## Variables
variable "database_name" {
   description = "The database name"
   default     = "DB_Name"
}

variable "database_username" {
   description = "The username of the database"
   default     = "postgres"
}

variable "database_password" {
   description = "The password of the database"
   default     = "1234567890"
}

...

## RDS
resource "aws_db_instance" "rds" {
  name                        = var.database_name
  identifier                  = "${var.environment}-database"
  username                    = var.database_username
  password                    = var.database_password
  port                        = "5432"
  engine                      = "postgres"
  engine_version              = "11"
  ...
}

## the task definition for the app
data "template_file" "app_task" {
   template = file("./templates/ecs/app_task.json.tpl")

   vars = {
      app_name             = "app"
      app_image            = var.app_image
      app_port             = var.app_port
      fargate_cpu          = 512
      fargate_memory       = 1024
      aws_region           = var.aws_region

      log_group            = aws_cloudwatch_log_group.app_log_group.name
      database_url         = "postgres://${var.database_username}:${var.database_password}@${local.database_endpoint}/${var.database_name}"
      ## database_url      = "postgresql://${var.database_username}:${var.database_password}@${local.database_endpoint}:5432/${var.database_name}"
      database_name        = var.database_name
      database_username    = var.database_username
      database_password    = var.database_password
      ...
}

## app_task.json.tpl
[
  {
    "name": "${app_name}",
    "image": "${app_image}",
    "cpu": ${fargate_cpu},
    "memory": ${fargate_memory},
    "networkMode": "awsvpc",
    "memoryReservation": 512,
    "logConfiguration": {
        "logDriver": "awslogs",
        "options": {
          "awslogs-group": "${log_group}",
          "awslogs-region": "${aws_region}",
          "awslogs-stream-prefix": "ecs"
        }
    },
    "portMappings": [
      {
        "containerPort": ${app_port},
        "hostPort": ${app_port}
      }
    ],
    "entryPoint": [
      "python", "main.py"
    ],
    "environment": [
      {
        "name": "POSTGRES_URL",
        "value": "${database_url}"
      },
      {
        "name": "POSTGRES_PORT",
        "value": "5432"
      },
      {
        "name": "POSTGRES_USER",
        "value": "${database_username}"
      },
      {
        "name": "POSTGRES_PASSWORD",
        "value": "${database_password}"
      },
      {
        "name": "POSTGRES_DB",
        "value": "${database_name}"
      },
      ...
   }
]