Moving EdgeDB deployment on AWS Aurora db.t3.medium to db.t3.small - RDS does not support creating a DB instance

Question

I am looking at a cloudformation template for EdgeDB : https://github.com/edgedb/edgedb-deploy/blob/891dee4b356ac02aa2eb8d7c69c18f7713bdff20/aws-cf/edgedb-aurora.yml

Within this the default database option was db.t3.medium, I was looking to see if I could deploy this under the AWS RDS Free tier, hence I was looking to try and use db.t3.small.

I modified the template like so:

AWSTemplateFormatVersion: 2010-09-09
Description: >
  This template will deploy an EdgeDB instance to ECS and connect it
  to Aurora Serverless PostgreSQL Cluster.

# https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/parameters-section-structure.html
Parameters:
  SuperUserPassword:
    Description: "The password that will be used for the database superuser"
    Type: String
    NoEcho: True
    MinLength: 6

  DockerImage:
    Description: "Name and tag of the EdgeDB Docker Image"
    Type: String
    Default: "edgedb/edgedb"

  InstanceName:
    Description: "The name of this EdgeDB service stack."
    Type: String
    Default: "project"
    AllowedPattern: "[a-z][0-9a-z-]{1,24}"
    ConstraintDescription: >
      must only contain lowercase letters and numbers, start with
      a lowercase letter, and be between 2 and 25 characters long

  PostgresVersion:
    Description: "The aurora-postgresql version"
    Type: String
    Default: "13.9"
    AllowedPattern: '\d+\.\d+'
    ConstraintDescription: Must contain two digits x.y.

  DBInstanceTypeParameter:
    Type: String
    Default: db.t3.micro
    AllowedValues:
      - db.t3.micro
      - db.t4g.micro
      - db.t3.medium
    Description: >
      Enter db.t3.micro, db.t4g.micro, db.t3.medium. Default is db.t3.micro.
      Free tier is: db.t3.micro & db.t4g.micro.

Resources:
  # https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-ec2-vpc.html
  VPC:
    Type: "AWS::EC2::VPC"
    Properties:
      CidrBlock: "10.0.0.0/16"
      EnableDnsSupport: true
      EnableDnsHostnames: true
      InstanceTenancy: default
      Tags:
        - Key: Name
          Value: !Sub "edgedb-${InstanceName}-vpc"

  # https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-ec2-internetgateway.html
  InternetGateway:
    Type: "AWS::EC2::InternetGateway"
    Properties:
      Tags:
        - Key: Name
          Value: !Sub "edgedb-${InstanceName}-internet-gateway"

  # https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-ec2-vpc-gateway-attachment.html
  VPCGatewayAttachment:
    Type: "AWS::EC2::VPCGatewayAttachment"
    Properties:
      VpcId: !Ref VPC
      InternetGatewayId: !Ref InternetGateway

  # ------------------
  # PUBLIC NETWORK ACL
  # ------------------

  # https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-ec2-networkacl.html
  NetworkAclPublic:
    Type: "AWS::EC2::NetworkAcl"
    Properties:
      VpcId: !Ref VPC
      Tags:
        - Key: Name
          Value: !Sub "edgedb-${InstanceName}-public-network-acl"

  # https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-ec2-networkaclentry.html
  NetworkAclEntryInPublicAllowInternet:
    Type: "AWS::EC2::NetworkAclEntry"
    Properties:
      NetworkAclId: !Ref NetworkAclPublic
      RuleNumber: 99
      Protocol: 6
      PortRange:
        From: 0
        To: 65535
      RuleAction: allow
      Egress: false
      CidrBlock: "0.0.0.0/0"

  # https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-ec2-networkaclentry.html
  NetworkAclEntryOutPublicAllowInternet:
    Type: "AWS::EC2::NetworkAclEntry"
    Properties:
      NetworkAclId: !Ref NetworkAclPublic
      RuleNumber: 99
      Protocol: 6
      PortRange:
        From: 0
        To: 65535
      RuleAction: allow
      Egress: true
      CidrBlock: "0.0.0.0/0"

  # -------------------
  # PRIVATE NETWORK ACL
  # -------------------

  # NOTE: there's no difference from the public ACL above, because the
  #       accessibility of the subnets is determined by MapPublicIpOnLaunch.
  #       We still create an explicit ACL for clarity.

