Purpose

Lambda does not allow parametrization. One way to add additional context is to pull context from a CloudFormation Stack that the function is a member of. By setting the Description field of a Lambda to the AWS::StackId inside your template and using this module, you can access Stack context without heavy listing or code generation.

Usage

Within the CloudFormation Stack Template

You must grant the Lambda the cloudformation:DescribeStacks, and lambda:GetFunctionConfiguration. Additionally, you must set the Description element of the Lambda Function to the StackId for the currently running Stack:

"DomainTrainingInvoker": {
  // ... 
  "Type": "AWS::Lambda::Function",
  "Properties" : {
    // ...
    "Description": {
      "Ref": "AWS::StackId" // Set the Description to Ref::StackId
    },
    // ...
  }
  // ...
},

Access Content Within Lambda

Install using npm i --save lambda-stack-context

Then, within your function, to apply the context:

var WithStackContext = require('lambda-stack-context');

// MyOriginalHandler would be whatever function you'd normally export
var MyContextualizedHandler = WithStackContext(MyOriginalHandler);

exports.handler = MyContexualizedHandler;


function MyOriginalHandler(event, context) {
  // Contains the the stack information
  console.log(context.Stack);
  // Rest of your code like normal
}

Structure of context.Stack

The context.Stack object has some helpful properties you can use in your code.

context.Stack.Parameters

Contains the Parameters from the template, as key-value pairs.

context.Stack.Outputs

Contains the Outputs from the template, as key-value pairs.

Technicalities

The system generates a function handler that looks like your original function. It grabs function context before the original handler executes, then runs your original function as a callback. This method is not recommended for latency-sensitive implementations, but is great for very simple rapid develop of connected systems.