Clarify docs: map and destroy

examples/simple/README.md

@@ -1,18 +1,44 @@
 This example demonstrates a basic usage of this module. 
 
-On a system, you have a root module tfstate-s3-manager which 
-includes terraform-aws-multi-stack-backends, and the main.tf
-defines the stacks to manage: 
-
-- there are 2 stacks, each with 2 root modules, so a total of
-  4 folders
-- the structure of the folder is irrelevant, as they are 
-  specified by path so wherever they are located on the local 
-  system, is where terraform-aws-multi-stack-backends will 
-  look for them
-
-First you `terraform apply` each root module of each stack. 
-The order does not matter. 
+On your system, 
+
+- you have a root module tfstate-s3-manager which 
+includes terraform-aws-multi-stack-backends
+- you have (in this example) 4 root modules, each in their own folder
+- two of those root modules belong to one stack, the other two belong
+  to the other stack
+- the `tfstate-s3-manager/main.tf` defines the stacks to manage: 
+  ```hcl
+  stacks_map = {
+    stack-1 = {
+      network = {
+        path = "../stack1-network"
+      }
+      cluster = {
+        path = "../stack1-cluster"
+      }
+    },
+    stack-2 = {
+      network = {
+        path = "../stack2-network"
+      }
+      cluster = {
+        path = "../stack2-cluster"
+      }
+    },
+  }
+  ```
+- the names are arbitrary (stack-1, network, etc)
+
+See the left green box in following diagram: 
+
+![Root modules on local filesystem (left green box)](stack-backends-setup.png)
+
+### Step 1
+
+Assume you have already run `terraform apply` in each root 
+module of each stack. The order does not matter. This step 
+can be done after step 2 as well. 
 
 Then `terraform apply` in the `tfstate-s3-manager` root module, 
 which will create the common bucket, its replica, dynamodb 
@@ -22,21 +48,50 @@ in the `main.tf`). See the following diagram.
 
 ![Step 1: setup common bucket and stack backend files](stack-backends-setup.png)
 
+### Step 2
+
 Then you should (although this is optional) move the 
 `tfstate-s3-manager` tfstate to s3, by running `terraform init`
 for that root module. This can be done before or after step 3.
 
 ![Step 2: move manager state to s3](move-manager-to-s3.png)
 
+### Step 3
+
 Finally you can go to each stack's root module's and `terraform
 init`. This will move each stack's root module's tfstate from
 local host to s3, using the `backend.tf` generated by the 
 `tfstate-s3-manager`. See the following diagram. 
 
 ![Step 3: move stack states to s3](move-stacks-to-s3.png)
 
+### Subsequently
+
 From then on, whenever you `terraform apply` in one of the 
 root modules associated with a stack, the *whole stack* will 
 get locked. See the following diagram. 
 
 ![Stack state locking in s3](stack-state-locking.png)
+
+To delete a root module from a stack, if the stack has other 
+root modules in main.tf
+
+- `terraform destroy` for the root module you no longer need
+- edit the `tfstate-s3-manager/main.tf` by removing the entry; 
+  ATTENTION: if there are no modules left for that stack in main.tf,
+  *leave the stack's empty map* so that the dynamodb table does not
+  get destroyed yet, eg
+  ```hcl
+  stacks_map = {
+    stack-3 = {}, // no longer needed
+  }
+  ```
+- depending on your situation:
+
+  - just delete the module folder, OR 
+  - `terraform apply` to remove the corresponding `backend.tf` then
+    `terraform init`
+
+Once a stack's module have all been removed, you can delete the empty
+stack map from stacks_map, and `terraform apply` to remove the dynamodb
+table for that stack.