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In this lesson, we’ll explore the benefits and trade-offs of using Community modules versus building Custom modules from scratch in Terraform. Mickey faces this choice on his project: should he leverage pre-built solutions or invest time crafting bespoke infrastructure components?

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The Dilemma

Community modules deliver speed and convenience with battle-tested building blocks for common infrastructure patterns. Custom modules, by contrast, give you full control and tailorability but demand more time and expertise. Is there a strategy that combines the best of both worlds?

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Custom Modules

Custom modules are developed in-house to address project-specific requirements and constraints. Key advantages include:
  • Full Control
    Define every variable, output, and resource to match exact specifications.
  • Tailored Fit
    Align module behavior with your organization’s unique infrastructure needs and policies.
  • Ownership & Governance
    Teams retain authority over updates, security audits, and versioning.

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When to Choose Custom Modules

  • You require specialized configurations not covered by existing modules.
  • Strict compliance with internal security or regulatory standards is mandatory.
  • You need seamless integration with legacy systems or proprietary tooling.

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Example: Simple Custom Module Structure

my-terraform-project/
├── modules/
│   └── custom_vpc/
│       ├── main.tf
│       ├── variables.tf
│       └── outputs.tf
└── environments/
    └── prod/
        └── main.tf

# environments/prod/main.tf
module "custom_vpc" {
  source       = "../../modules/custom_vpc"
  cidr_block   = var.vpc_cidr
  environment  = "prod"
}

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Community Modules

Community modules are shared on public registries and maintained by the broader Terraform user base. They offer:
  • Rapid Development
    Address common patterns—VPC, IAM roles, load balancers—without reinventing the wheel.
  • Collective Expertise
    Leverage bug fixes and improvements contributed by many users.
  • Continuous Updates
    Popular modules receive frequent enhancements and support for new provider features.

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When to Choose Community Modules

  • Your use case aligns with widely adopted infrastructure patterns.
  • You want to reduce development and maintenance overhead.
  • You’re comfortable with community-driven versioning and updates.

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Example: Consuming a Community Module

module "vpc" {
  source  = "terraform-aws-modules/vpc/aws"
  version = "3.10.0"
  name    = "my-vpc"
  cidr    = "10.0.0.0/16"
}

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Comparison Table

CriteriaCustom ModulesCommunity Modules
Control100% custom architectureConstrained by upstream design
Development SpeedLonger due to bespoke designFast—plug & play
MaintenanceOn your teamShared across community
ComplianceFully customizableAudit required before adoption
Update CadenceScheduled by your teamDepends on module author

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Hybrid Module Approach

For many projects, a hybrid strategy is ideal:
  1. Leverage Community Modules for standard components (e.g., networking, compute, storage).
  2. Wrap or Extend those modules to enforce naming conventions, tagging policies, or security guardrails.
  3. Develop Custom Modules only for truly unique business logic or specialized integrations.
module "secure_vpc" {
  source = "git::https://github.com/your-org/terraform-aws-vpc-wrapper.git?ref=v1.2.0"
  # Wraps the upstream VPC module with your policies
}

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Key Considerations

  • Security & Compliance
Always review third-party module code for hidden risks before deploying to production.
  • Version Locking
Pin module versions to avoid unexpected breaking changes:
module "vpc" {
  source  = "terraform-aws-modules/vpc/aws"
  version = "3.10.0"
}
  • Documentation & Support
    Verify that community modules are well-documented and actively maintained.
  • Team Expertise
    Assess your team’s Terraform skills to gauge how much custom work you can sustain.

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Conclusion

Choosing between Custom and Community modules isn’t an all-or-nothing decision. By understanding the strengths and limitations of each, you can craft a Terraform module strategy that accelerates development, enforces best practices, and adapts to your project’s unique needs.

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References

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