About Stacks ​

The stack concept is advised for several reasons. But before we go into why lets talk a little about what a stack is.

The stack concept is not defined by Hashicorp's Terraform tooling but just a convention used by the Terraform community, so a stack can be loosely defined as:

A terraform stack is a runnable terraform module that operates on a subset of
the infrastructure's resources.

By runnable it means it has everything needed to call terraform init/plan/apply . For example, a module used by other modules, ie, don't have the provider explicitly set, is not runnable hence it's not a stack.

If a module is missing any required components, such as provider settings, it cannot be considered a stack.

It's also worth noting that if a module creates your entire infrastructure, it's not a stack. Stacks are designed to break down your infrastructure into manageable pieces, so they operate on a specific subset of resources rather than your entire infrastructure.

Isolate code that changes frequently ​

If you make a lot of updates to your infrastructure, like deploying new code or changing settings often, doing everything in one Terraform project could cause problems. This might lead to some parts of your infrastructure needing to be rebuilt, which means more downtime.

Using stacks helps with this issue. Stacks let you only update the parts of your system that need changes, without affecting everything else. Be careful to make the stacks the right size to avoid unnecessary repetition.

In short, try to keep code that changes for different reasons separate, so you don't create problems when making updates.

Reduce the blast radius ​

A small change can lead to catastrophic events if you're not careful. For instance, forgetting a prevent_destroy attribute in the production database lifecycle declaration. Also, someone within your organisation or team may commit mistakes and it's better if you could reduce the impact of such errors. An extreme example is: avoiding a database instance destroy because of a DNS TTL change.

Reduce execution time ​

Using stacks allows you to reduce the runtime of Terraform drastically. This is especially beneficial when running Terraform in CI/CD environments like GitHub Actions since build time is usually billed per minute, which means faster execution time translates directly to lowered costs.

Ownership ​

In the case of a big organization you probably don't want a single person or team responsible for the whole infrastructure. The company's stacks can be spread over several repositories and managed by different teams.

By having this separation, it also makes it easier when you want separation by resource permissions, so having some stacks that can only be run by specific individuals or teams.

Others ​

There are lots of other opinionated reasons why stacks would be a good option: stacks per environment or deploy region location, stacks per topic (IAM vs actual resources) and so on.