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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 f

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.
      - swozey 3y ago
        @rcowley -- I'm going to preface this with I'm a Staff SRE at an adtech corp that does billions and have been a k8s and terraform contributor since 2015 (k8s 1.1 I forget the tf versions). I don't mean this to brag I just want to set my experience expectation since I'm a random name on hn who you'd never know. I think calling a service/stack (or whatever, app, etc) a "root module" is a very, very confusing thing to do. Terraform has actual micro objects called modules. We work with them every day. I get how you could consider encompassing an entire chunk of terrafrom code that calls various modules a "root module".. but I think this is just going to lead to absolute confusion to anyone not familiar with your terminology. I don't know every TF conversation but I can't think of a single time where I've heard root module in that context. Very good chance I've just missed those conversation and am ignorant to them. I'm currently hiring SRE 2s and 3s so I've been interviewing lots of terraform writers and one of my tech questions is to ask someone what makes them to decide to write a terraform module and what type of modules they've written - it's always ALBs, EKS, dbs, etc. components indepedently that go into creating a service/stack. I've definitely not heard anyone mention that they write "root modules" in the sense of an entire service/stack. I don't mean you're right or wrong, maybe more people are aware of that verbage than I am. I just wanted to mention that in my personal case I think it's confusing so I would assume that there are a lot of people in my shoes who would also be confused by it.
- linuxdude314 3y ago
  You’re not wrong. You can achieve the same results using modules well. It’s common for an entire environment for a whole biz unit or even company to be a single “stack”. This pretty much only works if the “terraform apply” is centrally orchestrated I.e gitops is the only way to trigger the terraform run.
  - xyzzy123 3y ago
    Agree, it's a complicated conversation because the tools support many different ways of working - and most of them can be made to work with different tradeoffs. A team that e.g owns its own microservice IaC can absolutely maintain a setup long term with app & db in the same state, it just requires care and love (it's easier in some ways, it just doesn't scale along certain dimensions). Maybe you have other controls / factors that can make it work for you. But my experience is that as you split things across more teams or have more complicated IaC "supply chains" (eg teams supplying modules to each other or have lots of ppl working on different bits of the same thing) you need to look at ways to make things more foolproof, easier to support and give yourself as much "wiggle room" as possible for upgrades. At this point having state split out is very helpful (almost essential). Because the terminology seems to be tripping up the conversation I'd be inclined to phrase it like this: a single "terraform apply" should be touching a precious, stateful stack or a disposable stateless stack and these should be clearly delineated. Ideally the stateful stacks should be as small as possible, as much stuff as possible should be in the stateless stacks.
    - linuxdude314 3y ago
      The way I see it is we only want to use TF for setting up "base infrastructure". This is things like our VPC networks, cloud routers, firewall rules, and finally our Kubernetes clusters. We still allow devs to use TF, but it's only for using cloud services they depend on, like say a SQL DB or something. I think you and I are 100% on the same page, but I think the word state (in the way you are using it) is causing a bit of confusion at least for me. A terraform stack will always have state, that's the point. In addition to the current "tfstate" there is a set of parameters that must be used with w/e modules in order to arrive at the tfstate. That's the state that causes problems, not the tfstate so much at least in my experience, as these often are _not_ version controlled. This is why it's critical to only allow terraform to be applied using automation. I mean don't make it a company policy, make it IAM policy so it's literally not possible.
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?

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.
    - swozey 3y ago
      @rcowley -- I'm going to preface this with I'm a Staff SRE at an adtech corp that does billions and have been a k8s and terraform contributor since 2015 (k8s 1.1 I forget the tf versions). I don't mean this to brag I just want to set my experience expectation since I'm a random name on hn who you'd never know. I think calling a service/stack (or whatever, app, etc) a "root module" is a very, very confusing thing to do. Terraform has actual micro objects called modules. We work with them every day. I get how you could consider encompassing an entire chunk of terrafrom code that calls various modules a "root module".. but I think this is just going to lead to absolute confusion to anyone not familiar with your terminology. I don't know every TF conversation but I can't think of a single time where I've heard root module in that context. Very good chance I've just missed those conversation and am ignorant to them. I'm currently hiring SRE 2s and 3s so I've been interviewing lots of terraform writers and one of my tech questions is to ask someone what makes them to decide to write a terraform module and what type of modules they've written - it's always ALBs, EKS, dbs, etc. components indepedently that go into creating a service/stack. I've definitely not heard anyone mention that they write "root modules" in the sense of an entire service/stack. I don't mean you're right or wrong, maybe more people are aware of that verbage than I am. I just wanted to mention that in my personal case I think it's confusing so I would assume that there are a lot of people in my shoes who would also be confused by it.
linuxdude314 3y ago
You’re not wrong. You can achieve the same results using modules well. It’s common for an entire environment for a whole biz unit or even company to be a single “stack”. This pretty much only works if the “terraform apply” is centrally orchestrated I.e gitops is the only way to trigger the terraform run.
- xyzzy123 3y ago
  Agree, it's a complicated conversation because the tools support many different ways of working - and most of them can be made to work with different tradeoffs. A team that e.g owns its own microservice IaC can absolutely maintain a setup long term with app & db in the same state, it just requires care and love (it's easier in some ways, it just doesn't scale along certain dimensions). Maybe you have other controls / factors that can make it work for you. But my experience is that as you split things across more teams or have more complicated IaC "supply chains" (eg teams supplying modules to each other or have lots of ppl working on different bits of the same thing) you need to look at ways to make things more foolproof, easier to support and give yourself as much "wiggle room" as possible for upgrades. At this point having state split out is very helpful (almost essential). Because the terminology seems to be tripping up the conversation I'd be inclined to phrase it like this: a single "terraform apply" should be touching a precious, stateful stack or a disposable stateless stack and these should be clearly delineated. Ideally the stateful stacks should be as small as possible, as much stuff as possible should be in the stateless stacks.
  - linuxdude314 3y ago
    The way I see it is we only want to use TF for setting up "base infrastructure". This is things like our VPC networks, cloud routers, firewall rules, and finally our Kubernetes clusters. We still allow devs to use TF, but it's only for using cloud services they depend on, like say a SQL DB or something. I think you and I are 100% on the same page, but I think the word state (in the way you are using it) is causing a bit of confusion at least for me. A terraform stack will always have state, that's the point. In addition to the current "tfstate" there is a set of parameters that must be used with w/e modules in order to arrive at the tfstate. That's the state that causes problems, not the tfstate so much at least in my experience, as these often are _not_ version controlled. This is why it's critical to only allow terraform to be applied using automation. I mean don't make it a company policy, make it IAM policy so it's literally not possible.

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?