Failing to write to DynamoDB using Lambda function with an S3 trigger

Question

I'm trying to write a lambda function that is triggered whenever a new image is written to an S3 bucket. The trigger is already setup using the correct S3 bucket so I know that's not the issue.

The lambda function itself has the roles s3:GetObject and dynamodb.* (which should be full access for DynamoDB writes).

The goal here is to simply write to a table that I've already created named 'art' and insert a primary key value (imageTitle) which i'm trying to obtain in var imageName. Then I want to assign to that key an attribute which is the url of that image, which I'm storing in var url.

This is just a simple exercise I'm trying to get down so that I can move on to more complex DB writes. But as of right now, I'm not getting anything written to the art table, even though I am adding new objects to the S3 bucket which sets off the trigger. Is it possible that the lambda function isn't deployed? I wrote it directly in the inline editor of the Lambda Console and saved it.

Here's the code:

const AWS = require('aws-sdk');
const docClient = new AWS.DynamoDB.DocumentClient({region: 'us-east-1'});
const s3 = new AWS.S3();

exports.handler = async (event, context, callback) => {
    //var sourceBucket = event.Records[0].s3.bucket.name;
    var sourceKey = event.Records[0].s3.object.key;
    var imageName = sourceKey.stringify;

    //generate imageURL
    var url = "https://s3.amazonaws.com/myapp-20181030214040-deployment/public/" + imageName;

    var params = {
        TableName : 'art',
        Item: {
            imageTitle: imageName,
            imageURL: url
        }
    };
    docClient.put(params, function(err, data) {
       if (err) console.log(err);
       else console.log(data);
    });
};

Can you share the cloudwatch logs? Btw, saving your lambda function would be enough to have it ready to work: They are not deployed in a classical manner and new instance is created every time you call it. — vahdet
– vahdet, Commented Nov 15, 2018 at 7:19
Click on your Lambda, click on the monitoring tab, then click on the 'View logs in CloudWatch' button. Find and open the most recent log stream. This is where your logs are written. Add some more console.log statements to your function if required to identify which line of code is failing. — F_SO_K
– F_SO_K, Commented Nov 15, 2018 at 8:45
Is there anything specific I should be seeing in the CloudWatch logs? I have a Start, End, and Report log each with the same requestID. Does my approach to placing an item in the table look correct to you guys though? I've had a hard time understanding exactly how I'd specify which item I want to place in as the primary key and which items should be attributes of that key so I tried placing the primary key first. I'll try placing some console.log statements in tomorrow morning and see what I can find. — KSamra
– KSamra, Commented Nov 15, 2018 at 9:11

thomasmichaelwallace · Accepted Answer · 2018-11-15 09:18:08Z

6

The problem here is that you're using an async lambda but returning nothing that is awaitable. This means that your lambda is terminating before the docClient.put operation is sent.

With an async handler you need to await and return, for example you could change this snippet to:

const data = await docClient.put(params).promise();
return data;

Or instead you could use the callback approach (note the signature of the handler does not contain async anymore):

exports.handler = (event, context, callback) => {
  // ... the rest of the code as was ...
  docClient.put(params, function(err, data) {
    if (err) {
      console.log(err);
      callback(err); // return 'lambda invoke failed because of the error' - will cause s3 to retry three times.
    } else {
      console.log(data);
      callback(null, data); // return 'nothing failed'; n.b. the s3 trigger ignores whatever you return.
    }
  });
};

answered Nov 15, 2018 at 9:18

thomasmichaelwallace

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2 Comments

KSamra Over a year ago

Thank you. So now that I've gotten rid of the asynchronous call the function actually runs according to the CloudWatch logs. However, it's failing due to the exception: ValidationException: One or more parameter values were invalid: Missing the key imageTitle in the item. But I believe I am entering a value for the key when I'm assigning imageTitle: imageName ? Is this not how you assign values to a primary key?

thomasmichaelwallace Over a year ago

No- you need to use { Key: { imageTitle: imageName }, Item: { imageURL: url }, TableName : 'art' }

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