  # https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-ec2-networkacl.html
  NetworkAclPrivate:
    Type: "AWS::EC2::NetworkAcl"
    Properties:
      VpcId: !Ref VPC
      Tags:
        - Key: Name
          Value: !Sub "edgedb-${InstanceName}-private-network-acl"

  # https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-ec2-networkaclentry.html
  NetworkAclEntryInPrivateAllowVPC:
    Type: "AWS::EC2::NetworkAclEntry"
    Properties:
      NetworkAclId: !Ref NetworkAclPrivate
      RuleNumber: 99
      Protocol: -1
      RuleAction: allow
      Egress: false
      CidrBlock: "0.0.0.0/0"

  # https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-ec2-networkaclentry.html
  NetworkAclEntryOutPrivateAllowVPC:
    Type: "AWS::EC2::NetworkAclEntry"
    Properties:
      NetworkAclId: !Ref NetworkAclPrivate
      RuleNumber: 99
      Protocol: -1
      RuleAction: allow
      Egress: true
      CidrBlock: "0.0.0.0/0"

  # -----------------------
  # PUBLIC SUBNET IN AZ "A"
  # -----------------------

  # https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-ec2-subnet.html
  SubnetAPublic:
    Type: "AWS::EC2::Subnet"
    Properties:
      AvailabilityZone: !Select [0, !GetAZs ""]
      CidrBlock: "10.0.0.0/20"
      MapPublicIpOnLaunch: true
      VpcId: !Ref VPC
      Tags:
        - Key: Name
          Value: !Sub "edgedb-${InstanceName}-subnet-a-public"
        - Key: Reach
          Value: public

  # https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-ec2-subnet-network-acl-assoc.html
  SubnetNetworkAclAssociationAPublic:
    Type: "AWS::EC2::SubnetNetworkAclAssociation"
    Properties:
      SubnetId: !Ref SubnetAPublic
      NetworkAclId: !Ref NetworkAclPublic

  # https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-ec2-routetable.html
  RouteTableAPublic:
    Type: "AWS::EC2::RouteTable"
    Properties:
      VpcId: !Ref VPC
      Tags:
        - Key: Name
          Value: !Sub "edgedb-${InstanceName}-route-table-a-public"

  # https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-ec2-route.html
  RouteTableAPublicInternetRoute:
    Type: "AWS::EC2::Route"
    DependsOn: VPCGatewayAttachment
    Properties:
      RouteTableId: !Ref RouteTableAPublic
      DestinationCidrBlock: "0.0.0.0/0"
      GatewayId: !Ref InternetGateway

  # https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-ec2-subnet-route-table-assoc.html
  RouteTableAssociationAPublic:
    Type: "AWS::EC2::SubnetRouteTableAssociation"
    Properties:
      SubnetId: !Ref SubnetAPublic
      RouteTableId: !Ref RouteTableAPublic

  # ------------------------
  # PRIVATE SUBNET IN AZ "A"
  # ------------------------

  # https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-ec2-subnet.html
  SubnetAPrivate:
    Type: "AWS::EC2::Subnet"
    Properties:
      AvailabilityZone: !Select [0, !GetAZs ""]
      CidrBlock: "10.0.16.0/20"
      VpcId: !Ref VPC
      Tags:
        - Key: Name
          Value: !Sub "edgedb-${InstanceName}-subnet-a-private"
        - Key: Reach
          Value: private

