Headers are received in Camel-Case

When running sam local start-api, I’m getting headers in Camel-Case, (see X-Api-Key and X-User-Id), however. the deployed lambda function is receiving the headers in lower-case (x-api-key and x-user-id).

My local code is working as of 2020-03-09 17:00 GMT+8, and I’m trying to understand why the behaviour is different now.

In sam local:

{
  'headers': {
    'X-Api-Key': '<redacted>',
    'X-User-Id': '<redacted>',
    'User-Agent': 'PostmanRuntime/7.22.0',
    'Accept': '*/*',
    'Cache-Control': 'no-cache',
    'Host': 'localhost:3000',
    'Accept-Encoding': 'gzip, deflate, br',
    'Connection': 'keep-alive',
    'X-Forwarded-Proto': 'http',
    'X-Forwarded-Port': '3000'
  },
  'multiValueHeaders': {
    'X-Api-Key': ['<redacted>'],
    'X-User-Id': ['<redacted>'],
    'User-Agent': ['PostmanRuntime/7.22.0'],
    'Accept': ['*/*'],
    'Cache-Control': ['no-cache'],
    'Host': ['localhost:3000'],
    'Accept-Encoding': ['gzip, deflate, br'],
    'Connection': ['keep-alive'],
    'X-Forwarded-Proto': ['http'],
    'X-Forwarded-Port': ['3000']
  }
}

In AWS Lambda:

{
  'headers': {
    'Accept': '*/*',
    'Accept-Encoding': 'gzip, deflate, br',
    'Cache-Control': 'no-cache',
    'Host': '<redacted>',
    'User-Agent': 'PostmanRuntime/7.22.0',
    'X-Amzn-Trace-Id': 'Root=1-5e6739e1-d6161c849faa6e9a369f680c',
    'x-api-key': '<redacted>',
    'X-Forwarded-For': '175.143.101.237',
    'X-Forwarded-Port': '443',
    'X-Forwarded-Proto': 'https',
    'x-user-id': '<redacted>'
  },
  'multiValueHeaders': {
    'Accept': ['*/*'],
    'Accept-Encoding': ['gzip, deflate, br'],
    'Cache-Control': ['no-cache'],
    'Host': ['<host>'],
    'User-Agent': ['PostmanRuntime/7.22.0'],
    'X-Amzn-Trace-Id': ['Root=1-5e6739e1-d6161c849faa6e9a369f680c'],
    'x-api-key': ['<redacted>'],
    'X-Forwarded-For': ['<IP address>'],
    'X-Forwarded-Port': ['443'],
    'X-Forwarded-Proto': ['https'],
    'x-user-id': ['<redacted>']
  }
}

Snippet of template.yaml I’m using.

Resources:
  BasePythonLayer:
    Type: AWS::Serverless::LayerVersion
    Properties:
      LayerName: pythonBaseLayer
      ContentUri: /app/LambdaCodes/LambdaLayers/PythonLayer
      CompatibleRuntimes:
        - python3.7
  VendorFunction:
    Type: AWS::Serverless::Function
    Properties:
      FunctionName: vendor-function
      Handler: vendorFunction.lambda_handler
      Runtime: python3.7
      Timeout: 120
      CodeUri: ./VendorFunction/
      Layers:
        - !Ref BasePythonLayer
      Events:
        Vendor:
          Type: Api
          Properties:
            Path: /vendor
            Method: ANY

1. OS: Using docker, python:alpine
2. sam --version: SAM CLI, version 0.40.0

[sapessi]

I’d recommend updating the function code to use a CaseInsensitiveDict object just like requests does. The Lambda PowerTools also handle this for you with the get_header_value method. In the HTTP protocol, headers are meant to be case insensitive. Right now it’s the x-api-key header but in the future the issues could apply to any other header.

[mastazi]

I was wrong, API Gateway doesn’t lower everything, in some cases headers remain capitalized. Thank you @jfuss for explaining this here https://github.com/aws/aws-sam-cli/issues/3083#issuecomment-887632702

I have closed #3117 for that reason.