Terraform: depends_on variable for cross module depenencies needed?

[BerndWessels]

Hi

We are encouraged to create re-usable modules to build our infrastructure. But this turns out do be difficult once it comes to cross module dependencies.

Here is my use case.

I have these re-usable modules in an infrastructure modules repository:

module PlatformAPI
- has a APIGateway REST API
- has a APIGateway REST API Deployment

module LambdaRestEndpoint
- has a Lambda function
- has APIGateway REST Resources

And I want to use them in my live infrastructure:

Live Production Module
module "PlatformAPI" {
  source = "git::[email protected]:xxx/infrastructure-modules.git//PlatformAPI"
}
module "LambdaRestEndpoint" {
  source = "git::[email protected]:xxx/infrastructure-modules.git//LambdaRestEndpoint"
}

To make this work the following order should be achieved through cross module dependencies:

• create > module PlatformAPI > APIGateway REST API
• then create > module LambdaRestEndpoint > Lambda and REST Resources
• then create > module PlatformAPI > APIGateway REST API Deployment

So basically the question is, how can a resource in one module can depend on a resource in another module if both of these modules don't know about each other?

This could be achieved elegantly with depends_on ["${var.ReadyToDeploy"]!

But unfortunately it is not possible to depend on input variables :(

Any suggestions? How is this supposed to work?

[jbardin]

Hi @BerndWessels,

While you can't use a variable in "depends_on" you can use the variable itself which creates the dependency. If you don't have anywhere at all to use the variable, you can put in a null_resource and depend on that.

As for explicit inter-nodule dependencies, we're tracking that feature request in #10462.

[BerndWessels]

@jbardin Thanks for the response.

Unfortunately that doesn't solve the main issue here, which is that we want to have resources in modules depend on resources in other modules - rather than just having modules depend on other modules.

As in my example above, to be able to have a re-usable module that encapsulates an APIGateway REST API and the corresponding APIGateway REST API Deployment and allowing other re-usable modules to provide REST API Resources to that REST API there must be a way to only make the APIGateway REST API Deployment dependent on the elsewhere defined REST API Resources.

A null_resource would not solve this problem.

Basically at the moment re-usable modules are limited to sets of resources that all depend on the same conditions - rather than each resource being able to depend on an injected condition.

[jbardin]

Hi @BerndWessels,

Sure you can, albeit a little convoluted since you can't reference them directly, but the dependency path of an example moduleA and moduleB imported into root would look like:

moduleA.resourceA -> moduleA.outputA -> root ->  moduleB.varB -> moduleB.resourceB

If you don't actually have a dependency in the configuration (i.e. there's no way to reference varB in resourceB), but need to force one with depends_on, that's where you can use the null_resource to give you a fake resource to target with depends_on.

Not only is depends_on rarely needed, but modules themselves are meant to be opaque, only exposing their outputs and variables, so targeting individual resources in a module isn't really feasible. Any enhancement that could be done in this areas can be further discussed in the referenced feature request.

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