Connect AWS mobile backend to DynamoDB - amazon-web-services

Question

I am trying to use AWS mobile backend (using lambda function) to insert into dynamoDB (also configured at the mobile backend) but with no success so far.
The relevant code:
'use strict';
console.log("Loading function");
const AWS = require('aws-sdk');
const docClient = new AWS.DynamoDB.DocumentClient({region:process.env.MOBILE_HUB_PROJECT_REGION});
exports.handler = function(event, context, callback) {
var responseCode = 200;
var requestBody, pathParams, queryStringParams, headerParams, stage,
stageVariables, cognitoIdentityId, httpMethod, sourceIp, userAgent,
requestId, resourcePath;
console.log("request: " + JSON.stringify(event));
// Request Body
requestBody = event.body;
if (requestBody !== undefined && requestBody !== null) {
// Set 'test-status' field in the request to test sending a specific response status code (e.g., 503)
responseCode = JSON.parse(requestBody)['test-status'];
}
// Path Parameters
pathParams = event.path;
// Query String Parameters
queryStringParams = event.queryStringParameters;
// Header Parameters
headerParams = event.headers;
if (event.requestContext !== null && event.requestContext !== undefined) {
var requestContext = event.requestContext;
// API Gateway Stage
stage = requestContext.stage;
// Unique Request ID
requestId = requestContext.requestId;
// Resource Path
resourcePath = requestContext.resourcePath;
var identity = requestContext.identity;
// Amazon Cognito User Identity
cognitoIdentityId = identity.cognitoIdentityId;
// Source IP
sourceIp = identity.sourceIp;
// User-Agent
userAgent = identity.userAgent;
}
// API Gateway Stage Variables
stageVariables = event.stageVariables;
// HTTP Method (e.g., POST, GET, HEAD)
httpMethod = event.httpMethod;
// TODO: Put your application logic here...
let params = {
Item:{
"prop1":0,
"prop2":"text"
},
TableName:"testTable"
};
docClient.put(params, function(data, err){
if(err)
responseCode = 500;
else
{
responseCode = 200;
context.succeed(data);
}
});
// For demonstration purposes, we'll just echo these values back to the client
var responseBody = {
requestBody : requestBody,
pathParams : pathParams,
queryStringParams : queryStringParams,
headerParams : headerParams,
stage : stage,
stageVariables : stageVariables,
cognitoIdentityId : cognitoIdentityId,
httpMethod : httpMethod,
sourceIp : sourceIp,
userAgent : userAgent,
requestId : requestId,
resourcePath : resourcePath
};
var response = {
statusCode: responseCode,
headers: {
"x-custom-header" : "custom header value"
},
body: JSON.stringify(responseBody)
};
console.log("response: " + JSON.stringify(response))
context.succeed(response);
};
this doesn't put the item to the table for some reason.
I gave the necessary permissions using the roles part, anything I am missing?
**responseCode is only for testing purposes.
Edit:
tried AWS node.js lambda request dynamodb but no response (no err, no return data) and doesn't work either.
Edit2:
Added the full handler code. (it the default generated code when creating first AWS lambda).

Answer 1

I have refactored some bits of your code to look much simpler and use async/await (make sure to select Node 8.10 as the running environment for your function) instead of callbacks. I also got rid of the context and callback parameters, as they were used for older versions of NodeJS. Once you're using Node 8+, async/await should be the default option.
Also, it is possible to chain a .promise() on docClient.putItem, so you can easily await on it, making your code way simpler. I have left only the DynamoDB part (which is what is relevant to your question)
'use strict';
console.log("Loading function");
const AWS = require('aws-sdk');
const docClient = new AWS.DynamoDB.DocumentClient({region:process.env.MOBILE_HUB_PROJECT_REGION});
exports.handler = async (event) => {
let params = {
Item:{
"prop0":1,
"prop2":"text"
},
TableName:"testTable"
};
try {
await docClient.put(params).promise();
} catch (e) {
console.log(e)
return {
messsage: e.message
}
}
return { message: 'Data inserted successfully' };
};
Things to keep in mind if still it does not work:
Make sure your Lambda function has the right permissions to insert items on DynamoDB (AmazonDynamoDBFullAccess will do it)
You ALWAYS have to provide the partition key when inserting items to DynamoDB. On your example, the JSON only has two properties: prop1 and prop2. If none of them are the partition key, your code will certainly fail.
Make sure you table also exists
If you code fails, just check CloudWatch logs as any exception is now captured and printed out on the console.

Answer 2

The reason why no data is written in the table is because the call to DynamoDB put is asynchronous and will return by calling your callback. But during that time, the rest of the code continues to execute and your function eventually finish before the call to DynamoDB has a chance to complete.
You can use the await / async keywords to make your code sychronous :
async function writeToDynamoDB(params) {
return new Promise((resolve,reject) => {
docClient.put(params, function(data, err){
if(err)
reject(500);
else
resolve(data);
});
});
}
let params = ...
var data = await writeToDynamoDB(params)
You can find sample code I wrote (in Typescript) at https://github.com/sebsto/maxi80-alexa/blob/master/lambda/src/DDBController.ts