Not today...

How to easily debug Terraform? This was one of my biggest problem when dealing with the tool. I followed instructions to use some output ressources or browse the “debug” logs. Disclaimer: none of it was working properly.

So here is what I am actually doing and I am perfectly happy with. Oh! And just before starting to enable debug logs you have to pass the environment variable TF_LOG=debug (because there is no man page and it is not written in the --help content).

Before starting

The way Terraform is working is by applying some changes to your infrastructure and writting it to a state file (I am simplifying here). Terraform will not display any information until this state has been written down. Namely: generated ressource ids, names or anything related to resource creation.

My way of debugging will then be pretty simple. I will apply my terraform changes until the point I am blocked (so all the remote resources will be created and saved in the state file), then ask Terraform to display this information nicely :).

The actual way

If you have read the previous section and knows a bit how terraform is working you will not be surprised here.

So for the sake of simplicity let’s say that I am working with a file looking like this:

resource "a" "this" {
	name = "foo"
}

resource "b" "this" {
	some_config = a.this.id
	another_config = a.this.name
}

Let’s say that when running my terraform apply I get an error saying that a.this.id does not exist. It is possible that I made a mistake and want to know what is actually held by the resource a to fix the instructions.

Here is what I will run:

# this will generate the state up until my error which is at the next step
terraform apply -target a.this

# and now I can display what is held by the resource a
terraform show

As simple as that, but it took me quite some time to figure this out. Hope it can help someone out there.

Bonus

Sometimes it is obvious but sometimes it may be complicated to understand the dependency path of a resource. Usually, if I want to visualize this I run the following command: terraform graph | dot -Tsvg > dependencies.svg