In a module call block, the optional providers meta-argument specifies which provider configurations from the parent module will be available inside the child module.
# The default "aws" configuration is used for AWS resources in the root
# module where no explicit provider instance is selected.
provider "aws" {
region = "us-west-1"
}
# An alternate configuration is also defined for a different
# region, using the alias "usw2".
provider "aws" {
alias = "usw2"
region = "us-west-2"
}
# An example child module is instantiated with the alternate configuration,
# so any AWS resources it defines will use the us-west-2 region.
module "example" {
source = "./example"
providers = {
aws = aws.usw2
}
}
If the child module does not declare any configuration aliases, the providers argument is optional. If you omit it, a child module inherits all of the default provider configurations from its parent module. (Default provider configurations are ones that don't use the alias argument.)
If you specify a providers argument, it cancels this default behavior, and the child module will only have access to the provider configurations you specify.
The value of providers is a map, where:
Both keys and values should be unquoted references to provider configurations. For default configurations, this is the local name of the provider; for alternate configurations, this is a <PROVIDER>.<ALIAS> reference.
Within a child module, resources are assigned to provider configurations as normal — either Terraform chooses a default based on the name of the resource type, or the resource specifies an alternate configuration with the provider argument. If the module receives a providers map when it's called, the provider configuration names used within the module are effectively remapped to refer the specified configurations from the parent module.
There are two main reasons to use the providers argument:
Most re-usable modules only use default provider configurations, which they can automatically inherit from their caller when providers is omitted.
However, in Terraform configurations that use multiple configurations of the same provider, you might want some child modules to use the default provider configuration and other ones to use an alternate. (This usually happens when using one configuration to manage resources in multiple different regions of the same cloud provider.)
By using the providers argument (like in the code example above), you can accommodate this without needing to edit the child module. Although the code within the child module always refers to the default provider configuration, the actual configuration of that default can be different for each instance.
In rare cases, a single re-usable module might require multiple configurations of the same provider. For example, a module that configures connectivity between networks in two AWS regions is likely to need both a source and a destination region. In that case, the root module may look something like this:
provider "aws" {
alias = "usw1"
region = "us-west-1"
}
provider "aws" {
alias = "usw2"
region = "us-west-2"
}
module "tunnel" {
source = "./tunnel"
providers = {
aws.src = aws.usw1
aws.dst = aws.usw2
}
}
Non-default provider configurations are never automatically inherited, so any module that works like this will always need a providers argument. The documentation for the module should specify all of the provider configuration names it needs.
For more details and guidance about working with providers inside a re-usable child module, see Module Development: Providers Within Modules.
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