# ###################################
##       Rule Specification        ##
#####################################
# 
# Rule Identifier:
#   rds_instance_enhanced_monitoring_enabled_check
# 
# Description:
#   This control checks whether enhanced monitoring is activated for Amazon RDS instances.
# 
# Reports on:
#   AWS::RDS::DBInstance
# 
# Evaluates:
#   AWS CloudFormation, AWS CloudFormation hook
# 
# Rule Parameters:
#   None
# 
# Scenarios:
#   Scenario: 1
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document does not contain any RDS DB instance resources
#      Then: SKIP
#   Scenario: 2
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains an RDS DB instance resource
#       And: 'Engine' is not one of 'aurora', 'aurora-mysql', 'aurora-postgresql', 'mariadb', 'mysql',
#            'oracle-ee', 'oracle-se2', 'oracle-se1', 'oracle-se', 'postgres', 'sqlserver-ee',
#            'sqlserver-se', 'sqlserver-ex', 'sqlserver-web'
#      Then: SKIP
#   Scenario: 3
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains an RDS DB instance resource
#       And: 'Engine' is one of 'aurora', 'aurora-mysql', 'aurora-postgresql', 'mariadb', 'mysql',
#            'oracle-ee', 'oracle-se2', 'oracle-se1', 'oracle-se', 'postgres', 'sqlserver-ee',
#            'sqlserver-se', 'sqlserver-ex', 'sqlserver-web'
#       And: 'MonitoringInterval' has not been specified
#      Then: FAIL
#   Scenario: 4
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains an RDS DB instance resource
#       And: 'Engine' is one of 'aurora', 'aurora-mysql', 'aurora-postgresql', 'mariadb', 'mysql',
#            'oracle-ee', 'oracle-se2', 'oracle-se1', 'oracle-se', 'postgres', 'sqlserver-ee',
#            'sqlserver-se', 'sqlserver-ex', 'sqlserver-web'
#       And: 'MonitoringInterval' has been specified
#       And: 'MonitoringInterval' has been set to '0'
#      Then: FAIL
#   Scenario: 5
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains an RDS DB instance resource
#       And: 'Engine' is one of 'aurora', 'aurora-mysql', 'aurora-postgresql', 'mariadb', 'mysql',
#            'oracle-ee', 'oracle-se2', 'oracle-se1', 'oracle-se', 'postgres', 'sqlserver-ee',
#            'sqlserver-se', 'sqlserver-ex', 'sqlserver-web'
#       And: 'MonitoringInterval' has been specified
#       And: 'MonitoringInterval' has not been set to a value from the list 1, 5, 10, 15, 30, 60
#      Then: FAIL
#   Scenario: 6
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains an RDS DB instance resource
#       And: 'Engine' is one of 'aurora', 'aurora-mysql', 'aurora-postgresql', 'mariadb', 'mysql',
#            'oracle-ee', 'oracle-se2', 'oracle-se1', 'oracle-se', 'postgres', 'sqlserver-ee',
#            'sqlserver-se', 'sqlserver-ex', 'sqlserver-web'
#       And: 'MonitoringInterval' has been specified
#       And: 'MonitoringInterval' has been set to a value from the list 1, 5, 10, 15, 30, 60
#       And: 'MonitoringRoleArn' has not been specified or specified as an empty string
#      Then: FAIL
#   Scenario: 7
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains an RDS DB instance resource
#       And: 'Engine' is one of 'aurora', 'aurora-mysql', 'aurora-postgresql', 'mariadb', 'mysql',
#            'oracle-ee', 'oracle-se2', 'oracle-se1', 'oracle-se', 'postgres', 'sqlserver-ee',
#            'sqlserver-se', 'sqlserver-ex', 'sqlserver-web'
#       And: 'MonitoringInterval' has been specified
#       And: 'MonitoringInterval' has been set to a value from the list 1, 5, 10, 15, 30, 60
#       And: 'MonitoringRoleArn' has been specified with a non-empty string or valid local reference
#      Then: PASS

