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Is a "stack" here a (root) folder on which you'd do a "terraform apply"? I've never know what to call those, surely they aren't "modules". And, so, you're sayi

Is a "stack" here a (root) folder on which you'd do a "terraform apply"? I've never know what to call those, surely they aren't "modules". And, so, you're saying: try to have a separate deployment (stack then?) that contains the state, so you can wipe away everything else if you want to, without having to manage the state?

xyzzy123 3y ago
It's not exactly about the folder, the IaC from a single folder / project can be instantiated in multiple places. Each time you do that, it has a unique state file, so I usually hear it referred to as a "state". In cfn you can similarly deploy the same thing lots of times and each instantiation is called a "stack", so stack/state tend to get used inter-changeably. And yes, that's a succinct rephrasing. When you first use iac it maybe seems logical to put your db and app server in the same "thing" (stack or state file) but now that thing is "pet like" and you have to take care of it forever. You can't safely have a "destroy" action in your pipeline as a last resort. If you put the stateful stuff in a separate stack you can freely modify the things in the stateless one with much less worry.
- swozey 3y ago
  Can you elaborate on this? I've never heard of this IAC structure and I'm trying to figure out what the benefit/cons are. Maybe it's just Friday and I'm checked out already. If you run a terraform apply and only update microservices but you also have your dbs/stateful things in the same stack/app, you're only updating the microservices so how would this affect the db/stateful at all? On the opposite end - I feel like there would be scenarios where I needed to update the stateful AND stateless services with the same terraform apply. Maybe I'm adding a new cluster and adding a db region/replica/securitygroup and that new cluster needs to point at the new db region. In your scenario I would have updated microservices trying to reach out to a db in a region that doesn't exist yet because I have to terraform apply two different stacks. How would you deal with a depends_on? Maybe I'm misunderstanding this.
  - rcrowley 3y ago
    (Hi, I’m one of the authors of the article at the root of this thread.) Considering your hypothetical stateless microservice change in the same root module as stateful services, problems arise when _someone else_ has merged changes that concern the stateful services, leaving you little room to apply your changes individually. It’s also worth remembering that, even if a stateless service and a stateful service are managed in the same root module, applying changes is absolutely not atomic. Applying tightly coupled changes to two services “at the same time” is likely to result in brief service interruptions, even if everything returns to normal as soon as the whole changeset is applied.
    - swozey 3y ago
      Ok I think we're talking about two separate things here - you're referencing a root module and not a "stack", as in a stack is a full service/application that uses multiple modules to deploy. Your db module, eks module, etc. All independent modules, not combined into one singular module. Say it's sitting in the /terraform/app1/services/db(&)app folders type of scenario. I think you're talking about putting stateful and stateless objects inside of a single module. So you've got /terraform/modules/mybigapp/main.tf that has your microservice + database inside of it. If I'm right and that's what you mean that's really interesting I don't think I've ever seen or done that but now I'm curious. I'm pretty sure I've never created an "app1" module with all of its resources. Am I totally off here?
      - rcrowley 3y ago
        I stuck with my typical term, root module, synonymous with how folks are using “stack” and “state” in various parts of this thread. A module is any directory with Terraform code in it. A root module is one that has a configuration for how to store and lock Terraform state, providers, and possibly tfvars files. Both modules and root modules may reference other modules. You run `terraform init|plan|apply` in root modules. I think my comment makes sense in that if you mix two services into the same root module (directly or via any amount of modules-instantiating-other-modules) you can end up with changes from two people affecting both services that you can’t easily sever. Happy to clarify further if I’m still not addressing your original comment.
- waffletower 3y ago
  That's right, stacks can be instantiated across repos even depending upon the organization (both meanings of 'organization' are valid here).
- pharmakom 3y ago
  what about prevent_destroy lifecycle?
GauntletWizard 3y ago
I have adopted the term "Root Module" vs "Submodule" because those line up with terraform's own definitions, but I agree that they're terribly, terribly named.
raffraffraff 3y ago
I have the same issue. They're all modules, but the ones at the tip of the directory tree (right at the end of your env/region/stack) are called root modules. Which makes no sense because the term "root" always implies that they are at the beginning, not the tippy-toe end. So I call mine "stacks". But as another answer suggested, "states" is also fine. Even though the actual state isn't inside that directory, it's probably in an object store. At the end of the day I don't care what other people call them.

