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.

Answer 2

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