Good day!
CloudFormation templates take parameters. There are several types. One of those types is Number. Here’s a super-simple demo template:
---
AWSTemplateFormatVersion: "2010-09-09"
Parameters:
CoolNumber:
Type: Number
Resources:
CoolResource:
Type: AWS::SSM::Parameter
Properties:
Name: CoolNumber
Type: String
Value: !Ref CoolNumber
It just passes the CF param into an SSM param. It’s a little funky that we can create an SSM param with type String
from a CF param with type Number
, but in CF all numbers become strings when you Ref
them (see the link above). That’s weird but it doesn’t break anything. Things break when we try to create a stack from the template.
Here’s a params file. We set our number to an integer:
[
{
"ParameterKey": "CoolNumber",
"ParameterValue": 1
}
]
If we try to launch the template, it fails:
aws cloudformation create-stack \
--template-body file://./cool_stack.yaml \
--stack-name CoolStack \
--parameters file://./params.json
Parameter validation failed:
Invalid type for parameter Parameters[0].ParameterValue, value: 1, type: <class 'int'>, valid types: <class 'str'>
But we declared a number! We gave it a number! Why does it want a string?
In CloudFormation, parameter types aren’t really types. They’re validators. CF expects to receive everything as a string, then it checks to see if that string looks like the parameter’s “type”. In our example, it wants a string that happens to contain characters that represent a number.
This works (the only difference is the new quote marks):
[
{
"ParameterKey": "CoolNumber",
"ParameterValue": "1"
}
]
(╯°□°)╯︵ ┻━┻
I’ve also seen this behavior from CI/CD pipelines that use SDK-driven plugins to launch stacks with params files.
There are cases where you won’t get this error. I think some input mechanisms end up converting values to strings for you.
Text box in the console works:
Inline params in CF’s CLI commands in bash work:
aws cloudformation deploy \
--template-file cool_stack.yaml \
--stack-name CoolStack \
--parameter-overrides CoolNumber=1
aws cloudformation create-stack \
--template-body file://./cool_stack.yaml \
--stack-name CoolStack \
--parameters ParameterKey=CoolNumber,ParameterValue=1
Specifically in CloudFormation, here’s what you should do: declare whatever parameter types you need, but always pass the values in a strings. Anywhere else I can think of that would cause an error. Only do this for CloudFormation.
Keep your automation weird!
Adam
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