AWS: How should we upgrade PostgreSQL 10.6 to 10.9 version of CloudFormation-controlled RDS instances

Shout out to Asmita V. for this answer.

Upgrade RDS PostgreSQL instances from 10.6 to 10.9 and in this process you want to understand that whether setting the "AllowMajorVersionUpgrades" flag in the CloudFormation template be sufficient and if during the process, existing instances will get replaced.

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RESPONSE
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Upgrading the PostgreSQL instance from version 10.6 to 10.9 is a minor version upgrade and hence would not require you to change the value of the parameter "AllowMajorVersionUpgrades" in your cloudformation template.

In order to upgrade your instances from 10.6 to 10.9 you can make a modification in your CFN stack by just specifying the "Engine Version" as 10.9 instead of 10.6 in your template. There is no replacement of the existing instance and hence there will be no loss of data. The existing instances will go to a "Modifying" State.

In order to confirm this behavior, I tried upgrading resources in my test environment and following are my observations:

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Testing
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Please refer the following set of steps that I took in order to upgrade my instance to 10.9 from 10.6:

Sample Stack
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{
  "AWSTemplateFormatVersion" : "2010-09-09",
  "Description" : "PostgreSQL RDS Template ",
  "Resources": {
    "pgDB" : {
      "Type" : "AWS::RDS::DBInstance",
      "Properties" : {
        "DBName" : "mydb",
        "DBInstanceClass" : "db.t2.small",
        "AllocatedStorage" : "20",
        "Engine" : "postgres",
        "EngineVersion": "10.6",
        "MasterUsername" : "username",
        "MasterUserPassword" : "password",
        "AutoMinorVersionUpgrade": false
      }
    }
  }
}

Step 1: Make Changes to the existing template
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1. On the Stacks page of the AWS CloudFormation console, click the name of the stack that you want to update.  https://console.aws.amazon.com/cloudformation
2. Select the Template tab and select View on Designer.
3. Modify the CFN Stack : "EngineVersion": "10.9"
        {
          "AWSTemplateFormatVersion" : "2010-09-09",
          "Description" : "PostgreSQL RDS Template ",
          "Resources": {
            "pgDB" : {
              "Type" : "AWS::RDS::DBInstance",
              "Properties" : {
                "DBName" : "mydb",
                "DBInstanceClass" : "db.t2.small",
                "AllocatedStorage" : "20",
                "Engine" : "postgres",
                "EngineVersion": "10.9",
                "MasterUsername" : "username",
                "MasterUserPassword" : "password",
                "AutoMinorVersionUpgrade": false
              }
            }
          }
        }
4. Validate you template.
5. Select Save. Save your template to S3 bucket.
6. Click on Save and copy the URL.
       
Ex. https://s3-external-1.amazonaws.com/cf-templates-us-east-1/template1
       
Step 2: Create Change Set for Current Stack
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1. On the Stacks page of the AWS CloudFormation console, click the name of the stack that you want to update.  https://console.aws.amazon.com/cloudformation
2. Go to Stack Actions and select "Create change set for current stack".
3. Select "Replace current template"
4. Input the URL that was copied in the Step 1:6.
5. On the Review page, click on Create Change Set.
6. In the preview page, under the Changes you will notice "Modify" under the Action column.
7. Click on Execute.

You can now check on the RDS Console. The status of the RDS instance would have gone to "Upgrading".

However, please note that engine version upgrade (major or minor) is always associated with some amount of downtime.

Even if your DB instance is in a Multi-AZ deployment, both the primary DB instance and standby DB instances are upgraded. The writer and standby DB instances are upgraded at the same time, and you experience an outage until the upgrade is complete.

Therefore it is always recommended to plan your upgrades in the non business hours.

[+] Upgrading the PostgreSQL DB Engine for Amazon RDS - https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_UpgradeDBInstance.PostgreSQL.html

[+] Modifying a DB Instance Running the PostgreSQL Database Engine  - Settings for PostgreSQL DB Instances - https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_ModifyPostgreSQLInstance.html#USER_ModifyInstance.Postgres.Settings

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Conclusion
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Therefore, as per my testing, I can confirm that there will be no replacement of the existing instance during the process of upgrading PostgreSQL instance from  v10.6 to 10.9.
You may also go ahead and follow the steps given above in order to upgrade your instances.

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REFERENCES
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[+] Updating Stacks Using Change Sets  - https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/using-cfn-updating-stacks-changesets.html

[+] Creating a Change Set  - https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/using-cfn-updating-stacks-changesets-create.html