Request body serialization differences when lambda function invoked via API Gateway v Lambda Console

Question

I have a simple API set up in AWS API Gateway. It is set to invoke a Python 2.7 lambda function via API Gateway Proxy integration.

I hit a strange error in that the lambda worked (processed the body correctly and updated a DB) when invoked locally and through the lambda test console, but not through curl or Postman.

Turns out that, when invoked through the lambda test console, the event['body'] object is coming through as a dict. When called via an HTTP client, it's coming through as a string (Unicode).

I can work around it of course, but I'd like to understand it, and I'd also prefer a proper Python object. I'd also like to be able to use the lambda test console, but currently I can't as it passes its input differently.

Is there a configuration switch I'm missing which will force API Gateway to serialize the request body (as well as all other params) as a python dict or proper object? The documentation on the specifics of what is passed is sparse, stating:

event – AWS Lambda uses this parameter to pass in event data to the handler. This parameter is usually of the Python dict type. It can also be list, str, int, float, or NoneType type.

I get that this blurb covers what I'm seeing, but it's not exactly helpful.

Apologies, but API Gateway is just calling the Lambda API directly. If you invoke your function manually (e.g. with the CLI) do you see similar behavior? — Bob Kinney
@BobKinney: I haven't tried it, only via their web interface (lambda console/test). To be honest I'm not sure it matters too much at this point. I can always call json.loads(event['body']) and go on with my life. I just don't understand the difference, or why the type is essentially unspecified (rather, one of six different unspecified options.) — Ed S.

Noel Llevares Noel Llevares · Accepted Answer · 2017-09-22T17:01:17

When you invoke the lambda locally or through the Lambda console, you are invoking that lambda directly and so your lambda receives exactly what you're sending.

When you invoke it through API Gateway, API Gateway creates the event object for you based on your HTTP request. It adds the HTTP headers, path, query strings, payload, etc.

Here's a summary of what you're getting as an event from an API Gateway invocation:

{
    "resource": "Resource path",
    "path": "Path parameter",
    "httpMethod": "Incoming request's method name"
    "headers": {Incoming request headers}
    "queryStringParameters": {query string parameters }
    "pathParameters":  {path parameters}
    "stageVariables": {Applicable stage variables}
    "requestContext": {Request context, including authorizer-returned key-value pairs}
    "body": "A JSON string of the request payload."
    "isBase64Encoded": "A boolean flag to indicate if the applicable request payload is Base64-encode"
}

Reference: http://docs.aws.amazon.com/apigateway/latest/developerguide/api-gateway-set-up-simple-proxy.html#api-gateway-simple-proxy-for-lambda-input-format

As you can see, the body will be sent to you as a string which you can parse using json.loads().

Request body serialization differences when lambda function invoked via API Gateway v Lambda Console

1 Answers