Part1

1. Introduction🔗

Terraform is written in Golang. Terraform uses graph theory to plot relationships between resources.

HCL - Hashicorp configuration language.

IaC tools:

AWS :: Cloudformation
Azure :: Resource Manager
GCP :: Deployment Manager
Terraform

2. Files in Terraform🔗

files:

provider.tf :: A provider in Terraform is a connection that allows Terraform to manage infrastructure using predefined interface
resources.tf :: define the resource - represent things in real world
variables.tf :: dynamic values - value set at runtime
data-source.tf :: used to fetch data from a resource that is not managed by terraform
outputs.tf :: outputs from provisioning

3. Terraform State🔗

NOTE: terraform.tfstate :: state according to terraform

Terraform state :: terraform.tfstate

record of all resources that it has created
it stores in local file terraform.tfstate
the file also stores dependency order of the resources that have been created

4. Resources🔗

allows you to define what you want to create in real world

Resource Block

describes one or more infrastructure objects ex: virtual network, compute instances, dns record

declares a resource of a: + given type "aws_instance" + given name "my_instance"

ex:

resource "aws_instance" "my_instance"{
    ami = "ami-1234qwerty"
    instance_type = "t2.micro"
}

where:: 1. resource type = aws_instance 2. local resource name = my_instance

argument name = ami, instance_type
argument value = ami-1234qwerty,t2.micro

Implicit dependency in Resource Block

Terraform will automatically manage Implicit dependency by find reference objects and creating an Implicit ordering

ex:

resource "aws_eip" "my_eip"{
    vpc = "true"
}

resource "aws_instance" "my_instance"{
    ami = "ami-1234qwerty"
    instance_type = "t2.micro"
    public_ip = aws_eip.my_eip.private_ip
}

Explicit dependency in Resource Block

is only required when resource relies on another resource behavior

ex:

resource "aws_s3_bucket" "example"{
        acl = "private"
}

resource "aws_instance" "my_instance"{
    instance_type = "t2.micro"
    depends_on = [aws_s3_bucket.example]
}

5. Terraform workflow/lifecycle🔗

terraform init :: Prepare your working directory for other command
terraform plan :: show changes required by the current configuration
terraform apply :: create, update, or delete infrastructure
terraform destroy :: destroy previously created infrastructure

6. Terraform debugging/troubleshooting🔗

enable logging in terraform with TF_LOG environment variable
presist logging output with TF_LOG_PATH

Values:

TF_LOG=TRACE :: Highest verbosity
TF_LOG=DEBUG
TF_LOG=INFO
TF_LOG=WARN
TF_LOG=ERROR :: lowest verbosity

ex:

export TF_LOG=TRACE
export TF_LOG_PATH=/logs/terraform.log

7. Data Sources🔗

used to fetch data from a resource that is not managed by terraform
allows data to be fetched or computed from elsewhere in terraform configuration

ex:

data "aws_ami" "example"{
    executable_users = ["self"]
    most_recent = true
    name_regex = "^myami-\\d{3}"
    owners = ["self"]
    filter {
        name="name"
        values = ["myami-*"]
    }
    filter {
        name="root-device-type"
        values=["ebs"]
    }
    filter {
        name="virtualization-type"
        values=["hvm"]
    }
}

8. Variables in Terraform🔗

something set at runtime
allows you to vary what terraform will do when passing in or using a dynamic value
variable with undefined values will not directly result in ERROR - NOTE: Terraform will ask you to supply a value IMPORTANT ex: variable custom_var {}

In version 0.12>= :: var.instance_type declaration
In version 0.11<= :: ${var.instance_type} declaration

3 types of variable types

Simple variable types - String - Number - boolean

variable "aws_region"{
    type=string #type enforcement
    default="us-west-2"
}
variable "instance_count"{
    type=Number
    default=2
}
variable "enable_vpn_gateway"{
    type=bool
    default=false
}

Collection Variable types (NOTE: all values of same type) - list :: [] - map :: {k1=v1,k2=v2} - set :: an unordered collection of unique Values

ex:

variable "private_subnet_cidr_blocks"{
  type = list(string)
  default=[
    "10.0.101.0/24",
    "10.0.102.0/24",
    "10.0.103.0/24"
  ]
}

  variable "resouce_tags"{
      type=map(string)
      default={
          project="project-alpha",
          environment="dev"
      }
  }

Structure Variable types - tuple :: fixed-length sequence of values of specified types - ["a",15,true] - object - A lookup table, matching a fixed set of keys to values of specified types

Variable Precendence Order (Highest to lowest)

-var or -var-file on command line

terraform apply -auto-approve \
    -var "aws_access_key=<KEY>" \
    -var "aws_secret_key="<KEY>"

*.auto.tfvars or *.auto.tfvars.json
terraform.tfvars ex: instance_type="t2.large"

variables.tf

variable "instance_type"{
        default="t2.micro"
}

environment variables - can be used to set a variable

- must be in format TF_VAR_name

- .bashrc export TF_VAR_instance_type=t2.micro export TF_VAR_ami=ami-12345abc

9. Splat Expressions🔗

The splat[*]

the special splat[*] symbol iterates over all the elements of a given list to its left and accesses from each one the attribute name given on its right.

ex:

resource "aws_iam_user" "lb"{
  name="loadbalancer.${count.index}"
  count=3
  path="/system/"
}

output "arns"{
    value=aws_iam_user.lb[*].arn
}

10. Functions in Terraform🔗

Built-in functions 1. Numeric Functions : abs/ceil/floor/log/max/min/parseint/pow/signum

String chomp/format/formatlist/indent/join/lower/regex/regexall/replace/split/strrev/substr/title/trim/trimprefix/trimsuffix/trimspace/upper
Collection alltrue/anytrue/chunklist/coalesce/coalescelist/compact/concat/contains/distinct/element/flatten/index/keys/length/list/lookup/map/matchkeys/merge/one/range/reverse/setintersection/setproduct/setsubtract/setunion/slice/sort/sum/transpose/values/zipmap
Encoding base64decode/base64encode/base64gzip/csvdecode/jsondecode/jsonencode/textdecodebase64/textencodebase64/urlencode/yamldecode/yamlencode
Filesystem abspath/dirname/pathexpand/basename/file/fileexists/fileset/filebase64/templatefile
Date and Time formatdate/timeadd/timestamp
Hash and crypto base64sha256/base64sha512/bcrypt/filebase64sha256/filebase64sha512/filemd5/filesha1/filesha256/filesha512/md5/rsadecrypt/sha1/sha256/sha512/uuid/uuidv5
IP network cidrhost/cidrnetmask/cidrsubnet/cidrsubnets
Type Conversion can/defaults/nonsensitive/sensitive/tobool/tolist/tomap/tonumber/toset/tostring/try/type

ex:

join :: produces a string by concatenating together all elements of a given list of strings with given delimeter

chomp :: removes newline characters at the end of a string
split :: produces a list by dividing string at all occurences of a given separator
max :: takes one or more numbers and returns the greatest number from the set
slice :: extracts some consecutive elements from within a list

NOTE: User defined functions are not supported NOTE: IMPORTANT - slice is NOT part of "string" function it is part of "Collection" function