MongoError: failed to connect to server [mongo:27017] on first connect - mongodb

Question

Docker config:
FROM node:8
WORKDIR /usr/src/app
COPY package*.json ./
RUN npm install
COPY . .
EXPOSE 8080
CMD [ "npm", "start" ]
Docker compose:
version: "2"
services:
web:
build: .
depends_on:
- mongo
mongo:
image: mongo
ports:
- "27017:27017"
package.json:
{
"name": "docker_web_app",
"version": "1.0.0",
"description": "",
"author": "Stepan Yakovenko <stiv.yakovenko#gmail.com>",
"main": "server.js",
"scripts": {
"start": "node server.js"
},
"dependencies": {"express": "^4.16.1","mongodb": "~3.0.1","monk": "~6.0.5" }
}
server.js:
var mongo = require('mongodb');
var monk = require('monk');
var db = monk('mongo:27017/nodetest1');
db.then(function(){console.log("hello"});
Most of the time it works, but if I purge docker cache, usually it doesn't work and gives me this:
web_1 | (node:15) UnhandledPromiseRejectionWarning: MongoError: failed to connect to server [mongo:27017] on first connect [MongoErr
or: connect ECONNREFUSED 172.18.0.2:27017]
The root cause I think that docker's depends_on doesn't guarantee me that mongo has started listening, because in this case I get listening message from mongo after this error. How can I fix this? Does docker has any fix for this situation? Or how can I ask mongo to try connecting forever?
Thanx

Answer 1

This is sample reconnect code, which worked for me:
var connect = function () {
var db = monk('mongo:27017/nodetest1');
db.then(function () {
console.log("connected");
}).catch(function () {
// sometimes node starts before mongo, so we have to reconnect in case of error
connect();
});
};
connect();