#
# Constants
#
let RDS_DB_INSTANCE_TYPE = "AWS::RDS::DBInstance"
let INPUT_DOCUMENT = this
let SUPPORTED_RDS_INSTANCE_ENGINES = [
    "aurora", "aurora-mysql", "aurora-postgresql", "mariadb", "mysql",
    "oracle-ee", "oracle-ee-cdb", "oracle-se2", "oracle-se2-cdb",
    "postgres", "sqlserver-ee", "sqlserver-se",
    "sqlserver-ex", "sqlserver-web"
]
let ALLOWED_EM_VALUES = [1, 5, 10, 15, 30, 60]

#
# Assignments
#
let rds_db_instances = Resources.*[ Type == %RDS_DB_INSTANCE_TYPE ]

#
# Primary Rules
#
rule rds_instance_enhanced_monitoring_enabled_check when is_cfn_template(%INPUT_DOCUMENT)
                                                         %rds_db_instances not empty {
    check(%rds_db_instances.Properties)
        <<
        [CT.RDS.PR.2]: Require an Amazon RDS database instance or cluster to have enhanced monitoring configured
        [FIX]: Set 'MonitoringInterval' to a supported value (1, 5, 10, 15, 30, 60), and set 'MonitoringRoleArn' to the ARN of an AWS IAM role.
        >>
}

rule rds_instance_enhanced_monitoring_enabled_check when is_cfn_hook(%INPUT_DOCUMENT, %RDS_DB_INSTANCE_TYPE) {
    check(%INPUT_DOCUMENT.%RDS_DB_INSTANCE_TYPE.resourceProperties)
        <<
        [CT.RDS.PR.2]: Require an Amazon RDS database instance or cluster to have enhanced monitoring configured
        [FIX]: Set 'MonitoringInterval' to a supported value (1, 5, 10, 15, 30, 60), and set 'MonitoringRoleArn' to the ARN of an AWS IAM role.
        >>
}

#
# Parameterized Rules
#
rule check(rds_db_instance) {
    %rds_db_instance [filter_engine(this)] {
        # Scenario: 3, 4, 5, 6 and 7
        MonitoringInterval exists
        MonitoringInterval in %ALLOWED_EM_VALUES
        # Scenario: 6 and 7
        MonitoringRoleArn exists
        check_for_valid_monitor_role_arn(MonitoringRoleArn)
    }
}

rule filter_engine(db_properties) {
    %db_properties {
        # Scenario: 2
        Engine exists
        Engine is_string
        Engine in %SUPPORTED_RDS_INSTANCE_ENGINES
    }
}

rule check_for_valid_monitor_role_arn(iam_role_arn) {
   %iam_role_arn {
      check_is_string_and_not_empty(this) or
      check_local_references(%INPUT_DOCUMENT, this, "AWS::IAM::Role")
    }
}

#
# Utility Rules
#
rule is_cfn_template(doc) {
    %doc {
        AWSTemplateFormatVersion exists  or
        Resources exists
    }
}

rule is_cfn_hook(doc, RESOURCE_TYPE) {
    %doc.%RESOURCE_TYPE.resourceProperties exists
}

rule check_is_string_and_not_empty(value) {
    %value {
        this is_string
        this != /\A\s*\z/
    }
}

rule check_local_references(doc, reference_properties, referenced_resource_type) {
    %reference_properties {
        'Fn::GetAtt' {
            query_for_resource(%doc, this[0], %referenced_resource_type)
                <<Local Stack reference was invalid>>
        } or Ref {
            query_for_resource(%doc, this, %referenced_resource_type)
                <<Local Stack reference was invalid>>
        }
    }
}

rule query_for_resource(doc, resource_key, referenced_resource_type) {
    let referenced_resource = %doc.Resources[ keys == %resource_key ]
    %referenced_resource not empty
    %referenced_resource {
        Type == %referenced_resource_type
    }
}