Add document

Author: chaspy

docs/rules/README.md

@@ -11,6 +11,7 @@ These rules warn of possible errors that can occur at `terraform apply`. Rules m
 |aws_alb_invalid_security_group|✔|
 |aws_alb_invalid_subnet|✔|
 |aws_db_instance_invalid_db_subnet_group|✔|
+|aws_db_instance_invalid_engine||
 |aws_db_instance_invalid_option_group|✔|
 |aws_db_instance_invalid_parameter_group|✔|
 |aws_db_instance_invalid_type||
@@ -52,6 +53,7 @@ These rules warn of possible errors that can occur at `terraform apply`. Rules m
 These rules suggest to better ways.
 
 - [aws_instance_previous_type](aws_instance_previous_type.md)
+- [aws_db_instance_invalid_engine](aws_db_instance_invalid_engine.md)
 - [aws_db_instance_previous_type](aws_db_instance_previous_type.md)
 - [aws_db_instance_default_parameter_group](aws_db_instance_default_parameter_group.md)
 - [aws_elasticache_cluster_previous_type](aws_elasticache_cluster_previous_type.md)

docs/rules/aws_db_instance_invalid_engine.md

@@ -0,0 +1,38 @@
+# aws_db_instance_invalid_engine
+
+Disallow using invalid engine name.
+
+## Example
+
+```hcl
+resource "aws_db_instance" "default" {
+  allocated_storage    = 20
+  engine               = "mysql57" // invalid engine name!
+  engine_version       = "5.7"
+  instance_class       = "db.t2.micro"
+  name                 = "mydb"
+  username             = "foo"
+  password             = "bar"
+  db_subnet_group_name = "my_database_subnet_group"
+  parameter_group_name = "default.mysql5.6"
+}
+```
+
+```
+$ tflint
+1 issue(s) found:
+
+Warning: "mysql57" is invalid engine. (aws_db_invalid_engine)
+
+  on template.tf line 3:
+   3:   engine               = "mysql57" // invalid engine name!
+ 
+```
+
+## Why
+
+Apply will fail. (Plan will succeed with the invalid value though)
+
+## How To Fix
+
+Select valid engine name according to the [document](https://docs.aws.amazon.com/cli/latest/reference/rds/describe-db-engine-versions.html#options)