  # https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-ec2-routetable.html
  RouteTableAPrivate:
    Type: "AWS::EC2::RouteTable"
    Properties:
      VpcId: !Ref VPC
      Tags:
        - Key: Name
          Value: !Sub "edgedb-${InstanceName}-route-table-a-private"

  # https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-ec2-subnet-network-acl-assoc.html
  SubnetNetworkAclAssociationAPrivate:
    Type: "AWS::EC2::SubnetNetworkAclAssociation"
    Properties:
      SubnetId: !Ref SubnetAPrivate
      NetworkAclId: !Ref NetworkAclPrivate

  # https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-ec2-subnet-route-table-assoc.html
  RouteTableAssociationAPrivate:
    Type: "AWS::EC2::SubnetRouteTableAssociation"
    Properties:
      SubnetId: !Ref SubnetAPrivate
      RouteTableId: !Ref RouteTableAPrivate

  # -----------------------
  # PUBLIC SUBNET IN AZ "B"
  # -----------------------

  # https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-ec2-subnet.html
  SubnetBPublic:
    Type: "AWS::EC2::Subnet"
    Properties:
      AvailabilityZone: !Select [1, !GetAZs ""]
      CidrBlock: "10.0.32.0/20"
      MapPublicIpOnLaunch: true
      VpcId: !Ref VPC
      Tags:
        - Key: Name
          Value: !Sub "edgedb-${InstanceName}-subnet-b-public"
        - Key: Reach
          Value: public

  # https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-ec2-subnet-network-acl-assoc.html
  SubnetNetworkAclAssociationBPublic:
    Type: "AWS::EC2::SubnetNetworkAclAssociation"
    Properties:
      SubnetId: !Ref SubnetBPublic
      NetworkAclId: !Ref NetworkAclPublic

  # https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-ec2-routetable.html
  RouteTableBPublic:
    Type: "AWS::EC2::RouteTable"
    Properties:
      VpcId: !Ref VPC
      Tags:
        - Key: Name
          Value: !Sub "edgedb-${InstanceName}-route-table-b-public"

  # https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-ec2-route.html
  RouteTableBPublicInternetRoute:
    Type: "AWS::EC2::Route"
    DependsOn: VPCGatewayAttachment
    Properties:
      RouteTableId: !Ref RouteTableBPublic
      DestinationCidrBlock: "0.0.0.0/0"
      GatewayId: !Ref InternetGateway

  # https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-ec2-subnet-route-table-assoc.html
  RouteTableAssociationBPublic:
    Type: "AWS::EC2::SubnetRouteTableAssociation"
    Properties:
      SubnetId: !Ref SubnetBPublic
      RouteTableId: !Ref RouteTableBPublic

  # ------------------------
  # PRIVATE SUBNET IN AZ "B"
  # ------------------------

  # https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-ec2-subnet.html
  SubnetBPrivate:
    Type: "AWS::EC2::Subnet"
    Properties:
      AvailabilityZone: !Select [1, !GetAZs ""]
      CidrBlock: "10.0.48.0/20"
      VpcId: !Ref VPC
      Tags:
        - Key: Name
          Value: !Sub "edgedb-${InstanceName}-subnet-b-private"
        - Key: Reach
          Value: private

  # https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-ec2-routetable.html
  RouteTableBPrivate:
    Type: "AWS::EC2::RouteTable"
    Properties:
      VpcId: !Ref VPC
      Tags:
        - Key: Name
          Value: !Sub "edgedb-${InstanceName}-route-table-b-private"

  # https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-ec2-subnet-network-acl-assoc.html
  SubnetNetworkAclAssociationBPrivate:
    Type: "AWS::EC2::SubnetNetworkAclAssociation"
    Properties:
      SubnetId: !Ref SubnetBPrivate
      NetworkAclId: !Ref NetworkAclPrivate

  # https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-ec2-subnet-route-table-assoc.html
  RouteTableAssociationBPrivate:
    Type: "AWS::EC2::SubnetRouteTableAssociation"
    Properties:
      SubnetId: !Ref SubnetBPrivate
      RouteTableId: !Ref RouteTableBPrivate

