform modules and environments in one repository describes monorepo. Use a single Terraform state file for all repositories is incorrect because state files are usually separate per repo. Combine Terraform and application code in the same repository mixes concerns.

3. Final Answer:

   Split Terraform code into separate repositories per environment or team -> Option B

4. Quick Check:

   Multi-repo = multiple repos for Terraform code [OK]

Hint: Multi-repo splits code by team or environment [OK]

Common Mistakes:

• Mixing monorepo and multi-repo definitions
• Assuming one state file for all repos
• Combining unrelated code in Terraform repos

3. Given a monorepo with Terraform code for two environments, what is a likely outcome when running terraform apply in the root directory without specifying a workspace or path?

medium

A. Terraform applies changes only to the default environment

B. Terraform applies changes to both environments at once

C. Terraform throws an error due to multiple state files

D. Terraform automatically detects and applies changes per environment

Solution

1. Step 1: Understand default Terraform behavior in monorepo

   Running terraform apply in root without workspace or path targets the default state and configuration.

2. Step 2: Analyze options

   Terraform applies changes only to the default environment is correct because Terraform applies only the default environment unless configured otherwise. Terraform applies changes to both environments at once is incorrect as Terraform does not apply multiple environments simultaneously by default. Terraform throws an error due to multiple state files is wrong; no error occurs unless misconfigured. Terraform automatically detects and applies changes per environment is false; Terraform does not auto-detect environments.

3. Final Answer:

   Terraform applies changes only to the default environment -> Option A

4. Quick Check:

   Default apply targets one environment [OK]

Hint: Without workspace, Terraform applies default environment only [OK]

Common Mistakes:

• Assuming Terraform applies all environments automatically
• Expecting errors without misconfiguration
• Thinking Terraform auto-detects multiple environments

4. You have a multi-repo Terraform setup but accidentally run terraform apply in the wrong repository. What is the best way to fix this mistake?

medium

A. Delete the entire repository and start over

B. Merge the repositories to avoid confusion

C. Run terraform destroy in that repository to remove unintended resources

D. Ignore it; Terraform will not create any resources without approval

Solution

1. Step 1: Identify the impact of wrong apply

   Running terraform apply in the wrong repo creates resources unintentionally.

2. Step 2: Choose corrective action

   Run terraform destroy in that repository to remove unintended resources is best: running terraform destroy removes those resources safely. Delete the entire repository and start over is extreme and unnecessary. Ignore it; Terraform will not create any resources without approval is wrong; apply creates resources after approval. Merge the repositories to avoid confusion is unrelated to fixing the mistake.

3. Final Answer:

   Run terraform destroy in that repository to remove unintended resources -> Option C

4. Quick Check:

   Destroy removes unintended resources safely [OK]

Hint: Use terraform destroy to undo wrong apply quickly [OK]

Common Mistakes:

• Deleting repos instead of destroying resources
• Ignoring accidental resource creation
• Thinking merging repos fixes mistakes

5. Your team wants to share Terraform modules easily but keep environment configurations isolated. Which approach best balances these needs?

hard

A. Use a monorepo for all code including modules and environments

B. Use a single repo with all environments but duplicate modules in each folder

C. Use multi-repo but copy modules into each environment repo manually

D. Use a multi-repo with one repo for shared modules and separate repos per environment

Solution

1. Step 1: Analyze sharing and isolation needs

   The team wants easy sharing of modules but isolated environment configs.

2. Step 2: Evaluate options for balance

   Use a multi-repo with one repo for shared modules and separate repos per environment fits best: shared modules in one repo, environments isolated in separate repos. Use a monorepo for all code including modules and environments mixes all code, reducing isolation. Use a single repo with all environments but duplicate modules in each folder duplicates modules, causing maintenance issues. Use multi-repo but copy modules into each environment repo manually copies modules manually, risking drift.

3. Final Answer:

   Use a multi-repo with one repo for shared modules and separate repos per environment -> Option D

4. Quick Check:

   Shared modules repo + separate env repos = best balance [OK]

Hint: Share modules in one repo, isolate environments in others [OK]

Common Mistakes:

• Mixing modules and environments in one repo
• Duplicating modules causing maintenance headaches
• Copying modules manually risking drift