govuk-aws: 12. Security Groups in Terraform
There are two methods of defining security groups for AWS in Terraform and they are distinguished by how you add rules: in-line and separate. Using in-line rules keeps the definition close to the resource but when ever a rule is changed Terraform will re-generate the entire resource. Using separate rules Terraform will only make the single rule change but there is greater boilerplate and separation between the group resource and the rule resource.
Security groups will be defined separate to their rules.
Additionally each security group will be defined, in its entirety, in a single file.
Terraform should only create the specific rules that have changed when run.
There will be a larger 'distance' between the definition of a group and its rules.
Separate files may be harder to follow if there is heavy cross-referencing of security groups.
There may be a performance impact to Terraform depending on how it treats forward references.