Document Terraform HTTP backend with GitLab API

doc/user/infrastructure/index.md

-# Infrastructure as Code
+# Infrastructure as code with GitLab managed Terraform State
 
-GitLab can be used to manage infrastructure as code. The following are some examples:
+[Terraform remote backends](https://www.terraform.io/docs/backends/index.html)
+enable you to store the state file in a remote, shared store. GitLab uses the
+[Terraform HTTP backend](https://www.terraform.io/docs/backends/types/http.html)
+to securely store the state files in local storage (the default) or
+[the remote store of your choice](../../administration/terraform_state.md).
 
-- [A generic tutorial for Terraform with GitLab](https://medium.com/@timhberry/terraform-pipelines-in-gitlab-415b9d842596).
-- [Terraform at GitLab](https://about.gitlab.com/blog/2019/11/12/gitops-part-2/).
+The GitLab managed Terraform state backend can store your Terraform state easily and
+securely, and spares you from setting up additional remote resources like
+Amazon S3 or Google Cloud Storage. Its features include:
+
+- Supporting encryption of the state file both in transit and at rest.
+- Locking and unlocking state.
+- Remote Terraform plan and apply execution.
+
+To get started, there are two different options when using GitLab managed Terraform State.
+
+- Use a local machine
+- Use GitLab CI
+
+## Get Started using local development
+
+If you are planning to only run `terraform plan` and `terraform apply` commands from your local machine, this is a simple way to get started.
+
+First, create your project on your GitLab instance.
+
+Next, define the Terraform backend in your Terraform project to be:
+
+```hcl
+terraform {
+  backend "http" {
+  }
+}
+```
+
+Finally, you need to run `terraform init` on your local machine and pass in the following options. The below example is using GitLab.com:
+
+```bash
+terraform init \
+    -backend-config="address=https://gitlab.com/api/v4/projects/<YOUR-PROJECT-ID>/terraform/state/<YOUR-PROJECT-NAME>" \
+    -backend-config="lock_address=https://gitlab.com/api/v4/projects/<YOUR-PROJECT-ID>/terraform/state/<YOUR-PROJECT-NAME>/lock" \
+    -backend-config="unlock_address=https://gitlab.com/api/v4/projects/<YOUR-PROJECT-ID>/terraform/state/<YOUR-PROJECT-NAME>/lock" \
+    -backend-config="username=<YOUR-USERNAME>" \
+    -backend-config="password=<YOUR-ACCESS-TOKEN>" \
+    -backend-config="lock_method=POST" \
+    -backend-config="unlock_method=DELETE" \
+    -backend-config="retry_wait_min=5"
+```
+
+This will initialize your Terraform state and store that state within your GitLab project.
+
+NOTE: YOUR-PROJECT-ID and YOUR-PROJECT-NAME can be accessed from the project main page.
+
+## Get Started using a GitLab CI
+
+Another route is to leverage GitLab CI to run your `terraform plan` and `terraform apply` commands.
+
+### Configure the CI variables
+
+To use the Terraform backend, [first create a Personal Access Token](../profile/personal_access_tokens.md) with the `api` scope. Keep in mind that the Terraform backend is restricted to tokens with [Maintainer access](../permissions.md) to the repository.
+
+To keep the Personal Access Token secure, add it as a [CI/CD environment variable](../../ci/variables/README.md). In this example we set ours to the ENV: `GITLAB_TF_PASSWORD`.
+
+If you are planning to use the ENV on a branch which is not protected, make sure to set the variable protection settings correctly.
+
+### Configure the Terraform backend
+
+Next we need to define the [http backend](https://www.terraform.io/docs/backends/types/http.html). In your Terraform project add the following code block in a `.tf` file such as `backend.tf` or wherever you desire to define the remote backend:
+
+```hcl
+terraform {
+  backend "http" {
+  }
+}
+```
+
+### Configure the CI YAML file
+
+Finally, configure a `.gitlab-ci.yaml`, which lives in the root of your project repository.
+
+In our case we are using a pre-built image:
+
+```yaml
+image:
+  name: hashicorp/terraform:light
+  entrypoint:
+    - '/usr/bin/env'
+    - 'PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin'
+```
+
+We then define some environment variables to make life easier. `GITLAB_TF_ADDRESS` is the URL of the GitLab instance where this pipeline runs, and `TF_ROOT` is the directory where the Terraform commands must be executed.
+
+```yaml
+variables:
+  GITLAB_TF_ADDRESS: ${CI_API_V4_URL}/projects/${CI_PROJECT_ID}/terraform/state/${CI_PROJECT_NAME}
+  TF_ROOT: ${CI_PROJECT_DIR}/environments/cloudflare/production
+
+cache:
+  paths:
+    - .terraform
+```
+
+In a `before_script`, pass a `terraform init` call containing configuration parameters.
+These parameters correspond to variables required by the
+[http backend](https://www.terraform.io/docs/backends/types/http.html):
+
+```yaml
+before_script:
+  - cd ${TF_ROOT}
+  - terraform --version
+  - terraform init -backend-config="address=${GITLAB_TF_ADDRESS}" -backend-config="lock_address=${GITLAB_TF_ADDRESS}/lock" -backend-config="unlock_address=${GITLAB_TF_ADDRESS}/lock" -backend-config="username=${GITLAB_USER_LOGIN}" -backend-config="password=${GITLAB_TF_PASSWORD}" -backend-config="lock_method=POST" -backend-config="unlock_method=DELETE" -backend-config="retry_wait_min=5"
+
+stages:
+  - validate
+  - build
+  - test
+  - deploy
+
+validate:
+  stage: validate
+  script:
+    - terraform validate
+
+plan:
+  stage: build
+  script:
+    - terraform plan
+    - terraform show
+
+apply:
+  stage: deploy
+  environment:
+    name: production
+  script:
+    - terraform apply
+  dependencies:
+    - plan
+  when: manual
+  only:
+    - master
+```
+
+### Push to GitLab
+
+Pushing your project to GitLab triggers a CI job pipeline, which runs the `terraform init`, `terraform validate`, and `terraform plan` commands automatically.
+
+The output from the above `terraform` commands should be viewable in the job logs.
+
+## Example project
+
+See [this reference project](https://gitlab.com/nicholasklick/gitlab-terraform-aws) using GitLab and Terraform to deploy a basic AWS EC2 within a custom VPC.