feat(terraform): allow traffic routing to be controlled by boolean

docs/infrastructure/terraform/vol-terraform-relationship.md

@@ -0,0 +1,60 @@
+---
+sidebar_position: 20
+---
+
+# Repositories
+
+The VOL infrastructure is divided into two repositories: [`vol-app`](https://github.com/dvsa/vol-app) and [`vol-terraform`](https://github.com/dvsa/vol-terraform).
+
+The rationale for this split is explained in [RFC-005](../../rfc/rfc-005-add-terraform-to-mono-repository.md). At a high level, this approach was the only identified feasible option for implementing the migration the VOL application to ECS and establishing CI/CD workflows.
+
+:::danger
+
+The `vol-app` repository depends on the `vol-terraform` repository. This relationship _must_ remain unidirectional to prevent circular dependencies. Circular dependencies can disrupt disaster recovery processes and complicate infrastructure re-creation.
+
+:::
+
+### Controlling Traffic Flow
+
+ECS-related resources created by `vol-app` were intentionally designed to coexist with the existing EC2 infrastructure provisioned by the `vol-terraform` repository. This approach ensures a smooth transition to the new infrastructure. Traffic flow can be managed by setting a listener rule with a higher priority (`0` being the highest priority).
+
+```mermaid
+sequenceDiagram
+    actor User
+    participant Load Balancer
+    participant EC2
+    participant ECS
+    User->>Load Balancer: Request
+    Load Balancer->>EC2: Route
+    EC2-->>User: Response
+    alt ECS listener rule added with higher priority
+        User->>Load Balancer: Request
+        Load Balancer->>ECS: Route
+        ECS-->>User: Response
+    end
+```
+
+:::tip
+
+The listener rule is controlled by the variable: `service.listener_rule_enable` variable.
+
+To push traffic to the ECS service, set `service.listener_rule_enable = true`. This can be controlled per environment _and_ service.
+
+```hcl showLineNumbers
+# vol-app/infra/terraform/environments/[env]/main.tf
+
+module "service" {
+  source = "../../modules/service"
+
+  # ...
+
+  services = {
+    "service" = {
+      # ...
+
+      // highlight-next-line
+      listener_rule_enable = true # default is `true`.
+    }
+  }
+}
+```

infra/terraform/modules/service/README.md

@@ -49,7 +49,7 @@
 | <a name="input_domain_name"></a> [domain\_name](#input\_domain\_name) | The domain name for the environment | `string` | n/a | yes |
 | <a name="input_environment"></a> [environment](#input\_environment) | The environment to deploy to | `string` | n/a | yes |
 | <a name="input_legacy_environment"></a> [legacy\_environment](#input\_legacy\_environment) | The legacy environment to deploy use | `string` | n/a | yes |
-| <a name="input_services"></a> [services](#input\_services) | The services to deploy | <pre>map(object({<br>    version    = string<br>    repository = string<br>    cpu        = number<br>    memory     = number<br>    task_iam_role_statements = list(object({<br>      effect    = string<br>      actions   = list(string)<br>      resources = list(string)<br>    }))<br>    add_cdn_url_to_env        = optional(bool, false)<br>    lb_listener_arn           = string<br>    listener_rule_priority    = optional(number, 10)<br>    listener_rule_host_header = optional(string, "*")<br>    security_group_ids        = list(string)<br>    subnet_ids                = list(string)<br>    vpc_id                    = optional(string, null)<br>  }))</pre> | `{}` | no |
+| <a name="input_services"></a> [services](#input\_services) | The services to deploy | <pre>map(object({<br>    version    = string<br>    repository = string<br>    cpu        = number<br>    memory     = number<br>    task_iam_role_statements = list(object({<br>      effect    = string<br>      actions   = list(string)<br>      resources = list(string)<br>    }))<br>    add_cdn_url_to_env = optional(bool, false)<br>    lb_listener_arn    = string<br>    // The reason for this was to enable the parallel running of ECS and EC2 services.<br>    // This boolean will control the flow of traffic. If `true`, traffic will go to ECS. If `false`, traffic will go to EC2.<br>    // Can be removed when EC2 services are removed.<br>    listener_rule_enable      = optional(bool, true)<br>    listener_rule_priority    = optional(number, 10)<br>    listener_rule_host_header = optional(string, "*")<br>    security_group_ids        = list(string)<br>    subnet_ids                = list(string)<br>    vpc_id                    = optional(string, null)<br>  }))</pre> | `{}` | no |
 | <a name="input_vpc_id"></a> [vpc\_id](#input\_vpc\_id) | The VPC ID | `string` | n/a | yes |
 
 ## Outputs

infra/terraform/modules/service/ecs.tf

@@ -20,7 +20,10 @@ resource "aws_lb_target_group" "this" {
 }
 
 resource "aws_lb_listener_rule" "this" {
-  for_each = var.services
+  for_each = {
+    for service, config in var.services : service => config
+    if config.listener_rule_enable
+  }
 
   listener_arn = each.value.lb_listener_arn
   priority     = each.value.listener_rule_priority

infra/terraform/modules/service/variables.tf

@@ -34,8 +34,12 @@ variable "services" {
       actions   = list(string)
       resources = list(string)
     }))
-    add_cdn_url_to_env        = optional(bool, false)
-    lb_listener_arn           = string
+    add_cdn_url_to_env = optional(bool, false)
+    lb_listener_arn    = string
+    // The reason for this was to enable the parallel running of ECS and EC2 services.
+    // This boolean will control the flow of traffic. If `true`, traffic will go to ECS. If `false`, traffic will go to EC2.
+    // Can be removed when EC2 services are removed.
+    listener_rule_enable      = optional(bool, true)
     listener_rule_priority    = optional(number, 10)
     listener_rule_host_header = optional(string, "*")
     security_group_ids        = list(string)

website/docusaurus.config.ts

@@ -69,6 +69,7 @@ const config: Config = {
     prism: {
       theme: lightTheme,
       darkTheme: darkTheme,
+      additionalLanguages: ["hcl"],
     },
   } satisfies Preset.ThemeConfig,