Support UsePreviousValue in parameters

[nathanreynolds-kcom]

CD scripts can update e.g ECS image refs with CloudFormation's UsePreviousValue flag. This ensures only the defined parameter is updated.

It'd be useful if Sceptre supported the same, to be sure we don't revert such changes on the next update-stack.

cfn-sphere has something similar in it's KeepOrUse macro.

[nathanreynolds-kcom]

Implementation-wise, could allow stack config parameters to accept dicts, e.g:

parameters:
  regular_style: 'my_value'
  verbose_style:
    value: 'my_value'
    use_previous_value: true

Stack._format_parameters may have to accept extra context, informing whether it's a create or update. If an update and use_previous_value is set, pass {"ParameterKey": ..., "UsePreviousValue": true} - otherwise, use the current behaviour.

[theseanything]

Hi @Nathkc,
Not sure I see a strong use case for this. All your parameter values are documented with the stack_config yaml file - so if you update your stack you should be passing the previous values anyway. Do you have a particular example of where/how this functionality is needed?

[nathanreynolds-kcom]

No problem, I'll do my best.

---

We have two teams: infrastructure and development.

Infrastructure have deployed a stack. Development now push AMI updates with:

• aws cloudformation update-stack --stack-name dev --use-previous-template --parameters ParameterKey=ami,ParameterValue=ami-xyz123

This only changes a parameter. It doesn't change the template, which is important:

• Minimal training for development team.
• No unwanted infrastructure changes from old branches / faulty merges.
• Infrastructure reasonably decoupled from development. Each team can independently improve their process.

---

Now infrastructure needs to update the stack.

Remember the development team hasn't updated the stack source. If we blindly deploy, it'll revert their changes.

Our options are:

1. Commit development parameter changes to the infrastructure repo. We don't want this for the previous reasons.
2. Merge in current parameter values before updating stack. That's where UsePreviousValue comes in.

[theseanything]

Hi @Nathkc, thanks for that! Personally, my opinion the issue is you have multiple deployment methods - AWS CLI and Sceptre. I recommend trying to stick to a single method across your teams- and possibly using Sceptre with in a CI/CD pipeline to automatically push AMI changes. It may also be advantageous to create a custom resolver to dynamically pick up the new AMI ID that is created by your developers.

[et304383]

@theseanything here's a valid use case:

Input parameter for RDS master password that comes from an environment variable or var argument.

I want to provide this on create, but use previous value on update.