RDS with Cloud Formation and AZ issues

Question

I am using cloud formation to create a setup containing an RDS instance.

I am having some difficulties creating the RDS Instance on the account of the following error:

Answer 1

Create a new subject which has a different availability zone with other subjects

Answer 2

Yes, even for a deployment entirely contained within a single Availability Zone [AZ], you must create an extra subnet in a different AZ and include it in your DB Subnet Group. The rationale for this requirement is to support high-availability Multi-AZ deployments, as noted in the Working with a DB Instance in a VPC section of the RDS User Guide:

For Multi-AZ deployments, defining a subnet for two or more Availability Zones in a region allows Amazon RDS to create a new standby in another Availability Zone should the need arise. You need to do this even for Single-AZ deployments, just in case you want to convert them to Multi-AZ deployments at some point.

As for not burdening your users with selecting another AZ just for this, there are ways to accomplish this. For example, you could select a secondary AZ automatically using the Fn::GetAZs and Fn::Select intrinsic functions. If you allow the user to select the primary AZ, you'll also need a Condition to ensure the secondary AZ doesn't equal the primary AZ selected.

Here's an example template snippet:

Parameters:
  PrimaryAZ:
    Type: AWS::EC2::AvailabilityZone::Name
    Description: Primary AZ
Conditions:
  IsFirstPrimaryAZ:
    Fn::Equals:
    - !Ref PrimaryAZ
    - Fn::Select [0, {Fn::GetAZs: ""}]
Resources:
  Subnet1:
    Type: "AWS::EC2::Subnet"
    Properties:
      AvailabilityZone: !Ref PrimaryAZ
      # ...
  Subnet2:
    Type: "AWS::EC2::Subnet"
    Properties:
      AvailabilityZone:
        Fn::If:
        - IsFirstPrimaryAZ
        - Fn::Select [1, {Fn::GetAZs: ""}]
        - Fn::Select [0, {Fn::GetAZs: ""}]
      # ...

Answer 3

Yes. Just delete one of the subnets and create again the same with different Availability Zone.

Answer 4

Just to add more details about this problem. I solved my problem evaluating subnet group at RDS > subnet group. (Subnet group is a concept that exists only in RDS context. Not in VPC context)

In my case there was a subnet group created automatically first time I tried to create a RDS Mysql selecting my personalized VPC (not default VPC) but I think RDS script created a subnet group selecting the first 3 subnets from my VPC:

• subnet1 region 1a,
• subnet2 region 1a,
• subnet3 region 1b

I created 3 private subnets to use with RDS:

• subnet4 region 1a,
• subnet5 region 1b,

• subnet6 region 1c

  Its not possible to modify a subnet group then I created one and then select 3 subnets with the same acl and route configuration, each one in a different region (1a,1b,1c). Then I deleted that another misconfigured subnet group. Back to RDS dashboard click in Create RDS, choose all database details you want, select the desired VPS and you see the selected subnet group name . Problem Solved.