  # ------------------
  # ECS Security Group
  # ------------------

  # https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-ec2-security-group.html
  EC2SecurityGroup:
    Type: "AWS::EC2::SecurityGroup"
    Properties:
      GroupName: !Sub "edgedb-${InstanceName}-ec2-security-group"
      GroupDescription: !Sub "Security group to control access to EC2 instances inside the ${InstanceName} stack"
      VpcId: !Ref "VPC"
      SecurityGroupIngress:
        - IpProtocol: "tcp"
          CidrIp: "0.0.0.0/0"
          FromPort: 5656
          ToPort: 5656
      Tags:
        - Key: "Name"
          Value: !Sub "edgedb-${InstanceName}-ec2-security-group"

  # ---------------------
  # Aurora Security Group
  # ---------------------

  # https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-ec2-security-group.html
  RDSSecurityGroup:
    Type: "AWS::EC2::SecurityGroup"
    Properties:
      GroupName: !Sub "edgedb-${InstanceName}-rds-security-group"
      GroupDescription: !Sub "Security group controlling access to RDS PostgreSQL instance inside the ${InstanceName} stack"
      VpcId: !Ref "VPC"
      SecurityGroupIngress:
        - IpProtocol: "tcp"
          SourceSecurityGroupId: !Ref "EC2SecurityGroup"
          FromPort: 5432
          ToPort: 5432

      Tags:
        - Key: "Name"
          Value: !Sub "edgedb-${InstanceName}-rds-security-group"

  # https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-rds-dbsubnet-group.html
  RDSSubnetGroup:
    Type: "AWS::RDS::DBSubnetGroup"
    Properties:
      DBSubnetGroupName: !Sub "edgedb-${InstanceName}-rds-subnet-group"
      DBSubnetGroupDescription: !Sub "EdgeDB RDS Subnet Group for ${InstanceName}"
      SubnetIds:
        - !Ref "SubnetAPrivate"
        - !Ref "SubnetBPrivate"

  # --------------
  # Aurora Cluster
  # --------------

  # https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-secretsmanager-secret.html
  DBPassword:
    Type: "AWS::SecretsManager::Secret"
    Properties:
      Name: !Sub "edgedb-${InstanceName}-password"
      SecretString: !Ref SuperUserPassword

  # https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-rds-dbcluster.html
  AuroraCluster:
    DependsOn: RDSSubnetGroup
    Type: "AWS::RDS::DBCluster"
    Properties:
      Engine: "aurora-postgresql"
      EngineVersion: !Ref PostgresVersion
      DBClusterIdentifier: !Sub "edgedb-${InstanceName}-postgres-cluster"
      DBSubnetGroupName: !Ref "RDSSubnetGroup"
      MasterUsername: "postgres"
      MasterUserPassword: !Ref "SuperUserPassword"
      Port: 5432
      VpcSecurityGroupIds:
        - !Ref "RDSSecurityGroup"

  AuroraInstanceA:
    Type: "AWS::RDS::DBInstance"
    Properties:
      AvailabilityZone: !GetAtt "SubnetAPrivate.AvailabilityZone"
      Engine: "aurora-postgresql"
      DBClusterIdentifier: !Ref AuroraCluster
      DBInstanceClass: !Ref DBInstanceTypeParameter
      DBSubnetGroupName: !Ref "RDSSubnetGroup"

  AuroraInstanceB:
    Type: "AWS::RDS::DBInstance"
    Properties:
      AvailabilityZone: !GetAtt "SubnetBPrivate.AvailabilityZone"
      Engine: "aurora-postgresql"
      DBClusterIdentifier: !Ref AuroraCluster
      DBInstanceClass: !Ref DBInstanceTypeParameter
      DBSubnetGroupName: !Ref "RDSSubnetGroup"

