Stack Parameter strategies

Currently used, valid strategy types are: - default - merge

Strategy type can be set both via command line, or overriding the default behaviour in your Moonshot::CLI subclass.

Command line: bin/environment update --parameter-strategy=merge

Inline: parameter_strategy :merge

Accepted both as a string or a symbol.

Setting precedence is the following: - command line option - inline default behaviour override - falling back to default

Default strategy

Default strategy is the legacy behaviour, works exactly as previously, prior to introducing different strategies. Each parameter is loaded from the parameter file (eg. cloud_formation/parameters/environment-name.yml), and all of them are overridden on update.

Word of caution: when using this strategy we encourage you to keep all per-environment tunings in the source repository acting as both a safety net and documentation of existing environments. A possible solution for using this strategy in a safe manner is the following:

When a stack update is performed, a parameter file is checked in cloud_formation/parameters/environment-name.yml. This file is YAML formatted and takes a hash of stack parameter names and values, for example:

AsgDesiredCap: 12
AsgMaxCap: 15
ELBCertificate: iam::something:star_example_com

If a file exists, it's used every time a CloudFormation change request is sent, so no configuration can revert back to defaults through this tool. It's highly recommended that you add these files back to source control as soon as possible and be in the habit of pulling latest changes before applying any infrastructure updates.

Merge strategy

Merge strategy is a new way of dealing with stack parameters on stack update. You only have to declare parameters you want to update in your parameter file, the remaining parameters are not updated, meaning it stays as it's current uploaded (live) state. This behaviour is achived by using CloudFormation's UsePreviousValue feature. This way you can avoid accidentally reverting parameter values with your outdated local parameter file.

Defining custom parameter strategy class

Defining and using your own custom parameter strategy class is possible if you are using Moonshot without the provided CLI. A parameter strategy class is a class which responds to a method called parameters. It receives two parameters: the first contains a hash of the existing, currently deployed stack parameters, the second one (also a hash) contains the parameters defined in the parameters file. The method should return an array of hashes of the following format:

  parameter_key: key,
  parameter_value: value,
  use_previous_value: false

You either supply a value for parameter_value or set use_previous_value to true, which leaves the parameter as it currently is.


class CustomStrategy
  def parameters(parameters, stack_parameters, template) do |k, v|
        parameter_key: k,
        parameter_value: v,
        use_previous_value: false

In order to use your custom strategy class, set a new instance of your class to ControllerConfig's parameter_strategy attribute.