# Set up a Lambda proxy integration with payload response streaming in API Gateway
<a name="response-transfer-mode-lambda"></a>

You can stream the response of a Lambda function to improve time to first byte (TTFB) performance and send partial responses back to the client as they become available. API Gateway requires you to use the [InvokeWithResponseStream](https://docs.aws.amazon.com/lambda/latest/api/API_InvokeWithResponseStream.html) Lambda API to invoke your Lambda function. API Gateway passes an event object to the Lambda function. The backend Lambda function parses the incoming request data to determine the response that it returns. In order for API Gateway to stream the Lambda output, the Lambda function must output the [format](#response-transfer-mode-lambda-format) required by API Gateway.

## Differences in Lambda proxy integrations between stream and buffered response transfer mode
<a name="response-transfer-mode-lambda-comparison"></a>

The following list describes the differences between a Lambda proxy integration and a Lambda proxy integration for response streaming:
+ API Gateway uses the [InvokeWithResponseStream](https://docs.aws.amazon.com/lambda/latest/api/API_InvokeWithResponseStream.html) API to invoke the Lambda proxy integration for response streaming. This results in a different URI, which is the following:

  ```
  arn:aws:apigateway:us-west-1:lambda:path/2021-11-15/functions/arn:aws:lambda:us-west-1:111122223333:function:my-function-name/response-streaming-invocations
  ```

  This ARN uses a different date for the API version and a different service action compared to Lambda proxy integration.

  If you use the API Gateway console for response streaming, the console uses the correct URI for you.
+ In a Lambda proxy integration, API Gateway sends the response to client only after it receives the full response from Lambda. In a Lambda proxy integration for response streaming, API Gateway begins the payload stream after it receives the valid metadata and delimiter from Lambda. 
+ Lambda proxy integration for response streaming uses the same input format as the proxy integration, but it requires a different output format.

## Lambda proxy integration format for response streaming
<a name="response-transfer-mode-lambda-format"></a>

When API Gateway invokes a Lambda function with response streaming, the input format is the same as the input format of a Lambda function for proxy integration. For more information, see [Input format of a Lambda function for proxy integration](set-up-lambda-proxy-integrations.md#api-gateway-simple-proxy-for-lambda-input-format). 

When Lambda streams a response to API Gateway, the response must adhere to the following format. This format uses a delimiter to separate the metadata JSON and the raw payload. In this case, the payload data is streamed as it is transmitted by your streaming Lambda function:

```
{
  "headers": {"headerName": "headerValue", ...},
  "multiValueHeaders": { "headerName": ["headerValue", "headerValue2", ...], ... },
  "cookies" : ["cookie1", "cookie2"],
  "statusCode": httpStatusCode
}<DELIMITER>PAYLOAD1 | PAYLOAD2 | PAYLOAD3
```

In the output:
+ The `headers`, `multiValueHeaders`, `cookies`, and `statusCode` keys can be unspecified if no extra response headers are to be returned.
+ The `headers` key can only contain single-value headers.
+ The output expects the headers to contain either `Transfer-Encoding: chunked` or `Content-length: number`. If your function doesn't return either of these headers, API Gateway appends `Transfer-Encoding: chunked` to the response header.
+ The `multiValueHeaders` key can contain multi-value headers as well as single-value headers. You can use the `multiValueHeaders` key to specify all of your extra headers, including any single-value ones.
+ If you specify values for both `headers` and `multiValueHeaders`, API Gateway merges them into a single list. If the same key-value pair is specified in both, only the values from `multiValueHeaders` will appear in the merged list.
+ The metadata must be valid JSON. Only `headers`, `multiValueHeaders`, `cookies` and the `statusCode` keys are supported.
+ You must provide a delimiter after the metadata JSON. The delimiter must be 8 null bytes and it must appear within the first 16KB of stream data.
+ API Gateway doesn't require a specific format for the method response payload.

If you're using a function URL to stream your Lambda function, you must modify the input and the output of your Lambda function to satisfy these requirements.

If your Lambda function output doesn't adhere to the requirements of this format, API Gateway might still invoke your Lambda function. The following table shows the combinations of API integration request settings and Lambda function code that is supported by API Gateway. This includes supported combinations for response transfer mode of buffered.


| Response transfer mode | Function code adheres to the required format | Lambda invoke API | Supported by API Gateway | 
| --- | --- | --- | --- | 
|  Stream  |  Yes  |   [InvokeWithResponseStream](https://docs.aws.amazon.com/lambda/latest/api/API_InvokeWithResponseStream.html)  |  Yes. API Gateway streams your response.  | 
|  Stream  |  No  |   [InvokeWithResponseStream](https://docs.aws.amazon.com/lambda/latest/api/API_InvokeWithResponseStream.html)  |  No. API Gateway invokes your Lambda function and return a 500 error response.  | 
|  Stream  |  Yes  |   [Invoke](https://docs.aws.amazon.com/lambda/latest/api/API_Invoke.html)  |  No. API Gateway doesn't support this integration configuration.  | 
|  Stream  |  No  |   [Invoke](https://docs.aws.amazon.com/lambda/latest/api/API_Invoke.html)  |  No. API Gateway doesn't support this integration configuration.  | 
|  Buffered  |  Yes  |   [InvokeWithResponseStream](https://docs.aws.amazon.com/lambda/latest/api/API_InvokeWithResponseStream.html)  |  No. API Gateway doesn't support this integration configuration.  | 
|  Buffered  |  No  |   [InvokeWithResponseStream](https://docs.aws.amazon.com/lambda/latest/api/API_InvokeWithResponseStream.html)  |  No. API Gateway doesn't support this integration configuration.  | 
|  Buffered  |  Yes  |   [Invoke](https://docs.aws.amazon.com/lambda/latest/api/API_Invoke.html)  |  API Gateway returns the HTTP headers and status code but not the response body.  | 
|  Buffered  |  No  |   [Invoke](https://docs.aws.amazon.com/lambda/latest/api/API_Invoke.html)  |  Yes. This is a Lambda proxy integration. For more information, see [Lambda proxy integration](set-up-lambda-proxy-integrations.md).  |