2021-05-terraform-centralized.md

Use Terraform in a Single Directory

• Status: approved
• Last Updated: 2021-05
• Builds on: Use Terraform for Infrastructure Management, Monorepo for Code Management
• Objective: Determine if Terraform should be centralized or decentralized

Context & Problem Statement

Terraform for a project generally lives in a terraform/ directory. However, our repository includes multiple components as standalone services. Should infrastructure ownership be standalone, or should each service own it's own dependencies?

Priorities & Constraints

• Delivery across the project has a single lead
• Dependencies are more easily managed with a centralized terraform definition

Considered Options

1. Terraform at /terraform: Terraform is contained in a single directory
2. Terraform per Component: Each component (website, content-api, and delivery) has independent terraform definition & state
3. Terraform Controller & Modules per Component: Terraform has centralized resources at /terraform and each component supplies it's own module. Each component may optionally provide it's own terraform entrypoint to be independently tested.

Decision

Chosen option: "[option 1] Terraform at /terraform".

With a single engineer overseeing responsible for terraform excellence and knowledge unevenly spread across the team, we want to avoid "premature optimization" of the components.

Expected Consequences

• We have an "infrastructure monolith"
• The infrastructure monolith further risks us falling into the "distributed monolith" anti-pattern for the application
• As an example terraform implementation, we will be less effective at showing how service teams can own their own infrastructure

Research

• https://www.terraform.io/docs/language/modules/develop/composition.html
• https://opencredo.com/blogs/terraform-infrastructure-design-patterns/
• https://www.hashicorp.com/resources/evolving-infrastructure-terraform-opencredo

Links

• User Journey: #25