Write to S3 bucket using Async/Await in AWS Lambda

Question

I have been using the code below (which I have now added await to) to send files to S3. It has worked fine with my lambda code but as I move to transfer larger files like MP4 I feel I need async/await.

How can I fully convert this to async/await?

exports.handler = async (event, context, callback) => {
...
// Copy data to a variable to enable write to S3 Bucket
var result = response.audioContent;
console.log('Result contents ', result);

// Set S3 bucket details and put MP3 file into S3 bucket from tmp
var s3 = new AWS.S3();
await var params = {
Bucket: 'bucketname',
Key: filename + ".txt",
ACL: 'public-read',
Body: result
};

await s3.putObject(params, function (err, result) {
if (err) console.log('TXT file not sent to S3 - FAILED'); // an error occurred
else console.log('TXT file sent to S3 - SUCCESS');    // successful response
context.succeed('TXT file has been sent to S3');
});

You don't need to use await var params = ... await is only useful for promises — djheru
– djheru, Commented Mar 5, 2019 at 22:35

djheru · Accepted Answer · 2019-03-06 21:17:11Z

36

You only await functions that return a promise. s3.putObject does not return a promise (similar to most functions that take a callback). It returns a Request object. If you want to use async/await, you need to chain the .promise() method onto the end of your s3.putObject call and remove the callback (https://docs.aws.amazon.com/AWSJavaScriptSDK/latest/AWS/Request.html#promise-property)

try { // You should always catch your errors when using async/await
  const s3Response = await s3.putObject(params).promise();
  callback(null, s3Response);
} catch (e) {
  console.log(e);
  callback(e);
}

edited Mar 6, 2019 at 21:17

answered Mar 5, 2019 at 22:34

djheru

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4 Comments

Michael - sqlbot Over a year ago

I think this is right on track, except you probably also don't want to use the deprecated context.succeed() here, since that's mixing async/await and callbacks. Return a response or a promise, since you're inside an async function (the handler), yes?

Gracie Over a year ago

What should context.succeed(s3Response); be replaced with in an async/await situation?

Gracie Over a year ago

Perhaps callback(null, s3Response);?

djheru Over a year ago

Yes, I wasn't sure about the context.succeed. I did a little research, and the context object is really just to provide additional information about the lambda request (docs.aws.amazon.com/lambda/latest/dg/…). Instead of context.succeed you want to call callback(null, s3Response). I'll update my answer

fagiani · Accepted Answer · 2021-04-20 06:15:15Z

9

As @djheru said, Async/Await only works with functions that return promises. I would recommend creating a simple wrapper function to assist with this problem.

const putObjectWrapper = (params) => {
  return new Promise((resolve, reject) => {
    s3.putObject(params, function (err, result) {
      if(err) reject(err);
      if(result) resolve(result);
    });
  })
}

Then you could use it like this:

const result = await putObjectWrapper(params);

Here is a really great resource on Promises and Async/Await:

https://javascript.info/async

edited Apr 20, 2021 at 6:15

fagiani

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answered Apr 1, 2020 at 14:27

Richard

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1 Comment

Hemant Kumar Over a year ago

i guess small change needed if(err) resolve(err); should be replaced by if(err) reject(err);

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