[Terraform](https://www.terraform.io/intro): "HashiCorp Terraform is an infrastructure as code tool that lets you define both cloud and on-prem resources in human-readable configuration files that you can version, reuse, and share. You can then use a consistent workflow to provision and manage all of your infrastructure throughout its lifecycle. Terraform can manage low-level components like compute, storage, and networking resources, as well as high-level components like DNS entries and SaaS features."
- Full automation: In the past, resource creation, modification and removal were handled manually or by using a set of tooling. With Terraform or other IaC technologies, you manage the full lifecycle in an automated fashion.<br>
- Modular and Reusable: Code that you write for certain purposes can be used and assembled in different ways. You can write code to create resources on a public cloud and it can be shared with other teams who can also use it in their account on the same (or different) cloud><br>
- Improved testing: Concepts like CI can be easily applied on IaC based projects and code snippets. This allow you to test and verify operations beforehand
- Declarative: Terraform uses the declarative approach (rather than the procedural one) in order to define end-status of the resources
- No agents: as opposed to other technologies (e.g. Puppet) where you use a model of agent and server, with Terraform you use the different APIs (of clouds, services, etc.) to perform the operations
- Community: Terraform has strong community who constantly publishes modules and fixes when needed. This ensures there is good modules maintenance and users can get support quite quickly at any point
1. Write Terraform definitions: `.tf` files written in HCL that described the desired infrastructure state (and run `terraform init` at the very beginning)
This is a manual process. Most of the time this is automated so user submits a PR/MR to propose terraform changes, there is a process to test these changes and once merged they are applied (`terraform apply`).
- Infra provisioning and management: You need to automated or code your infra so you are able to test it easily, apply it and make any changes necessary.
- Consistent environments: You manage environments such as test, production, staging, ... and looking for a way to have them consistent so any modification in one of them, applies to other environments as well
<summary>What's the difference between Terraform and technologies such as Ansible, Puppet, Chef, etc.</summary><br><b>
Terraform is considered to be an IaC technology. It's used for provisioning resources, for managing infrastructure on different platforms.
Ansible, Puppet and Chef are Configuration Management technologies. They are used once there is an instance running and you would like to apply some configuration on it like installing an application, applying security policy, etc.
To be clear, CM tools can be used to provision resources so in the end goal of having infrastructure, both Terraform and something like Ansible, can achieve the same result. The difference is in the how. Ansible doesn't saves the state of resources, it doesn't know how many instances there are in your environment as opposed to Terraform. At the same time while Terraform can perform configuration management tasks, it has less modules support for that specific goal and it doesn't track the task execution state as Ansible. The differences are there and it's most of the time recommended to mix the technologies, so Terraform used for managing infrastructure and CM technologies used for configuration on top of that infrastructure.
<summary>Explain the following block of Terraform code
```
resource "aws_instance" "some-instance" {
ami = "ami-201720221991yay"
instance_type = "t2.micro
}
```
</summary><br><b>
It's a resource of type "aws_instance" used to provision an instance. The name of the resource (NOT INSTANCE) is "some-instance".
The instance itself will be provisioned with type "t2.micro" and using an image of the AMI "ami-201720221991yay".
</b></details>
<details>
<summary>What do you do next after writing the following in main.tf file?
```
resource "aws_instance" "some-instance" {
ami = "ami-201720221991yay"
instance_type = "t2.micro
}
```
</summary><br><b>
Run `terraform init`. This will scan the code in the directory to figure out which providers are used (in this case AWS provider) and will download them
</b></details>
<details>
<summary>You've executed <code>terraform init</code> and now you would like to move forward to creating the resources but you have concerns and would like to make be 100% sure on what you are going to execute. What should you be doing?</summary><br><b>
Execute `terraform plan`. That will provide a detailed information on what Terraform will do once you apply the changes.
</b></details>
<details>
<summary>You've downloaded the providers, seen the what Terraform will do (with terraform plan) and you are ready to actually apply the changes. What should you do next?</summary><br><b>
Run `terraform apply`. That will apply the changes described in your .tf files.
</b></details>
<details>
<summary>Explain the meaning of the following strings that seen at the beginning of each line When you run <code>terraform apply</code>
* '+'
* '-'
* '-/+'
</summary><br><b>
* '+' - The resource or attribute is going to be added
* '-' - the resource or attribute is going to be removed
* '-/+' - the resource or attribute is going to be replaced
</b></details>
### Dependencies
<details>
<summary>Sometimes you need to reference some resources in the same or separate .tf file. Why and how it's done?</summary><br><b>
Why: because resources are sometimes connected or need to be connected. For example, you create an AWS instance with "aws_instance" resource but, at the same time you would like also to allow some traffic to it (because by default traffic is not allowed). For that you'll create a "aws_security_group" resource and then, in your aws_instance resource, you'll reference it.
<summary>Does it matter in which order Terraform creates resources?</summary><br><b>
Yes, when there is a dependency between different Terraform resources, you want the resources to be created in the right order and this is exactly what Terraform does.
To make it ever more clear, if you have a resource X that references the ID of resource Y, it doesn't makes sense to create first resource X because it won't have any ID to get from a resource that wasn't created yet.
</b></details>
<details>
<summary>Is there a way to print/see the dependencies between the different resources?</summary><br><b>
Yes, with `terraform graph`
The output is in DOT - A graph description language.
