Added terraform notes to the knowledge base

knowledge base/terraform.md

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+# Terraform
+
+## TL;DR
+
+```shell
+# init
+terraform init
+terraform init -reconfigure
+
+# check what would be done
+terraform plan
+terraform plan -out path/to/file.tfstate -parallelism 50
+
+# make the changes
+terraform apply
+terraform apply -auto-approve -backup -parallelism 25 path/to/plan.tfstate
+
+# destroy everything
+terraform destroy
+
+# check files formatting
+terraform fmt
+terraform fmt -check -diff -recursive
+
+# validate files
+terraform validate
+
+# create a graph
+# requires dot from graphviz for image generation
+terraform graph
+terraform graph | dot -Tsvg > graph.svg
+
+# show a created resource
+terraform state show 'packet_device.worker'
+terraform state show 'packet_device.worker["example"]'
+terraform state show 'module.foo.packet_device.worker'
+
+# recursively update all needed modules
+terraform get -update -no-color
+```
+
+## Modules
+
+Include a module in the configuration with the `module` keyword:
+
+```hcl
+module "remote_vpc_module" {
+
+  # module settings
+  source  = "terraform-aws-modules/vpc/aws"  # required
+  version = "2.21.0"
+
+  # module variables
+  …
+
+}
+
+module "local_vpc_module" {
+
+  # module settings
+  source = "./modules/aws_vpc"  # required
+
+  # module variables
+  …
+
+}
+```
+
+Run `terraform init` or `terraform get` to install the modules.  
+Modules are installed in the `.terraform/modules` directory inside the configuration's working directory; local modules are symlinked from there.
+
+When terraform processes a module block, that block will inherit the provider from the enclosing configuration.
+
+A module's output can be accessed from the configuration that calls the module through the syntax `module.$moduleName.$outputName`. Module outputs are read-only attributes.
+
+## Gotchas
+
+### `count` vs `for_each`
+
+`count` creates an unordered list of objects, while `for_each` creates a map.
+
+`count` is sensitive to any changes in the list order and this means that if for some reason order of the list is changed terraform will force the replacement of all resources for which the index in the list has changed:
+
+```diff
+variable "my_list" {
+-  default = ["first", "second", "third"]
++  default = ["zeroth", "first", "second", "third"]
+}
+```
+
+```text
+Terraform will perform the following actions:
+
+# null_resource.default[0] must be replaced
+-/+ resource "null_resource" "default" {
+      ~ id       = "4074861383382414527" -> (known after apply)
+      ~ triggers = { # forces replacement
+            "list_index" = "0"
+          ~ "list_value" = "first" -> "zeroth"
+        }
+    }
+…
+# null_resource.default[3] will be created
+  + resource "null_resource" "default" {
+      + id       = (known after apply)
+      + triggers = {
+          + "list_index" = "3"
+          + "list_value" = "third"
+        }
+    }
+```
+
+## Further readings
+
+- [CLI Documentation]
+- [for_each vs count]
+- [Providers best practices]
+- [Version constraints]
+
+[for_each vs count]: https://medium.com/@business_99069/terraform-count-vs-for-each-b7ada2c0b186
+
+[cli documentation]: https://www.terraform.io/docs/cli/
+[providers best practices]: https://www.terraform.io/language/providers/requirements#best-practices-for-provider-versions
+[version constraints]: https://www.terraform.io/language/expressions/version-constraints