Support for relative paths Design Doc in SPK (#459)

* rebasing off master

* updated docs format - standardized

Co-authored-by: Dennis Seah <dennis.seah@gmail.com>

technical-docs/designs/infra/relativepaths.md

@@ -0,0 +1,256 @@
+# Relative Paths for Bedrock Terraform Infrastructure
+
+| Revision | Date         | Author         | Remarks                     |
+| -------: | ------------ | -------------- | --------------------------- |
+|      0.1 | Mar-23, 2020 | Nathaniel Rose | Initial Draft               |
+|      0.2 | Mar-26, 2020 | Nathaniel Rose | Changes Reflected from Team |
+
+## 1. Overview
+
+Terraform modules use a source parameter to locate the resource module target to
+download in order to initialize a terraform environment for deployment. A module
+can call other modules, which lets you include the child module's resources into
+the configuration in a concise way. Modules can also be called multiple times,
+either within the same configuration or in separate configurations, allowing
+resource configurations to be packaged and re-used.
+
+This document outlines multiple solution that seeks to validate alternative ways
+in which relative paths for child module may be supported in Bedrock
+implementation and the SPK cli tooling.
+
+```
+module "servers" {
+  source = "./app-cluster"
+
+  servers = 5
+}
+```
+
+This design shall only target supporting relative paths in spk scaffolding and
+generation.
+
+### Issues Addressed:
+
+1. As an SRE, I want to use module sources that are versioned and localalized to
+   the terraform templates repo.
+2. As an Operator, I want automated CI pipelines that can handle internal
+   references for incoming PR that modify my custom Bedrock modules.
+
+## 2. Solution
+
+In the current implementation on SPK infra practices, the tool is embedded into
+a generation pipeline that provisions terraform files respective to the modified
+infra HLD. `spk infra generate` provisions a terraform deployment template files
+based on the key values provided in the target directory's `definition.yaml`. If
+the `source` template in the `definition.yaml` uses relative paths in respects
+to the source, `infra generate` command will produce a terraform file with the
+module source out of scope from the provisioned terraform files.
+
+When executing a `terraform init` on the generated deployment files it produces
+an error similar to the following:
+
+```
+Initializing modules...
+- aks-gitops in
+- provider in
+- vnet in
+
+Error: Unreadable module directory
+
+Unable to evaluate directory symlink: lstat ../../azure: no such file or
+directory
+```
+
+### 2.1 Munge together URL with relative Path in SPK Generate - **ACCEPTED DESIGN**
+
+One option to address this issue is to directly modify the generated `.tf` files
+to reflect the respective module source. Inside the `infra generate` command we
+can use the `source`, `version`, and `template` values to modify the terraform
+child module source to a remote module source for terraform to download upon
+initialization.
+
+Example:
+
+**`Definition.yaml`**
+
+```
+name: fabrikam
+source: "https://github.com/Microsoft/bedrock.git"
+template: cluster/environments/azure-single-keyvault
+version: nate.infra.relative.paths
+.
+.
+.
+```
+
+Use a function to get the module path from root of remote repo in cached
+directory. (`pathFunction`)
+
+> source= ~~https://~~`source`?ref=`version`//`pathFunction()`"
+
+**Output `Main.tf`**
+
+```
+terraform {
+  backend "azurerm" {}
+}
+
+data "azurerm_client_config" "current" {}
+
+data "azurerm_resource_group" "cluster_rg" {
+  name = var.resource_group_name
+}
+
+data "azurerm_resource_group" "keyvault" {
+  name = var.keyvault_resource_group
+}
+
+data "azurerm_subnet" "vnet" {
+  name                 = var.subnet_name
+  virtual_network_name = var.vnet_name
+  resource_group_name  = data.azurerm_resource_group.keyvault.name
+}
+
+module "aks-gitops" {
+  source = "github.com/microsoft/bedrock?ref=master//cluster/azure/aks-gitops"
+}
+
+# Create Azure Key Vault role for SP
+module "keyvault_flexvolume_role" {
