Terraform

Terraform is an open-source infrastructure as a code (hashicorp configuration language) software tool created by HashiCorp. Similar to CloudFormation, terraform is designed as a provisioning tool, which is different than tools like chef and puppet, which are primarily designed for configuration management. Definitely, there is some overlap between configuration management tools and provisioning tools, but it's important to understand what they are best at. Terraform and CloudFormation follows declarative state language, which means you specify end state and tool will figure out sequence or dependencies of the task. Tools like ansible are procedural-based automation tools, where you write code and specify the sequence of steps to achieve end state. 

The main purpose of the Terraform language is declaring resources. All other feature exists to make the definition of resources more flexible and convenient. A Terraform configuration consists of a root module, where evaluation begins, along with a tree of child modules created when one module calls another.

Terraform Components

variable

Input variables serve as parameters for a Terraform module. When you declare variables in the root module of your configuration, you can set their values using CLI options (-var="my_var=something"), or in .tfvars file or in environment variables. When you declare them in child modules, the calling module should pass values in the module block. The value assigned to a variable can be accessed (var.my_var) only from expressions within the module where it was declared. Ex string, number, bool, list, map, set

provider

Provider (like aws or google) requires configuration of their own, like authentication settings, etc. You can have multiple providers give them an alias and reference it by alias name (<PROVIDER NAME>.<ALIAS>). Every time a new provider is added, you need to download it by initializing it.

resource

Resource block describes one or more infrastructure objects. Each resource is associated with a single resource type. Terraform handles most resource dependencies automatically but in the rare cases where you need to explicitly define you can use depends_on metadata argument. You can use a count meta argument to create multiple resources. You can add Provisioners to any resource and are used to execute scripts on a local or remote machine 

output

This is like the return of a module that can be used in the child module or for printing from the root module.

module 

This is a container for multiple resources while it helps reusability of the code. Every Terraform configuration has at least a root module, which consists of the resources defined in the .tf files in the main working directory.

data

Data sources allow fetching data defined outside of Terraform. One of the very common use case of this is getting aws AZ

data "aws_availability_zones" "available" {
  state = "available"
}

In the above example, we are storing aws AZs list which can be accessed anywhere in the code as follow

"${data.aws_availability_zones.available.names[0]}"

State

Terraform state can be considered some sort of database to map Terrform config to the real world. This also tracks resource dependencies.  This can be saved locally or remotely and location can be configured by -state. It locks state while applying for all operations that could write state.

Terraform Console

This provides an interactive way to evaluate an expression. This becomes very handy to test with build-in function and also with interpolation. Just type terraform console on the command prompt and then play with it.