node.js - 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 server goes down for any reason, the application doesn't reconnect. To get this right, I based my tests on the code in this official tutorial.

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

MongoClient.connect('mongodb://localhost:27017/test', 

// Optional: uncomment if necessary
// { db: { bufferMaxEntries: 3 } },


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)
});

The idea is to run this script, and then stop mongod, and then restart it. So, here we go:

TEST 1: stopping mongod for 10 seconds

Stopping MongoDb for 10 seconds does the desired result: it will stop running the queries for those 10 seconds, and then will run all of them once the server is back ip

TEST 2: stopping mongod for 30 seconds

After exactly 30 seconds, I start getting:

{ [MongoError: topology was destroyed] name: 'MongoError', message: 'topology was destroyed' }
insert

{ [MongoError: topology was destroyed] name: 'MongoError', message: 'topology was destroyed' }

The trouble is that from this on, when I restart mongod, the connection is not re-establised.

Solutions?

Does this problem have a solution? If so, do you know what it is? Once my app starts puking "topology was destroyed", the only way to get everything to work again is by restarting the whole app...

Answer 1

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.