Reliably reconnect to MongoDB

Question

UPDATE: I am using the 2.1 version on the driver, against 3.2

I have a node application that uses MongoDB. The problem I have is that if the MongoDB

Answer 1

If you was using Mongoose for your Schemas, it would be worth considering my option below since mongoose was never retrying to reconnect to mongoDB implicitly after first attempt failed.

Kindly note I am connecting to Azure CosmosDB for MongoDB API. On yours maybe on the local machine.

Below is my code.

const mongoose = require('mongoose');

// set the global useNewUrlParser option to turn on useNewUrlParser for every connection by default.
mongoose.set('useNewUrlParser', true);
// In order to use `findOneAndUpdate()` and `findOneAndDelete()`
mongoose.set('useFindAndModify', false);

async function mongoDbPool() {
// Closure.
return function connectWithRetry() {
    // All the variables and functions in here will Persist in Scope.
    const COSMODDBUSER = process.env.COSMODDBUSER;
    const COSMOSDBPASSWORD = process.env.COSMOSDBPASSWORD;
    const COSMOSDBCONNSTR = process.env.COSMOSDBCONNSTR;

    var dbAuth = {
        auth: {
            user: COSMODDBUSER,
            password: COSMOSDBPASSWORD
        }
    };
    const mongoUrl = COSMOSDBCONNSTR + '?ssl=true&replicaSet=globaldb';

    return mongoose.connect(mongoUrl, dbAuth, (err) => {
        if (err) {
            console.error('Failed to connect to mongo - retrying in 5 sec');
            console.error(err);
            setTimeout(connectWithRetry, 5000);
        } else {
            console.log(`Connected to Azure CosmosDB for MongoDB API.`);
        }
    });
};}

You may decide to export and reuse this module everywhere you need to connect to db via Dependency Injection. But instead I will only show how to access the database connection for now.

(async () => {
    var dbPools = await Promise.all([mongoDbPool()]);
    var mongoDbInstance = await dbPools[0]();

    // Now use "mongoDbInstance" to do what you need.
})();

Answer 2

Behavior may differ with different versions of driver. You should mention your driver version.

driver version : 2.2.10 (latest) mongo db version : 3.0.7

Below code will extend the time mongod can take to come back up.

var MongoClient = require('mongodb').MongoClient
  , f = require('util').format;

function connectCallback(err, db) {
  var col = db.collection('t');

  setInterval(function() {
    col.insert({a:1}, function(err, r) {
      console.log("insert")
      console.log(err)

      col.findOne({}, function(err, doc) {
        console.log("findOne")
        console.log(err)
      });
    })
  }, 1000)
}
var options = { server: { reconnectTries: 2000,reconnectInterval: 1000 }} 
MongoClient.connect('mongodb://localhost:27017/test',options,connectCallback);

2nd argument can be used to pass server options.

Answer 3

By default the Mongo driver will try to reconnect 30 times, one every second. After that it will not try to reconnect again.

You can set the number of retries to Number.MAX_VALUE to keep it reconnecting "almost forever":

    var connection = "mongodb://127.0.0.1:27017/db";
    MongoClient.connect(connection, {
      server : {
        reconnectTries : Number.MAX_VALUE,
        autoReconnect : true
      }
    }, function (err, db) {

    });

Answer 4

There are 2 connection options that control how mongo nodejs driver reconnects after connection fails

• reconnectTries: attempt to reconnect #times (default 30 times)
• reconnectInterval: Server will wait # milliseconds between retries (default 1000 ms)

reference on mongo driver docs

Which means that mongo will keep trying to connect 30 times by default and wait 1 second before every retry. Which is why you start seeing errors after 30 seconds.

You should tweak these 2 parameters based on you needs like this sample.

var MongoClient = require('mongodb').MongoClient,
    f = require('util').format;

MongoClient.connect('mongodb://localhost:27017/test', 
    {
        // retry to connect for 60 times
        reconnectTries: 60,
        // wait 1 second before retrying
        reconnectInterval: 1000
    },

    function(err, db) {
        var col = db.collection('t');

        setInterval(function() {
            col.insert({
                a: 1
            }, function(err, r) {
                console.log("insert")
                console.log(err)

                col.findOne({}, function(err, doc) {
                    console.log("findOne")
                    console.log(err)
                });
            })
        }, 1000)
    });

This will try 60 times instead of the default 30, which means that you'll start seeing errors after 60 seconds when it stops trying to reconnect.

Sidenote: if you want to prevent the app/request from waiting until the expiration of the reconnection period you have to pass the option bufferMaxEntries: 0. The price for this is that requests are also aborted during short network interruptions.

Answer 5

package.json: "mongodb": "3.1.3"

Reconnect existing connections

To fine-tune the reconnect configuration for pre-established connections, you can modify the reconnectTries/reconnectInterval options (default values and further documentation here).

Reconnect initial connection

For the initial connection, the mongo client does not reconnect if it encounters an error (see below). I believe it should, but in the meantime, I've created the following workaround using the promise-retry library (which uses an exponential backoff strategy).

const promiseRetry = require('promise-retry')
const MongoClient = require('mongodb').MongoClient

const options = {
  useNewUrlParser: true,
  reconnectTries: 60,
  reconnectInterval: 1000,
  poolSize: 10,
  bufferMaxEntries: 0
}

const promiseRetryOptions = {
  retries: options.reconnectTries,
  factor: 1.5,
  minTimeout: options.reconnectInterval,
  maxTimeout: 5000
}

const connect = (url) => {
  return promiseRetry((retry, number) => {
    console.log(`MongoClient connecting to ${url} - retry number: ${number}`)
    return MongoClient.connect(url, options).catch(retry)
  }, promiseRetryOptions)
}

module.exports = { connect }

Mongo Initial Connect Error: failed to connect to server [db:27017] on first connect

Answer 6

With mongodb driver 3.1.10, you can set up your connection as

MongoClient.connect(connectionUrl, {
    reconnectInterval: 10000, // wait for 10 seconds before retry
    reconnectTries: Number.MAX_VALUE, // retry forever
}, function(err, res) {
    console.log('connected') 
})

You do not have to specify autoReconnect: true as that's the default.