Serverless + Firebase: API is not reply message - node.js

Question

I am creating an api which will update a Firebase Realtime database, using Nodejs, Serverless, Javascript.
my .js file:
var admin = require("firebase-admin");
var serviceAccount = require("../xxxxxxx-firebase-adminsdk-xxxxxxx.json");
module.exports.saveState = (event, context, callback) => {
const body = JSON.parse(event.body);
var timestamp_create = body.timestamp_create;
try {
admin.initializeApp({
credential: admin.credential.cert(serviceAccount),
databaseURL: "https://xxxxxxx.firebaseio.com"
});
} catch (err) {
console.log('Firebase initialization error', err.stack);
}
var db = admin.database();
var ref = db.ref('item/');
ref.child(timestamp_create).set({
state: 'empty'
}, function (error) {
if (error) {
console.log("Data could not be saved." + error);
} else {
console.log("Data saved successfully.");
return callback(null, {
headers: {
"Access-Control-Allow-Origin": "*",
},
statusCode: 200,
body: JSON.stringify({ msg: "Data saved successfully." })
});
}
});
};
I deploy API to AWS and test on Postman:
Problem and my attempts:
After saving item to Firebase Realtime Database, the api didn't reply the message. Api fell into timeout.
I checked CloudWatch, the log is printed ("Data saved successfully."), but it didn't execute the return callback at all.
I tried api in "serverless offline", it works normally, and returns the message.
The API doesn't return callback in AWS environment(lambda).
Any Suggestion is appreciated.

Answer 1

After long google and test, I came to a solution for this, leave it here for reference:
The firebase database connection is not released after saving data, so that AWS Lambda cannot return the callback. The simple solution is close database connection after task
var ref = db.ref('item/');
ref.child(timestamp_create).set({
state: 'empty'
}, function (error) {
if (error) {
console.log("Data could not be saved." + error);
} else {
console.log("Data saved successfully.");
db.gpOffline(); // this line will release database connection
return callback(null, {
headers: {
"Access-Control-Allow-Origin": "*",
},
statusCode: 200,
body: JSON.stringify({ msg: "Data saved successfully." })
});
}
});