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Learn how to return an HTTP response from AWS Lambda immediately using response streaming while continuing background execution — ideal for Slack integrations with tight timeouts.
This blog was originally written and published by Trek10, which is now part of Caylent.
A feature I’ve always wanted from AWS Lambda functions is the ability to return a response and continue executing. One use case for this is Slack Slash command handler which must return an initial response within 3 seconds. Of course, you could do something similar to this by invoking a second Lambda function (or the same one) or an AWS step function, but both of these significantly increase the complexity. For simple use cases, it would be very convenient to be able to return a response and keep executing.
When AWS Lambda extensions were released I hoped that they would update the internal Lambda API to allow this. Unfortunately, they did not. I had the same hope when Lambda streaming responses were released and this time I was not disappointed. While it’s not documented you can in fact return a response and continue executing in certain circumstances.
In this post, I’ll explain how to do this using the Node.js Lambda runtime.
But first, I’d like to explain how Lambda streaming responses work. According to the documentation, to enable streaming responses you have to turn on function urls and enable the streaming response. In your code, you have to wrap your handler function with a wrapper provided by the AWS Lambda Node.js runtime. This wrapper is accessible on the Javascript global object.
Here are the CloudFormation resource definitions I used to create a test function:
Resources:
Trek10LambdaTestFunctionWithUrl:
Type: AWS::Serverless::Function
Properties:
Handler: index.handler
FunctionName: Trek10-Lambda-Test-Function-With-Url
CodeUri: ./src
Runtime: nodejs18.x
Timeout: 30
Trek10LambdaTestUrl:
Type: AWS::Lambda::Url
Properties:
AuthType: NONE
InvokeMode: RESPONSE_STREAM
TargetFunctionArn: !GetAtt Trek10LambdaTestFunctionWithUrl.Arn
Trek10LambdaTestUrlPermission:
Type: AWS::Lambda::Permission
Properties:
Action: lambda:InvokeFunctionUrl
FunctionUrlAuthType: NONE
Principal: '*'
FunctionName: !GetAtt Trek10LambdaTestFunctionWithUrl.ArnAnd here is the handler code:
const util = require('util')
const sleep = util.promisify(setTimeout)
// awslambda is patched into the global object by the lambda runtime
module.exports.handler = awslambda.streamifyResponse(async function(event, responseStream, context) {
responseStream.write('Hello my request id is: ' + context.awsRequestId)
responseStream.end();
console.log('response sent')
// the response has been sent and the request to lambda should return very quickly.
// now we simulate doing some work by sleeping
await sleep(5000)
console.log('done waiting')
})As you can see from the code above all you have to do to continue executing after returning is simply end the stream but don’t return from your function.
Here is the output from calling the lambda URL as well as the corresponding execution logs from CloudWatch:
$ time curl https://wrwtktb7psotxknjhvwnllkpuu0cxvtb.lambda-url.us-east-1.on.aws/
Hello my request id is: 654c7282-1216-496c-956b-e9ad124752c7
real 0m0.240s
user 0m0.015s
sys 0m0.015s
$ aws logs filter-log-events --log-group-name /aws/lambda/Trek10-Lambda-Test-Function-With-Url --filter-pattern '"654c7282-1216-496c-956b-e9ad124752c7"' --output text --query 'events[*].message'
START RequestId: 654c7282-1216-496c-956b-e9ad124752c7 Version: $LATEST
2023-11-05T03:01:34.355Z 654c7282-1216-496c-956b-e9ad124752c7 INFO response sent
2023-11-05T03:01:39.356Z 654c7282-1216-496c-956b-e9ad124752c7 INFO done waiting
END RequestId: 654c7282-1216-496c-956b-e9ad124752c7
REPORT RequestId: 654c7282-1216-496c-956b-e9ad124752c7 Duration: 5003.61 ms Billed Duration: 5004 ms Memory Size: 128 MB Max Memory Used: 69 MBAs you can see from the time output the Lambda URL invocation took 240ms, but you can see that at the whole Lambda duration was 5004ms.
Now you may be asking “What if I want to continue executing after returning a response, but also want to invoke via the standard API (or via an ApiGateway proxy integration) and not via the Function URL?”
No problem. Just do it. In fact, you don’t even need to have the Function URL enabled for this to work.
Here is another CloudFormation resource definition, notice there is no URL configuration or permission:
Trek10LambdaTestFunctionWithNoUrl:
Type: AWS::Serverless::Function
Properties:
Handler: index.handler
FunctionName: Trek10-Lambda-Test-Function-With-No-Url
CodeUri: ./src
Runtime: nodejs18.x
Timeout: 30This function uses the same handler code as the previous function. Here is the output when calling via the API/CLI:
$ time aws lambda invoke --function-name Trek10-Lambda-Test-Function-With-No-Url result_no_url.txt
{
"StatusCode": 200,
"ExecutedVersion": "$LATEST"
}
real 0m0.690s
user 0m0.221s
sys 0m0.072s
$ cat result_no_url.txt
Hello my request id is: 145b188e-dc16-49fb-9ec5-1b2b270d350c
$ aws logs filter-log-events --log-group-name /aws/lambda/Trek10-Lambda-Test-Function-With-No-Url --filter-pattern '"145b188e-dc16-49fb-9ec5-1b2b270d350c"' --output text --query 'events[*].message'
START RequestId: 145b188e-dc16-49fb-9ec5-1b2b270d350c Version: $LATEST
2023-11-05T03:12:56.715Z 145b188e-dc16-49fb-9ec5-1b2b270d350c INFO response sent
2023-11-05T03:13:01.720Z 145b188e-dc16-49fb-9ec5-1b2b270d350c INFO done waiting
END RequestId: 145b188e-dc16-49fb-9ec5-1b2b270d350c
REPORT RequestId: 145b188e-dc16-49fb-9ec5-1b2b270d350c Duration: 5084.14 ms Billed Duration: 5085 ms Memory Size: 128 MB Max Memory Used: 68 MBAgain you can see that the API call took much less time than the Lambda invocation. (Also note you can return non-JSON results with the streaming response).
Conclusion
To summarize, your Lambda can return a response yet continue executing by simply wrapping the handler with the streaming response and then just doing more things after ending the stream.
There is one caveat here: if you invoke your Lambda via the API and request the last 4kb of logs via the --log-type=Tail option, you don’t get a response until the Lambda execution ends.
Is this possible without using the streaming response wrapper? Not as far as I can tell if you are using the AWS-provided Node.js runtime. I hope to be able to do it in a custom runtime or with an internal Lambda extension. I’m hoping to test that in the future, and I’ll post what I find here if it’s interesting.
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Trek10 Team
Founded in 2013, Trek10 helped organizations migrate to and maximize the value of AWS by designing, building, and supporting cloud-native workloads with deep technical expertise. In 2025, Trek10 joined Caylent, forming one of the most comprehensive AWS-only partners in the ecosystem, delivering end-to-end services across strategy, migration and modernization, product innovation, and managed services.
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