<summary>Explain what is a "provider"</summary><br><b>
[terraform.io](https://www.terraform.io/docs/language/providers/index.html): "Terraform relies on plugins called "providers" to interact with cloud providers, SaaS providers, and other APIs...Each provider adds a set of resource types and/or data sources that Terraform can manage. Every resource type is implemented by a provider; without providers, Terraform can't manage any kind of infrastructure."
<summary>Write a configuration of a Terraform provider (any type you would like)</summary><br><b>
AWS is one of the most popular providers in Terraform. Here is an example of how to configure it to use one specific region and specifying a specific version of the provider
Since a provisioner can run a variety of actions, it's not always feasible to plan and understand what will happen when running a certain provisioner. For this reason, it's usually recommended to use Terraform built-in option, whenever's possible.
<code>terraform taint resource.id</code> manually marks the resource as tainted in the state file. So when you run <code>terraform apply</code> the next time, the resource will be destroyed and recreated.
<summary>What are output variables and what <code>terraform output</code> does?</summary><br><b>
Output variables are named values that are sourced from the attributes of a module. They are stored in terraform state, and can be used by other modules through <code>remote_state</code>
State locking is a mechanism that blocks an operations against a specific state file from multiple callers so as to avoid conflicting operations from different team members. Once the first caller's operation's lock is released the other team member may go ahead to
carryout his own operation. Nevertheless Terraform will first check the state file to see if the desired resource already exist and
[Terraform.io](https://www.terraform.io/language/modules/develop): "A module is a container for multiple resources that are used together. Modules can be used to create lightweight abstractions, so that you can describe your infrastructure in terms of its architecture, rather than directly in terms of physical objects."
</b></details>
<details>
<summary>How do you test a terraform module?</summary><br><b>
There are multiple answers, but the most common answer would likely to be using the tool <code>terratest</code>, and to test that a module can be initialized, can create resources, and can destroy those resources cleanly.
<summary>There's a discussion in your team whether to store modules in one centralized location/repository or have them in each of the projects/repositories where they are used. What's your take on that?</summary><br><b>
You might have a different opinion but my personal take on that, is to keep modules in one centralized repository as any maintenance or updates to the module you need to perform, are done in one place instead of multiple times in different repositories.
<summary>What variables are good for in Terraform?</summary><br><b>
Variables allow you define piece of data in one location instead of repeating the hardcoded value of it in multiple different locations. Then when you need to modify the variable's value, you do it in one location instead of changing each one of the hardcoded values.
<summary>How would you enforce users that use your variables to provide values with certain constraints? For example, a number greater than 1</summary><br><b>
Using `validation` block
```
variable "some_var" {
type = number
validation {
condition = var.some_var > 1
error_message = "you have to specify a number greater than 1"
}
}
```
</b></details>
<details>
<summary>What is the effect of setting variable as "sensitive"?</summary><br><b>
It doesn't show its value when you run `terraform apply` or `terraform plan` but eventually it's still recorded in the state file.
You can use something like the `-var` option to provide the value and avoid being prompted to insert a value. Another option is to run `export TF_VAR_<VAR_NAME>=<VALUE>`.
</b></details>
#### Variables Hands-On
<details>
<summary>Demonstrate variable definition with type, description and default parameters</summary><br><b>
[Terraform.io](https://www.terraform.io/language/state): "Terraform must store state about your managed infrastructure and configuration. This state is used by Terraform to map real world resources to your configuration, keep track of metadata, and to improve performance for large infrastructures."
<summary>Why does it matter where you store the tfstate file? Where would you store it?</summary><br><b>
- tfstate contains credentials in plain text. You don't want to put it in publicly shared location
- tfstate shouldn't be modified concurrently so putting it in a shared location available for everyone with "write" permissions might lead to issues. (Terraform remote state doesn't has this problem).
- tfstate is in important file. As such, it might be better to put it in a location that has regular backups.
As such, tfstate shouldn't be stored in git repositories. secured storage such as secured buckets, is a better option.
Terraform import is used to import existing infrastructure. It allows you to bring resources created by some other means (eg. manually launched cloud resources) and bring it under Terraform management.
Yes, It can also be stored remotely, which works better in a team environment. Given condition that remote location is not publicly accessible since tfstate file contain sensitive information as well. Access to this remote location must be only shared with team members.
<summary>How and why concurrent edits of the state file should be avoided?</summary><br><b>
If there are two users or processes concurrently editing the state file it can result in invalid state file that doesn't actually represents the state of resources.<br>
To avoid that, Terraform can apply state locking if the backend supports that. For example, AWS s3 supports state locking and consistency via DynamoDB. Often, if the backend support it, Terraform will make use of state locking automatically so nothing is required from the user to activate it.
</b></details>
<details>
<summary>Describe how you manage state file(s) when you have multiple environments (e.g. development, staging and production)</summary><br><b>
There is no right or wrong here, but it seems that the overall preferred way is to have a dedicated state file per environment.
</b></details>
<details>
<summary>How to write down a variable which changes by an external source or during <code>terraform apply</code>?</summary><br><b>
You use it this way: <code>variable “my_var” {}</code>
</b></details>
<details>
<summary>You've deployed a virtual machine with Terraform and you would like to pass data to it (or execute some commands). Which concept of Terraform would you use?</summary><br><b>
<summary>You have a Git repository with Terraform files but no .gitignore. What would you add to a .gitignore file in Terraform repository?</summary><br><b>
```
.terraform
*.tfstate
*.tfstate.backup
```
You don't want to store state file nor any downloaded providers in .terraform directory. It also doesn't makes sense to share/store the state backup files.