xyzzy123 3y ago

It's not exactly about the folder, the IaC from a single folder / project can be instantiated in multiple places. Each time you do that, it has a unique state file, so I usually hear it referred to as a "state". In cfn you can similarly deploy the same thing lots of times and each instantiation is called a "stack", so stack/state tend to get used inter-changeably. And yes, that's a succinct rephrasing. When you first use iac it maybe seems logical to put your db and app server in the same "thing" (stack or state file) but now that thing is "pet like" and you have to take care of it forever. You can't safely have a "destroy" action in your pipeline as a last resort. If you put the stateful stuff in a separate stack you can freely modify the things in the stateless one with much less worry.

swozey 3y ago
Can you elaborate on this? I've never heard of this IAC structure and I'm trying to figure out what the benefit/cons are. Maybe it's just Friday and I'm checked out already. If you run a terraform apply and only update microservices but you also have your dbs/stateful things in the same stack/app, you're only updating the microservices so how would this affect the db/stateful at all? On the opposite end - I feel like there would be scenarios where I needed to update the stateful AND stateless services with the same terraform apply. Maybe I'm adding a new cluster and adding a db region/replica/securitygroup and that new cluster needs to point at the new db region. In your scenario I would have updated microservices trying to reach out to a db in a region that doesn't exist yet because I have to terraform apply two different stacks. How would you deal with a depends_on? Maybe I'm misunderstanding this.
- rcrowley 3y ago
  (Hi, I’m one of the authors of the article at the root of this thread.) Considering your hypothetical stateless microservice change in the same root module as stateful services, problems arise when _someone else_ has merged changes that concern the stateful services, leaving you little room to apply your changes individually. It’s also worth remembering that, even if a stateless service and a stateful service are managed in the same root module, applying changes is absolutely not atomic. Applying tightly coupled changes to two services “at the same time” is likely to result in brief service interruptions, even if everything returns to normal as soon as the whole changeset is applied.
  - swozey 3y ago
    Ok I think we're talking about two separate things here - you're referencing a root module and not a "stack", as in a stack is a full service/application that uses multiple modules to deploy. Your db module, eks module, etc. All independent modules, not combined into one singular module. Say it's sitting in the /terraform/app1/services/db(&)app folders type of scenario. I think you're talking about putting stateful and stateless objects inside of a single module. So you've got /terraform/modules/mybigapp/main.tf that has your microservice + database inside of it. If I'm right and that's what you mean that's really interesting I don't think I've ever seen or done that but now I'm curious. I'm pretty sure I've never created an "app1" module with all of its resources. Am I totally off here?
    - rcrowley 3y ago
      I stuck with my typical term, root module, synonymous with how folks are using “stack” and “state” in various parts of this thread. A module is any directory with Terraform code in it. A root module is one that has a configuration for how to store and lock Terraform state, providers, and possibly tfvars files. Both modules and root modules may reference other modules. You run `terraform init|plan|apply` in root modules. I think my comment makes sense in that if you mix two services into the same root module (directly or via any amount of modules-instantiating-other-modules) you can end up with changes from two people affecting both services that you can’t easily sever. Happy to clarify further if I’m still not addressing your original comment.
waffletower 3y ago
That's right, stacks can be instantiated across repos even depending upon the organization (both meanings of 'organization' are valid here).
pharmakom 3y ago
what about prevent_destroy lifecycle?

GauntletWizard 3y ago

I have adopted the term "Root Module" vs "Submodule" because those line up with terraform's own definitions, but I agree that they're terribly, terribly named.

raffraffraff 3y ago

I have the same issue. They're all modules, but the ones at the tip of the directory tree (right at the end of your env/region/stack) are called root modules. Which makes no sense because the term "root" always implies that they are at the beginning, not the tippy-toe end. So I call mine "stacks". But as another answer suggested, "states" is also fine. Even though the actual state isn't inside that directory, it's probably in an object store. At the end of the day I don't care what other people call them.