[cristim]

Summary

• RDS auto-minor-upgraded the dev instance from 16.6 to 16.13; Terraform then attempted to downgrade back to 16.6, which RDS rejected: InvalidParameterCombination: Cannot find upgrade path from 16.13 to 16.6.
• Root cause: all six engine_version pins were set to the exact minor "16.6". Any RDS auto-minor upgrade creates an irreconcilable drift -- Terraform sees a config mismatch and tries a downgrade that the API refuses.
• Fix: change all six pins from "16.6" to "16" (major-only). The AWS provider v5 (5.100.0, confirmed in lockfile) stores the real minor version in engine_version_actual and suppresses diff when the configured value is a major-only prefix, so no downgrade is ever attempted and future auto-minor upgrades remain transparent.
• auto_minor_version_upgrade = true is now set explicitly in the module resource with a comment, making the intent self-describing.

Files changed

• terraform/modules/database/aws/main.tf -- added explicit auto_minor_version_upgrade = true with explanatory comment
• terraform/modules/database/aws/variables.tf -- pin changed to "16", description updated
• terraform/environments/aws/variables.tf -- pin changed to "16", description updated
• terraform/environments/aws/dev.tfvars.example -- pin changed to "16"
• terraform/environments/aws/github-dev.tfvars -- pin changed to "16"
• terraform/environments/aws/github-prod.tfvars -- pin changed to "16"
• terraform/environments/aws/github-staging.tfvars -- pin changed to "16"

Gate results

• terraform fmt -check -recursive on both touched dirs: exit 0
• terraform init -backend=false && terraform validate in terraform/modules/database/aws: exit 0
• terraform init -backend=false && terraform validate in terraform/environments/aws: exit 0

Test plan

• CI "Validate Terraform (aws)" passes on this PR
• terraform plan in the dev environment shows no engine_version change for the existing 16.13 instance
• Next deploy does not attempt a downgrade

Addresses the AWS deploy failure in #1372.

[cristim]

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📥 Commits

Reviewing files that changed from the base of the PR and between 157f9e2 and 32ff7ae.

📒 Files selected for processing (7)

• terraform/environments/aws/dev.tfvars.example
• terraform/environments/aws/github-dev.tfvars
• terraform/environments/aws/github-prod.tfvars
• terraform/environments/aws/github-staging.tfvars
• terraform/environments/aws/variables.tf
• terraform/modules/database/aws/main.tf
• terraform/modules/database/aws/variables.tf

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🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/rds-engine-version-drift

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[cristim]

Merged to main: RDS engine_version pins switched from exact-minor 16.6 to major-only 16 across all six locations (module default, env default, 3 github tfvars, example), with auto_minor_version_upgrade explicit and the convention documented. Provider v5.100.0 prefix-suppression verified, so the live 16.13 instance satisfies the pin and no downgrade is ever attempted. This fixes the AWS-half of #1372 (Deploy to AWS Lambda failing on ModifyDBInstance 16.13->16.6). Next AWS deploy on main will confirm; GCP half already confirmed fixed.