Cannot connect Node to mongoDB in docker container on first try - node.js

Question

I'm trying to dockerize Node.js application which connects to MongoDB using mongoose. It succeeds anytime I run node index.js from the shell when the connection URL is: mongodb://localhost:27017/deposit.
If I restart my computer and then try to run the dockerized project (with mongo instead of localhost in url) with the command docker-compose up it fails to connect to MongoDB. But after I try again the same command, then it succeeds.
So my question is why node cannot connect to MongoDB on first try after the computer is restarted?
PS. Docker is running when I'm trying it
connection.js
const mongoose = require('mongoose');
const connection = "mongodb://mongo:27017/deposit";
const connectDb = () => {
mongoose.connect(connection, {useNewUrlParser: true, useUnifiedTopology: true}).then(res => console.log("Connected to DB"))
.catch(err => console.log('>> Failed to connect to MongoDB, retrying...'));
};
module.exports = connectDb;
Dockerfile
FROM node:latest
RUN mkdir -p /app
WORKDIR /app
#/usr/src/app
COPY package.json /app
RUN npm install
COPY . /app
EXPOSE 7500
# ENTRYPOINT ["node"]
CMD ["node", "src/index.js"]
docker-compose.yml
version: "3"
services:
deposit:
container_name: deposit
image: test/deposit
restart: always
build: .
network_mode: host
ports:
- "7500:7500"
depends_on:
- mongo
mongo:
container_name: mongo
image: mongo
volumes:
- /data:/data/db
network_mode: host
ports:
- '27017:27017'

Answer 1

In your case, node application starts before mongo being ready. There are two approaches to tackle this problem: To handle it in your docker-compose or your application.
You can use wait-for-it.sh or write a wrapper script (both described here) to make sure that your node application starts after the db is ready.
But as quoted from docker documentation, it is better to handle this in your application:
To handle this, design your application to attempt to re-establish a connection to the database after a failure. If the application retries the connection, it can eventually connect to the database.
The best solution is to perform this check in your application code,
both at startup and whenever a connection is lost for any reason
You can implement mongo retry as below (Described in this answer):
var connectWithRetry = function() {
return mongoose.connect(mongoUrl, function(err) {
if (err) {
console.error('Failed to connect to mongo on startup - retrying in 5 sec', err);
setTimeout(connectWithRetry, 5000);
}
});
};
connectWithRetry();

Answer 2

As mentioned in the comment there might be the possibility the DB is not yet ready to accept the connection, so one way is to add retry logic or the other option is
serverSelectionTimeoutMS -
With useUnifiedTopology, the MongoDB driver will try to find a server
to send any given operation to, and keep retrying for
serverSelectionTimeoutMS milliseconds. If not set, the MongoDB driver
defaults to using 30000 (30 seconds).
So try with below option
const mongoose = require('mongoose');
const uri = 'mongodb://mongo:27017/deposit?retryWrites=true&w=majority';
mongoose.connect(uri, {
useNewUrlParser: true,
useUnifiedTopology: true,
serverSelectionTimeoutMS: 50000
}).catch(err => console.log(err.reason));
But again if you init DB script getting bigger it will take more time, so you go with retry logic if that did not work. in the above script it will wait for 50 seconds.