Testing a Lambda function locally against DynamoDB on AWS.

1_instructions.md

Local development with SAM

This method allows you to iterate very quickly against Lambda code without having to run AWS resources locally.

Requirements

• AWS CLI
• SAM CLI

Step 1: Create and deploy a simple SAM Template (see template.yaml)

• must include the environment varibales on the Lambda to pass the SAMPLE_TABLE in (see line 17)
• use sam deploy --guided for the first time deploy

Ths will create the table in the AWS dev account and will be accessible to the locally invoked Lambda function via the AWS CLI credentials

Step 2: Create a local environments file (See locals.json)

• Ensure it contains the name of the actual table created in the deploy
• I also have an "experimental" python script that will build this for you. (see build-params.py)

Step 3: Test locally

• requires an AWS CLI profile to be configured with CRUD access to dynamodb

A single invoke

sam local invoke getAllItemsFunction -n locals.json

As a listening API Gateway emulator

sam local start-api -n locals.js

More Info

https://docs.aws.amazon.com/serverless-application-model/latest/developerguide/serverless-sam-cli-using-invoke.html

build-params.py

# Use at your own risk.
# Set path/to/python
# requires boto3
# requires an AWS CLI profile to be configured

#!/path/to/python
import sys, json
import boto3

if len(sys.argv) == 1:
   print('CloudFormation stack required')
   sys.exit()

cloudformation_client = boto3.client('cloudformation')
lambda_client = boto3.client('lambda')

page = {}

# Grab stack from AWS
try:
    stack_resources = cloudformation_client.list_stack_resources(
        StackName=sys.argv[1]
    )
except Exception as e:
    print(e)
    sys.exit()

# Grab environment vars from each lambda in the stack
for resource in stack_resources['StackResourceSummaries']:
    if(resource['ResourceType']=='AWS::Lambda::Function'):
        lambda_config = lambda_client.get_function_configuration(
            FunctionName=resource['PhysicalResourceId']
        )
        if 'Environment' in lambda_config and 'Variables' in lambda_config['Environment']:
            page[resource['LogicalResourceId']] = lambda_config['Environment']['Variables']

# Print to console
print(json.dumps(page, indent=2))

locals.json

{
    "getAllItemsFunction": {
        "SAMPLE_TABLE": "SampleTable-3MNBRQBENB1F"
    }
}

template.yaml

AWSTemplateFormatVersion: 2010-09-09
Description: My App
Transform: AWS::Serverless-2016-10-31

Resources:
  getAllItemsFunction:
    Type: AWS::Serverless::Function
    Properties:
      Handler: src/handlers/get-all-items.getAllItemsHandler
      Runtime: nodejs10.x
      MemorySize: 128
      Timeout: 100
      Description: A simple example includes a HTTP get method to get all items from a DynamoDB table.
      Policies:
        - DynamoDBCrudPolicy:
            TableName: !Ref SampleTable
      Environment:
        Variables:
          SAMPLE_TABLE: !Ref SampleTable
      Events:
        Api:
          Type: Api
          Properties:
            Path: /
            Method: GET

  # DynamoDB table to store item:
  SampleTable:
    Type: AWS::Serverless::SimpleTable

Outputs:
  WebEndpoint:
    Description: "API Gateway endpoint URL for Prod stage"
    Value: !Sub "https://${ServerlessRestApi}.execute-api.${AWS::Region}.amazonaws.com/Prod/"