Lambda malformed proxy response (Unknown reason)

Question

There's probably a simple solution to this but I'm stuck. My lambda function being called by API Gateway should return my json response but it's giving me the same "Malformed proxy response" error

I've fiddled with the response a dozen times but as far as i can tell, there's nothing wrong with it? It's the same response as i'd get from a postman API. What am I doing wrong here?

The following screenshot is the response when testing in cloud9, but when fetching from API gateway using proxy response parameters, i get the "Malformed proxy response" error.

 Response
{
    "statusCode": 200,
    "list": [
        {
            "id": 8000048,
            "name": "ESTACAO DE SERVICO TETE",
            "addr": "EN7, Bairro Samora Machel, Unidade Mpadue",
            "lat": -16.195047,
            "lon": 33.585467,
            "amen": [
                false,
                false,
                "",
                false,
                false,
                false,
                false,
                false,
                false,
                "",
                false,
                false,
                false,
                false
            ],
            "fuel": [
                null,
                null,
                null,
                null,
                null
            ],
            "loc": [
                "Tete",
                "Center",
                "Mozambique",
                "MZ"
            ]
        }
    ]
}

404 · Accepted Answer · 2021-01-27 13:09:12Z

1

The response you're returning is invalid. As per the documentation,

In Lambda proxy integration, API Gateway requires the backend Lambda function to return output according to the following JSON format:
{
    "isBase64Encoded": true|false,
    "statusCode": httpStatusCode,
    "headers": { "headerName": "headerValue", ... },
    "multiValueHeaders": { "headerName": ["headerValue", "headerValue2", ...], ... },
    "body": "..."
}

Not all of those fields are required, but you can't just add random stuff to the response; rather you should set a stringified version of the object as the body field, for example (using js as I don't know python) body: JSON.stringify(info) or body: JSON.stringify({ list: info })

answered Jan 27, 2021 at 13:09

404

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2 Comments

Griffin Maree Over a year ago

I tried to add simplejson.dumps(info) to turn the result into a string, but I get back "list": "[{\"id\": 8000048, \"name\": \"ESTACAO DE SERVICO TETE\", \"addr This suggests that the dictionary is double compressed. It's a list of json dictionaries. So now how would I compress this list of dictionaries into a string that API Gateway would accept as a response?

404 Over a year ago

@GriffinMaree You'll need to show more code, as that should not happen. For one thing in the code posted in your question, info does not contain a "list" property, so the structure you are stringifying isn't the same as in your example. What you need to do is stringify only the content of the body, not the content of list. For example, where info = [{ a: 1, b: 2 }], then body: JSON.stringify(info) will return a response of [{"a":1,"b":2}], and body: JSON.stringify({ list: info }) will return {"list":[{"a":1,"b":2}]}

Griffin Maree · Accepted Answer · 2021-02-01 09:36:56Z

0

Sorry for the lack of info in this question, due to legal reasons I could not share everything. But the solution to this ended up being double compressing the json response. Let me explain ...

API gateway decompresses the response before returning it to the client. Due to this, responding to the event in lambda requires you to not only stringify the response but to kinda double stringify (or i call compress) the response. Basically, i had to ...

simplejson.dumps(response)
simplejson.dumps(response)

I'm not sure i understand this correctly or if I'm using the correct terms, so take all this with a pinch of salt. Thanks, everyone.

answered Feb 1, 2021 at 9:36

Griffin Maree

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