Terraform has two mechanisms for provisioning multiple resources (and modules
since version 0.13): count and for_each. The count feature predates
for_each. Now that for_each is available, there is no reason to use
count.
The Terraform documentation says “If your instances are almost identical, count is appropriate. If some of their arguments need distinct values that can’t be directly derived from an integer, it’s safer to use for_each.”
This is bad advice. The behavior of terraform apply with for_each is
entirely different, and better, than the behavior of count.
When a resource uses count, Terraform creates a list of resources. Suppose
you have count = 4 for some resource, and you need to destroy the resource
at index 2. What does the plan look like?
The resources are a list. This means the resources are ordered. Removing one from the middle of the order causes all resources later in the order to shift to new positions. In our scenario, the apply plan will destroy resources at indexes 2 and 3, and then recreate the resource formerly at index 3 at its new index of 2. You will not want to encounter this issue more than once!
Fortunately for_each does not have this problem. There is no order to
resources under for_each, and creating or destroying individual resources
leaves the others in place.
It is trivial to convert a list to a map in Terraform:
{for k,v in toset(mylist): k => v}
So one can easily provide inputs appropriate to for_each instead of count.
Unfortunately, converting to for_each is not quite so simple. To preserve
existing resource you must remove them from the state and then add them again
under the new for_each style reference. Fortunately, you need do this only
once, rather than every time you want to destroy a middle-of-the-list
resource.
Back to the advice from the Terraform
documentation, when should you use count rather than for_each? The answer is “never”.
Questions
Have you adopted Terraform 0.13 use of for_each for modules?
How do you use dependency inversion in your Terraform modules?
Have you used Terragrunt for any projects?
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