DynamoDB always returns "Response: null" on AWS Lambda

Question

Consider the code :

const AWS = require('aws-sdk');
const dynamoDB = new AWS.DynamoDB({
    region : 'us-east-2'  , 
    apiVersion: '2012-08-10'
});


exports.handler = async (event , context , callback) => {
    const type = event.type;
    if (type === 'all') { 
      // Which table we want to scan
      const params = {
          TableName : 'compare-yourself'
      };
      await dynamoDB.scan(params ,(err , data) => {
         if (err) {
             callback(err);
         }
         else{
             callback(null , data);
         }
         
      });
        // callback(null , "All Data");
    }
    else if (type === 'single') {
        callback(null , "Single data");
    }
    else{
        callback(null , "Everything else...");    
    }
    
};

When I test this piece of code on AWS Lambda with the value :

The result is always "Response: null" , even through there is data in the DynamoDB table compare-yourself.

What might be the problem here ?

Why are you doing await and also at the same time waiting for callback in dynamoDB.scan ? — Deekshith Hegde
– Deekshith Hegde, Commented Jul 25, 2020 at 11:21

Shiyas · Accepted Answer · 2020-07-25 21:36:54Z

2

Promise will solve the issue. I prefer documentclient.


const AWS = require('aws-sdk');
const documentClient = new AWS.DynamoDB.DocumentClient();

exports.handler = async (event, context, callback) => {
    const type = event.type;
    if (type === 'all') {
        const params = {
            TableName: 'compare-yourself'
        };
        let data = await documentClient.scan(params).promise();
        callback(null, data);
    } else if (type === 'single') {
        callback(null, "Single data");
    } else {
        callback(null, "Everything else...");
    }
};

The data will be an object with table values in the Items,

{
 Items: [],
 Count:
 ScannedCount:

  .....

}

answered Jul 25, 2020 at 21:36

Shiyas

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Comments

Mauro Valc · Accepted Answer · 2020-07-25 16:02:19Z

1

try use DocumentClient();

var documentClient = new AWS.DynamoDB.DocumentClient();

const params = {
          TableName : 'compare-yourself'
      };

documentClient.scan(params, function(err, data) {
   if (err) console.log(err);
   else console.log(data);
});

answered Jul 25, 2020 at 16:02

Mauro Valc

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Comments

Sai · Accepted Answer · 2020-07-25 16:31:13Z

Mistake:

The problem is you are doing both callback and await at the same.

Fix:

So you could put the scan function in a try block, so on error we do callback error in the catch block and stop the function without proceeding further (return) else you will send the data.

Code:

const AWS = require('aws-sdk');
const dynamoDB = new AWS.DynamoDB({
    region : 'us-east-2'  , 
    apiVersion: '2012-08-10'
});


exports.handler = async (event , context , callback) => {
    const type = event.type;
    if (type === 'all') { 
      // Which table we want to scan
      const params = {
          TableName : 'compare-yourself'
      };
      
      try {
        await dynamoDB.scan(params).promise();
      } catch (e) {
        callback(e);
        return;
      }
      callback(null , data);
    }

    else if (type === 'single') {
        callback(null , "Single data");
    }
    else{
        callback(null , "Everything else...");    
    }
    
};

Note:

Just wanted to add this, when performing scan be aware of the limits, if the limit exceeds you won't get the entire entries in the table.

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