  # https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-secretsmanager-secret.html
  AuroraDSN:
    DependsOn: [AuroraInstanceA, AuroraInstanceB]
    Type: "AWS::SecretsManager::Secret"
    Properties:
      Name: !Sub "edgedb-${InstanceName}-backend-dsn"
      SecretString:
        !Join [
          "",
          [
            "postgres://",
            "postgres",
            ":",
            !Ref "SuperUserPassword",
            "@",
            !GetAtt "AuroraCluster.Endpoint.Address",
            ":5432/",
            "postgres",
          ],
        ]

  # ------------------
  # EdgeDB ECS Cluster
  # ------------------

  # https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-iam-role.html
  ExecutionRole:
    Type: AWS::IAM::Role
    Properties:
      RoleName: !Sub "edgedb-${InstanceName}-execution-role"
      AssumeRolePolicyDocument:
        Statement:
          - Effect: Allow
            Principal:
              Service: ecs-tasks.amazonaws.com
            Action: "sts:AssumeRole"
      ManagedPolicyArns:
        - "arn:aws:iam::aws:policy/service-role/AmazonECSTaskExecutionRolePolicy"
      Policies:
        - PolicyName: Secrets-Access-Policy
          PolicyDocument:
            Statement:
              - Effect: Allow
                Action:
                  - "secretsmanager:GetSecretValue"
                Resource:
                  - !Ref DBPassword
                  - !Ref AuroraDSN

  # https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-iam-role.html
  TaskRole:
    Type: AWS::IAM::Role
    Properties:
      RoleName: !Sub "edgedb-${InstanceName}-task-role"
      AssumeRolePolicyDocument:
        Statement:
          - Effect: Allow
            Principal:
              Service: ecs-tasks.amazonaws.com
            Action: "sts:AssumeRole"

  # https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-logs-loggroup.html
  LogGroup:
    Type: AWS::Logs::LogGroup
    Properties:
      LogGroupName: !Sub "/ecs/edgedb/${InstanceName}"

  # https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-ecs-cluster.html
  Cluster:
    Type: AWS::ECS::Cluster
    Properties:
      ClusterName: !Sub "edgedb-${InstanceName}-server-cluster"

  # https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-ecs-taskdefinition.html
  TaskDefinition:
    Type: AWS::ECS::TaskDefinition
    DependsOn: LogGroup
    Properties:
      # These are invariant for Fargate
      RequiresCompatibilities: [FARGATE]
      NetworkMode: awsvpc
      ExecutionRoleArn: !Ref ExecutionRole
      TaskRoleArn: !Ref TaskRole

      # These are configurable
      Family: !Sub "edgedb-${InstanceName}-task-definition"
      Cpu: 1024
      Memory: "2GB"
      ContainerDefinitions:
        - Name: !Ref InstanceName
          Image: !Ref DockerImage
          PortMappings:
            - ContainerPort: 5656
          Command:
            - "edgedb-server"
          Environment:
            - Name: "EDGEDB_SERVER_TLS_CERT_MODE"
              Value: "generate_self_signed"
          Secrets:
            - Name: "EDGEDB_SERVER_PASSWORD"
              ValueFrom: !Ref "DBPassword"
            - Name: "EDGEDB_SERVER_BACKEND_DSN"
              ValueFrom: !Ref "AuroraDSN"
          LogConfiguration:
            LogDriver: awslogs
            Options:
              awslogs-region: !Ref AWS::Region
              awslogs-group: !Ref LogGroup
              awslogs-stream-prefix: ecs

