terraform.txt

################## Overview ###################

Terraform is a tool for building infrastructure (IaC - Infrastructure as a code)
It is idempotent and it uses high level syntax, allows for deployment of several layers of architecture

#Documentation
https://www.terraform.io/docs/index.html

## For installation download the package and unzip it

################# Usage ##################

#Terraform files end in .tf extension 
#example main.tf

# Pull centos and http images and run httpd container
resource "docker_image" "centos" {
  name = "centos:latest"
}

resource "docker_image" "apache" {
  name = "httpd:latest"
}

resource "docker_container" "tera-test" {
  name = "tera-test"
  image = "${docker_image.apache.latest}"
  ports {
    internal = "80"
    external = "9000"
  }

}


#to initialize the working directory for terraform
terraform init

#To generate execution plan 
terraform plan

#to apply the declared resources
terraform apply 

#to check and show your resources
terraform show

#to destroy the infrastructure
terraform destroy

#to taint a resource for redeployment
terraform taint docker_container.tera-test
#untaint to revert

## To output something to the console in main.tf
# Output name and ip address
output "IP Address" {
  value = "${docker_container.tera-test.ip_address}"
}

output "Name" {
  value = "${docker_container.tera-test.name}"
}

## Terraform variables in main.tf

variable "image" {
  description = "image for container"
  default = "httpd:latest"
}

#to reference it in the declaration ${var.image}
#If there is no default value the variable will be interactive

#In order to use modularized set up in main.tf

module "image" {
 source = "./image"
 image = "${var.image}"
}

module "container" {
  source = "./container"
  image = "${module.image.image.out}"  ## image_out - output from image module
}


#Modules encapsulate their resources. 
#A resource in one module cannot directly depend on resources or attributes in other modules, 
#unless those are exported through outputs. 
#These outputs can be referenced in other places in your configuration

## Map variables can be used to lookup default values for other variables
#variables.tf

## env variable for map and lookup
variable "env" {
  description = "env: dev or prod"
}

variable "port" {
  description = "External port for the container"
  type = "map"
  default = {
    dev = "12000"
    prod = "10000"
  }
}

#main.tf
module "container" {
  source = "./containers"
  image = "${module.image.image_out}"
  port = "${lookup(var.port, var.env)}"
}

## lookup var.port depending on var.env
## variables can be provided with -var flag on terrafom command also
terraform apply -var 'foo=bar'

#Variable values can also be placed in file terraform.tfvars , definitions can remain in variables.tf
#example variables.tf
variable "port" {
  description = "External port for the container"
  type = "map"
}
#terraform.tfvars
port = {
  dev = "12000"
  prod = "10000"
}


########### Workspaces ############

#In order to have multiple environments/deployments at the same time 
#new workspace should be created

terraform workspace new NAME
#to switch between
terraform workspace select NAME  # default for the default workspace

### to execute commands on the host or on other resources 
# null resource is used like the following:

resource "null_resource" "command" {
  provisioner "local-exec" {
    command = "echo 'This is a test exec command' > file.txt"
  }
}

## To format the code in all files
terraform fmt --diff