+  source = "github.com/microsoft/bedrock?ref=master//cluster/azure/keyvault_flexvol_role"
+}
+
+# Deploy central keyvault flexvolume
+module "flex_volume" {
+  source = "github.com/microsoft/bedrock?ref=master//cluster/azure/keyvault_flexvol"
+}
+```
+
+## 3. Dependencies
+
+An existing pull request for Bedrock currently exists that verifies relative
+path implementation with the use of Bedrock Provided Templates in
+[#1189](https://github.com/microsoft/bedrock/pull/1189). This design document
+seeks to propose interoperability of the relative paths with SPK tooling and
+adjustment of infrastructure pipeline.
+
+## 4. Risks & Mitigations
+
+Limitations to the solution include:
+
+- Manipulating user input data and output terraform files
+- Potential Regex parsing for URL validation to detect relative paths
+
+## 5. Documentation
+
+Yes. A brief update to the
+[`spk-infra-generation-pipeline.md`](/guides/infra/spk-infra-generation-pipeline.md)
+detailing relative path support for module sources.
+
+## 6. Alternatives
+
+### 6.1 Copy Modules to Alternative Configured Generated Directory
+
+Another option is to copy modules from cached remote directory to the generated
+folder. This allows SPK to directly reference the parent module source with the
+generated terraform templates. In addition, this would also require the
+templates to be placed accordingly in respects to relative paths to parent
+modules. In Bedrock, the template folders are 3 levels down
+(../../../cluster/azure/keyvault_flex_vol")
+
+```
+fabrikam/
+    definition.yaml
+    fabrikam-central/
+        definition.yaml
+    fabrikam-east/
+        definition.yaml
+    fabrikam-west/
+        definition.yaml
+fabrikam-generated/
+    cluster
+        azure
+        common
+        minikube
+        environments
+            deployments
+                fabrikam-central/
+                    main.tf
+                    variables.tf
+                    keyvault.tf
+                    terraform.tfvars
+                    backend.tfvars
+                fabrikam-east/
+                    main.tf
+                    variables.tf
+                    keyvault.tf
+                    terraform.tfvars
+                    backend.tfvars
+                fabrikam-west/
+                    main.tf
+                    variables.tf
+                    keyvault.tf
+                    terraform.tfvars
+                    backend.tfvars
+```
+
+Limitations:
+
+- Very coupled to current organization of Bedrock and will have breaking changes
+  when introducing new folders at template /module levels
+- Copying modules twice local to agent, once for cache and again during generate
+
+### 6.2 Use a symlink to reference the modules
+
+A symbolic link, also termed a soft link, is a special kind of file that points
+to another file, much like a shortcut in Windows or a Macintosh alias. This will
+allow generate the alias the cached repo for all `spk infra generate` commands.
+
+`ln -s modules /path/to/modules`
+
+Inside of spk, the terraform files will now reference the symlink which is
+cached.
+
+**Output `Main.tf`**
+
+```
+terraform {
+  backend "azurerm" {}
+}
+
+data "azurerm_client_config" "current" {}
+
+data "azurerm_resource_group" "cluster_rg" {
+  name = var.resource_group_name
+}
+
+data "azurerm_resource_group" "keyvault" {
+  name = var.keyvault_resource_group
+}
+
+data "azurerm_subnet" "vnet" {
+  name                 = var.subnet_name
+  virtual_network_name = var.vnet_name
+  resource_group_name  = data.azurerm_resource_group.keyvault.name
+}
+
+module "aks-gitops" {
+  source = "modules/azure/aks-gitops"
+}
+
+# Create Azure Key Vault role for SP
+module "keyvault_flexvolume_role" {
+  source = "modules/azure/keyvault_flexvol_role"
+}
+
+# Deploy central keyvault flexvolume
+module "flex_volume" {
+  source = "modules/azure/keyvault_flexvol"
+}
+```
+
+Limitations:
+
+- Modifying the terraform output files.
+- Added complexity with symlink alias which prevents ease of simply running
+  terraform commands on native machine