Params Layering Support

Intro: Common Layers

Params support layering for the configs/demo/params folder. Params files are layered if they exist. Let’s say you have the following params directory structure:

├── base.txt
├── development.txt
└── production.txt
  • base.txt is always evaluated.
  • development.txt or production.txt is evaluated based on LONO_ENV.

In this case, you want to define your common params used for templates in the base.txt and overrides in production.txt. For example, let’s say we’re setting the min and max size of an autoscaling group. We could have something like this:





Lono will use the InstanceType=m5.large parameter value when launching the stack with LONO_ENV=production. Lono will use InstanceType=t3.micro for all other LONO_ENV values. Example:

$ lono cfn deploy demo # InstanceType=t3.micro
$ LONO_ENV=production lono cfn deploy demo # InstanceType=m5.large

Remember params can be used to affect templates at run-time. Here’s the lifecycle flow to see when the run-time phase happens.

Stack Created

Full Layering: Conventional Requested

Additionally, layering also considers params files that match the stack name. Here is a list of the full layering possibilities.

Description Path Comments
base configs/BLUEPRINT/params/base.txt always evaluated
env configs/BLUEPRINT/params/LONO_ENV.txt evaluated based on LONO_ENV value
params level configs/BLUEPRINT/params/REQUESTED.txt common evaluated based on REQUESTED
env level configs/BLUEPRINT/params/LONO_ENV/REQUESTED.txt generally recommended

The BLUEPRINT, LONO_ENV, and TEMPLATE placeholders are self-explanatory. REQUESTED requires a little more explanation. REQUESTED is usually the requested stack name. Here’s a concrete example with stack name:

lono cfn deploy my-stack --blueprint demo # REQUESTED=my-stack

The layers would be:

  1. configs/demo/params/base.txt
  2. configs/demo/params/development.txt
  3. configs/demo/params/my-stack.txt
  4. configs/demo/params/development/my-stack.txt (recommended)

If you need to use a different param file that does not match the stack name, you can explicitly specify the param file with the --param option. Here’s an example with the --param option. Here REQUESTED comes from --param my-param:

lono cfn deploy my-stack --blueprint demo --param my-param # REQUESTED=my-param

These params files are layered on top of the “Common” base.txt and development.txt layers.

  1. configs/demo/params/base.txt
  2. configs/demo/params/development.txt
  3. configs/demo/params/my-param.txt
  4. configs/demo/params/development/my-param.txt (recommended)

Direct Locations

You can also specify relative and full paths for the --param value. Example:

lono cfn deploy my-stack --blueprint demo --param configs/demo/params/my-param.txt

You can specify params files that exist outside of the lono project too.

lono cfn deploy my-stack --blueprint demo --param /tmp/my-param.txt

Relative and full paths generally remove the “Conventional” lookups for the layers. So you’ll end up with this:

  1. configs/demo/params/base.txt
  2. configs/demo/params/development.txt
  3. /tmp/my-param.txt

Pro tip: Use the <- and -> arrow keys to move back and forward.

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