Return data from async AWS Lambda

Question

How do I get the data back from a lambda invoked with as an event to the calling function?

Essentially the lambda function I have is:

exports.handler = function(event, context, callback) {
    var data = {};
    data.foo ='hello';
    callback(null, data)
}

and the invoking function looks like this:

var AWS = require('aws-sdk');

var lambda = new AWS.Lambda();
var params = {
    FunctionName: 'SomeFunction',
    InvocationType: 'Event'
};

lambda.invoke(params, function (err, data) {
    if (err) {
        console.log(err, err.stack); // an error occurred
    } else {
        console.log(JSON.stringify(data, null, 2));
    }
});

However the only thing I get back from the function is

{
  "StatusCode": 202,
  "Payload": ""
}

I thought the point of the callback parameter was to allow the invoking function to get the data when the function has finished. Am I using it wrong or is what I am asking not possible with Lambdas?

idbehold · Accepted Answer · 2016-07-21 21:13:19Z

9

When you invoke the Lambda function you need to set the InvocationType to 'RequestResponse' instead of 'Event'.

When using the Event type your callback is invoked when the payload has been received by AWS's servers. When using the RequestResponse type your callback is invoked only after the Lambda function has completed and you will receive the data it provided to its callback. It is not possible to do what you want with the Event type.

edited Jul 21, 2016 at 21:13

answered Jul 21, 2016 at 20:45

idbehold

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7 Comments

user2127726 Over a year ago

Is it not possible to do it with 'Event' type? I thought the point of the callback parameter was for the invoking function to be able to get the data from the lambda when it finished executing?

user2127726 Over a year ago

Shame. You can use the optional callback to return information to the caller, otherwise return value is null, this made it sound like I could use the callback to return result of the invocation

idbehold Over a year ago

@user2127726 I mean, you can… just use the right InvocationType. Why is changing that an issue?

user2127726 Over a year ago

RequestResponse is synchronous right? So there would have to be another thread to prevent blocking the calling one right? Just some people don't seem to like that extra step

idbehold Over a year ago

@user2127726 no, using the RequestResponse type will not make the call synchronous.

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f.cipriani · Accepted Answer · 2022-10-20 21:10:15Z

0

As @MatthewPope commented in this answer, if you do need the results from an asynchronous Lambda execution, you should consider having the Lambda function write its results to S3 (or something to that effect) at a known location where clients of the Lambda function can periodically check for the existence of the result.

answered Oct 20, 2022 at 21:10

f.cipriani

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