  # https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-ecs-service.html
  Service:
    Type: AWS::ECS::Service
    DependsOn:
      - Listener5656
    Properties:
      ServiceName: !Ref InstanceName
      Cluster: !Ref Cluster
      TaskDefinition: !Ref TaskDefinition
      DeploymentConfiguration:
        MinimumHealthyPercent: 100
        MaximumPercent: 200
      DesiredCount: 2
      HealthCheckGracePeriodSeconds: 120
      LaunchType: FARGATE
      NetworkConfiguration:
        AwsvpcConfiguration:
          AssignPublicIp: ENABLED
          Subnets:
            - !Ref "SubnetAPublic"
            - !Ref "SubnetBPublic"
          SecurityGroups:
            - !Ref "EC2SecurityGroup"
      LoadBalancers:
        - ContainerName: !Ref InstanceName
          ContainerPort: 5656
          TargetGroupArn: !Ref TargetGroup

  # --------------------
  # EdgeDB Load Balancer
  # --------------------

  # https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-elasticloadbalancingv2-targetgroup.html
  TargetGroup:
    Type: AWS::ElasticLoadBalancingV2::TargetGroup
    Properties:
      HealthCheckIntervalSeconds: 10
      HealthCheckPath: /server/status/ready
      HealthCheckProtocol: HTTPS
      UnhealthyThresholdCount: 2
      HealthyThresholdCount: 2
      Name: !Sub "edb-${InstanceName}-tg"
      Port: 5656
      Protocol: TCP
      TargetGroupAttributes:
        - Key: deregistration_delay.timeout_seconds
          Value: 60
      TargetType: ip
      VpcId: !Ref VPC

  # https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-elasticloadbalancingv2-listener.html
  Listener5656:
    Type: AWS::ElasticLoadBalancingV2::Listener
    Properties:
      DefaultActions:
        - TargetGroupArn: !Ref TargetGroup
          Type: forward
      LoadBalancerArn: !Ref LoadBalancer
      Port: 5656
      Protocol: TCP

  # https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-elasticloadbalancingv2-loadbalancer.html
  LoadBalancer:
    Type: AWS::ElasticLoadBalancingV2::LoadBalancer
    DependsOn: VPCGatewayAttachment
    Properties:
      Type: network
      Name: !Sub "edb-${InstanceName}-lb"
      Scheme: internet-facing
      Subnets:
        - !Ref "SubnetAPublic"
        - !Ref "SubnetBPublic"

Outputs:
  PublicHostname:
    Description: "Host name of EdgeDB instance"
    Value: !GetAtt LoadBalancer.DNSName

  SubnetAPublic:
    Description: "Subnet A public."
    Value: !Ref SubnetAPublic
    Export:
      Name: !Sub "${AWS::StackName}-SubnetAPublic"

  SubnetBPublic:
    Description: "Subnet B public."
    Value: !Ref SubnetBPublic
    Export:
      Name: !Sub "${AWS::StackName}-SubnetBPublic"

  EC2SecurityGroupID:
    Description: "The ID of the EC2 Security group"
    Value: !GetAtt EC2SecurityGroup.GroupId
    Export:
      Name: !Sub "${AWS::StackName}-EC2-ID"

However, I get an error when trying to deploy this:

Resource handler returned message:
"RDS does not support creating a DB instance with the following combination:
DBInstanceClass=db.t3.micro,
Engine-aurora-postgresql,
EngineVersion=13.9,
LicenseModel=postgresql-license. 
For supported combinations of instance class and database engine version,
see the documentation. 
(Service: Rds, Status Code: 400, Request ID: X)"
(RequestToken: dl Y,HandlerErrorCode: InvalidRequest)

I can see from the Aurora docs https://docs.aws.amazon.com/AmazonRDS/latest/AuroraUserGuide/Concepts.DBInstanceClass.html that PostgresSQL is simply "No" under available versions of Postgres-SQL,

However, due to the wording of the error message I would think that for RDS it is available, as per the AWS-RDS docs: https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/Concepts.DBInstanceClass.html#Concepts.DBInstanceClass.Support

So I wonder then if it is possible at all to deploy this under the free tier? My understanding was that Aurora utilises RDS and that I should not be running into such an issue regarding DB engine compatability? https://docs.aws.amazon.com/prescriptive-guidance/latest/migration-postgresql-planning/rds.html