NodeJS in AWS Lambda to Mongo Atlas - connection fails with no logs - node.js

Question

I am JS newbie so this may be some silly trouble. I have a lambda written in NodeJS 10.x and I am trying to add MongoDB Atlas insertion. I have started with this tutorial: https://docs.atlas.mongodb.com/best-practices-connecting-to-aws-lambda/
This is my code:
const MongoClient = require('mongodb').MongoClient;
let cachedDb = null;
function connectToDatabase (uri) {
console.log('Connect to mongo database');
if (cachedDb) {
console.log('Using cached database instance');
return Promise.resolve(cachedDb);
}
return MongoClient.connect(uri)
.then(db => {
console.log('Successful connect');
cachedDb = db;
return cachedDb;
}).catch(err => {
console.log('Connection error occurred: ', err);
callback(err);
});
}
function insertUser(db, email) {
console.log('=> modify database');
return db.collection('users').insertOne({"email" : email})
.then(() => { callback(null, result); })
.catch(err => {
console.log('Insert error occurred: ', err);
callback(err);
});
}
exports.handler = (payload, context, callback) => {
const { email, password } = JSON.parse(payload.body);
context.callbackWaitsForEmptyEventLoop = false;
connectToDatabase(MONGODB_URI)
.then(db => {
console.log('Mongo connected')
insertUser(db, email);
})
.then(result => {
console.log('Mongo insert succeeded', result);
})
.catch(err => {
console.log('Mongo insert failed', err);
return responses.INTERNAL_SERVER_ERROR_500(err, callback, response);
});
console.log('finished mongo stuff');
I can see the following logs in CloudWatch:
START RequestId: 0338d336-7d33-40d5-abc7-1511f1c9ea4c Version: $LATEST
2020-01-11T12:18:00.808Z 0338d336-7d33-40d5-abc7-1511f1c9ea4c INFO Connect to mongo database
2020-01-11T12:18:00.855Z 0338d336-7d33-40d5-abc7-1511f1c9ea4c INFO finished mongo stuff
2020-01-11T12:18:01.416Z 0338d336-7d33-40d5-abc7-1511f1c9ea4c ERROR (node:8) DeprecationWarning: current Server Discovery and Monitoring engine is deprecated, and will be removed in a future version. To use the new Server Discover and Monitoring engine, pass option { useUnifiedTopology: true } to the MongoClient constructor.
END RequestId: 0338d336-7d33-40d5-abc7-1511f1c9ea4c
The item is not inserted in Atlas. I have added more verbose logging but it is not shown. If the mongo connect failed there shall be an error. But it seems that the error handlers are ignored. Where is the problem?

Answer 1

You are very close. Few things are missing
callback function needs to be called from your handler function as the insert function doesn't know about callback.
When you do a .then on a promise and you use curly braces you need to return whatever is in there to send it to the next then (unless you do it in a single line).
db in insert method needs to be db.db()
I would recommend to to use async/await instead of callback style. Here is the equivalent code
exports.handler = async (payload) => {
try {
const { email, password } = JSON.parse(payload.body);
const db = await connectToDatabase(MONGODB_URI);
console.log("Mongo connected");
const result = await insertUser(db, email);
console.log("Mongo insert succeeded", result);
return result;
} catch(err) {
console.error(err);
}
};