AWS Lambda function not writing to DynamoDB

Question

I have a lambda function that's suppose to be writing to a database. When I run it on my local machine it works but then when I upload it to lambda and test it It doesn't put anything in the database. The role I have the function using has full access to DynamoDB and its the exact same code that works fine when I run it from my laptop. Any idea why that would be the case?

Here's my lambda. The dao class contains the code that actually accesses dynamo. I'm just trying to upload some constant strings right now.

const DAO = require('./PostStatusDAO.js');

exports.handler = async (event, context, callback) => {
    var dao = new DAO();
    dao.post("this is a test", "@jordan", "@matt", "none");       
    const response = {
        statusCode: 200,
        body: {
            result: "good"
        }
    };
    return response;
};

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

const ddb = new AWS.DynamoDB.DocumentClient({region: 'us-west-2'});

class PostStatusDAO {
    post(in_text, in_user, in_author, in_attachment) {
        var params = {
            Item: {
                user: String(in_user),
                timestamp: Date.now(),
                author: String(in_author),
                text: String(in_text),
                attachment: String(in_attachment),
            },

            TableName: 'Feed',
        };

        console.log(params);

        var result = ddb.put(params, (err, data) => {
            console.log("callback");
            if(err) {
                console.log("Error: ", err);
            } else {
                console.log("Data: ", data);
            }
        });

        // console.log(result);
    }
}

module.exports = PostStatusDAO;

You should wait for the ddb.put operation to be finished before returning the response from lambda. — Soubhik Mondal
– Soubhik Mondal, Commented Nov 16, 2019 at 4:28

Jorge Garcia · Accepted Answer · 2019-11-16 08:33:33Z

1

To see the reason why your function is failing you have to either run it synchronously or return the promise back to the caller/runtime like this:

const DAO = require('./PostStatusDAO.js');

exports.handler = async(event, context, callback) => {
    var dao = new DAO();

    // Return new promise 
    return new Promise(function(resolve, reject) {
     // Do async job
        dao.post("this is a test", "@jordan", "@matt", "none", function(err, data) {
            if (err) {
                console.log("Error: ", err);
                reject(err);
            }
            else {
                console.log("Data: ", data);
                resolve(data);
            }
        })
    })
};

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

const ddb = new AWS.DynamoDB.DocumentClient({region: 'us-west-2'});

class PostStatusDAO {
    async post(in_text, in_user, in_author, in_attachment, callback) {
        var params = {
            Item: {
                user: String(in_user),
                timestamp: Date.now(),
                author: String(in_author),
                text: String(in_text),
                attachment: String(in_attachment),
            },

            TableName: 'Feed',
        };

        console.log(params);

        return ddb.put(params, callback).promise();
    }
}

module.exports = PostStatusDAO;

answered Nov 16, 2019 at 8:33

Jorge Garcia

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2 Comments

apil.tamang Over a year ago

May I ask WHY we need to run synchronously, or use a async/wait pattern? When the ddb.put(...) method is called, the underlying infrastructure should just proceed to write the result, no? Not sure if this is a javascript thing, or something special about lambda and/or AWS infra

Jorge Garcia Over a year ago

All requests made through the SDK are asynchronous and use a callback interface. It's just the way AWS designed it. Lambda support both, async and non-async handlers, it's up to you, see more here: docs.aws.amazon.com/lambda/latest/dg/nodejs-handler.html

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