Converting SSM Value To Number - amazon-sns

Question

I am trying to deploy an SNS subscription through Serverless Framework to AWS using a range filter policy. My resource definition looks something like this:
MySusbscription:
Type: AWS::SNS::Subscription
Properties:
TopicArn:
Ref: MyTopic
Endpoint: !GetAtt MyQueue.Arn
Protocol: sqs
RawMessageDelivery: true
FilterPolicy:
percentage:
- numeric:
- '>='
- ${ssm:/path/to/ssm/key}
RedrivePolicy:
deadLetterTargetArn: !GetAtt MyQueueDlq.Arn
The problem is that Serverless pulls all SSM values as strings, so the compiled version of the deployment config will be:
- numeric:
- '>='
- '0.25'
This will fail to deploy as SNS requires 0.25 to be a number instead of a string. Serverless has a strToBool function, but I haven't seen documentation to indicate there is an equivalent function for converting to a number/float.
I'm sure I can get around this by using env variables, but we store all configurations in SSM and I'm hoping to not have to do a one-off just to get past this issue.

Answer 1

The first solution is not the cleanest one, but will work for sure.
In the directory define your custom JS file ssmToNumber.js with following script:
const AWS = require('aws-sdk');
module.exports = async (serverless) => {
# Setup AWS SDK with region and profile
const { region } = serverless.processedInput.options;
AWS.config.update({ region });
if (serverless.processedInput.options['aws-profile']) {
process.env.AWS_PROFILE = serverless.processedInput.options['aws-profile'];
}
const ssm = new AWS.SSM({ apiVersion: '2014-11-06' });
# Get SSM params details from serverless.yml
const ssmKey = serverless.service.custom.ssmKeys.percentageNumericThreshold.path;
const decryption = serverless.service.custom.ssmKeys.percentageNumericThreshold.decryption;
# Get SSM parameter value and convert it to int
const result = await ssm.getParameter({
Name: ssmKey,
WithDecryption: decryption,
}).promise();
if (result.Parameter) {
return parseInt(result.Parameter.Value, 10);
}
throw new Error(`Failed to read SSM parameter for key ${ssmKey}`);
};
Now in serverless.yml define following values:
custom:
ssmKeys:
percentageNumericThreshold:
path: '/path/to/ssm/key'
decryption: false
And in the place, where you want to have your number value from SSM, simply invoke it this way:
FilterPolicy:
percentage:
- numeric:
- '>='
- ${file(./ssmToNumber.js)}
How it works?
Serverless Framework can start any JavaScript/TypeScript file during the build and put it's output into serverless.yml file.
And this is exactly what we do here. We define our ssmToNumber.js script, that simply reads SSM parameter from SSM and then converts it to integer and returns the value.
It knows which SSM path to use, thanks to the custom.ssmKeys section in serverless.yml file.
Of course if you want to customise ssmToNumber.js file, to make it more verbose and fault tolerant, then you simply need to edit the JavaScript file.
Another, more elegant way
It requires more work. Check out the official example for Serverless with Typescript.
As you can see, it's possible to use serverless.ts or serverless.js instead of YAML file.
It would require from you some work about refactoring existing YML file, but writing helper function, that would convert it to number, is very easy and elegant way to achieve your use case.
It have some downsides, like problems with including directly other YAML templates, but still you can define your CloudFormation YAML templates in separate files and then simply import those using import call from TS/JS code.