[Mine] ESP32 -> AWS IoT -> DynamoDB via MQTT WiTh AWS Lambda

index.js

/* Env translated constants */
const TABLE_NAME = process.env.TABLE_NAME;
const IOT_ENDPOINT = process.env.IOT_ENDPOINT;
const IOT_PUBLISH_TOPIC = process.env.IOT_PUBLISH_TOPIC;

/* AWS related constants */
const AWS = require('aws-sdk');
const dynamodb = new AWS.DynamoDB();
const iotdata = new AWS.IotData({endpoint: IOT_ENDPOINT})

exports.handler = (event, context, callback) => { //inb4 "lol noob not using async". our operations
    // are mostly synchronous; you cannot know if you need to scan the table again until you scan the table
    // because the lambda is written for the sole purpose
    // of AWS IoT calling it, therefore, we can assume
    // that AWS IoT is the one calling it.

    // what that means is that we don't have to care
    // about the event variable

    // note that dynamodb.describeTable is not good enough;
    // it updates the item count every 6 hours, which is unindeal.

    // also, before you roast me about the amount of database throughput this will
    // generate, i'm aware. it's the most basic tutorial for the most basic needs.
    // gimmie a break.

    // if you want to fix this issue, then maintain a count of the table somewhere else
    // then you won't waste throughput
    
    let totalCount = 0;
    let callback_function = (err, data) => {
	if (err !== null) {
	    callback({ statusCode: 500, body: "Can't seem to do DynamoDB things" });
	    return;
	}
	
	totalCount += data.Count; //adds to the total count
	console.log(data);
	if (!('LastEvaluatedKey' in data) || data.LastEvaluatedKey === {}) {
	    let ledOn = Math.min(1, totalCount % 10); //remember that 0 is on and 1 is off
	    iotdata.publish({ topic: IOT_PUBLISH_TOPIC, payload: ledOn.toString() }, (err, data) => {
		if (err !== null) {
		    callback({ statusCode: 500, body: "Can't seem to do IoT things" });
		    return;
		}
		callback(null, { statusCode: 200, body: "Right on the money." });
	    });
	} else { //we haven't scanned the whole database
	    dynamodb.scan({TableName: TABLE_NAME, ExclusiveStartKey: data.LastEvaluatedKey}, callback_function);
	}
    }
    
    dynamodb.scan({TableName: TABLE_NAME}, callback_function);
};