Explanation
Businesses are making a transition where traditionally-managed infrastructure can no longer meet the demands
of today's businesses. IT organizations are quickly adopting the public cloud, which is predominantly
API-driven. To meet customer demands and save costs, application teams are architecting their applications to
support a much higher level of elasticity, supporting technology like containers and public cloud resources.
These resources may only live for a matter of hours; therefore the traditional method of raising a ticket to
request resources is no longer a viable option Pointing and clicking in a management console is NOT scale and
increases the change of human error.

Explanation
Terraform CLI defines the following OPTIONAL arguments for variable declarations:
default - A default value which then makes the variable optional.
type - This argument specifies what value types are accepted for the variable.
description - This specifies the input variable's documentation.
validation - A block to define validation rules, usually in addition to type constraints.
sensitive - Limits Terraform UI output when the variable is used in configuration.
nullable - Specify if the variable can be null within the module.
https://www.terraform.io/language/values/variables#arguments

Explanation
For small infrastructures, Terraform can query your providers and sync the latest attributes from all your
resources. This is the default behavior of Terraform: for every plan and apply, Terraform will sync all
resources in your state.
For larger infrastructures, querying every resource is too slow. Many cloud providers do not provide APIs to
query multiple resources at once, and the round trip time for each resource is hundreds of milliseconds. On top
of this, cloud providers almost always have API rate limiting so Terraform can only request a certain number
of resources in a period of time. Larger users of Terraform make heavy use of the -refresh=false flag as well as
the -target flag in order to work around this. In these scenarios, the cached state is treated as the record of
truth.
https://www.terraform.io/docs/state/purpose.html

Explanation
Modules do not inherit variables from the parent module. All modules are self-contained units. So you have to
explicitly define variables in the child module, and then explicit set these variables in the parent module, when
you instantiate the child module.