Yan Cui for AWS Heroes

Posted on Oct 6, 2023 • Originally published at theburningmonk.com on Oct 6, 2023

Help! How do I set DeletionPolicy to Retain for production only?

#aws #cdk #sam #serverless

It’s a good practice to use CloudFormation’s DeletionPolicy to protect stateful resources such as DynamoDB tables or RDS databases from accidental deletions. Such as when someone accidentally deletes a CloudFormation stack!

As I discussed previously [1], this is a much better way to guard against these accidental data losses than separating stateful and stateless resources into different stacks. After all, how we think about infrastructure vs. code needs to evolve in the serverless era [2]. That separation is no longer so clear-cut.

The problem with DeletionPolicy

But using DeletionPolicy on all your stateful resources causes friction with another common practice in serverless development – the use of ephemeral (or temporary) environments [3].

If you set DeletionPolicy to retain on all your stateful resources, then they will all linger when you delete the temporary environment.

In fact, you only need to use DeletionPolicy in production. That’s the only environment where it’s necessary.

So, how do we do that?

The CDK solution

CDK gives you the full power of a general-purpose programming language (even though it’s often needed [4]). So it’s trivial to do this in a CDK application:

Pass the environment name as either a CloudFormation parameter or context variable.
Set the removalPolicy inside an if block like this:

if (environment === 'prod') {
  myTable.removalPolicy = cdk.RemovalPolicy.RETAIN
}

The Serverless framework solution

The easiest way to do this with the Serverless framework is to use the serverless-plugin-ifelse plugin.

Once you have installed the plugin as a dev dependency, you can use it to remove the DeletionPolicy on stateful resources like this:

plugins:
  - serverless-plugin-ifelse

custom:
  serverlessIfElse:
    - If: '"${sls:stage}" != "prod"'
      Exclude:
        - resources.Resources.MyTable.DeletionPolicy

resources:
  Resources:
    MyTable:
      Type: AWS::DynamoDB::Table
      DeletionPolicy: Retain
      Properties:
        ...

Notice that DeletionPolicy is enabled by default and is removed if the current stage (i.e. environment name) is not prod?

This is defensive coding. So that if I get anything wrong, the worst that can happen is I have to manually delete the table from temporary environments.

With the reverse approach (i.e. add DeletionPolicy only for the prod stage), the worst-case scenario is far worse.

If you want to see this solution in action, then check out this repo [5].

SAM / CloudFormation solution

For SAM or CloudFormation, this is the best solution I have found:

Add an Environment CloudFormation Parameter to your template.
Add a map of DeletionPolicy value to use for each environment.
Use the Fn:FindInMap intrinsic function to find the DeletionPolicy value to use for the current environment.

Something like this:

Parameters:
  Environment:
    Type: String
    Default: dev
    Description: Environment name, e.g. dev, test, prod

Mappings:
  DeletionPolicyMap: 
   dev: 
     value: Delete
   prod: 
     value: Retain

Resources:
  MyTable:
    Type: AWS::DynamoDB::Table
    DeletionPolicy: !FindInMap [ DeletionPolicyMap, !Ref Environment, value ]
    Properties:
      ...

This solution is not ideal because it requires the environment names to be explicitly configured. You have to update the DeletionPolicyMap every time you create a new environment.

As such, it doesn’t work well with ephemeral environments where environment names are usually dynamic. If you know of a better way to do this with SAM and CloudFormation, please let me know!

DynamoDB DeletionProtectionEnabled attribute

I have used DynamoDB in the examples above. But it’s worth pointing out that DeletionPolicy is actually not the most effective protection against data loss for DynamoDB.

For starters, it doesn’t guard against someone deleting a DynamoDB table in the console or programmatically through the AWS CLI or SDK.

Fortunately, DynamoDB also has the DeletionProtectionEnabled [6] attribute. When enabled, it protects the table from accidental deletion by any user or process.

Additionally, DynamoDB offers other protection against data loss, such as point-in-time recovery and the ability to export data to S3.

Having said that, DeletionPolicy is still a very useful tool for other services such as S3, EventBridge, SQS, SNS and so on.

Shout out to Hala

I want to give a shout-out to Hala Al Aali for asking me about this in the Testing Serverless Architectures [7] course’s forum.

It’s a common problem that many students have come across. I hope this article has helped you. If you want to learn more about building serverless applications for the real world, check out my upcoming workshops [8].