Managing Connections with the MongoDB Node.js Driver

Managing Connections with the MongoDB Node.js Driver

Managing Connections with the MongoDB Node.js Driver - The MongoDB Node.js driver is very good about handling connection issues automatically. If you lose a node in your replica set or you restart your single server, the driver will usually recover on its own with no external help needed.

Too often, MongoDB REST API developers don't think about handling database outages until they have an outage in production. Usually you can get away with this because version 2.x of the MongoDB Node.js driver does most of the work for you: it handles attempting to reconnect and can buffer operations for you so you don't get errors during a temporary outage. However, the MongoDB Node.js driver has a lot of tunable options and corresponding subtleties that you need to be aware of. In this article, I'll cover the basics of what happens when your backend MongoDB topology goes down for single server and replica sets, so you can configure the driver to do the right thing for your use case.

Handling Single Server Outages

The  mongodb-topology-manager npm package is indispensible for testing MongoDB connection logic. This lib can stop and start MongoDB servers, replica sets, and sharded clusters from Node.js. The  MongoDB driver Node.js uses it for testing internally.

First, let's see what happens when the MongoDB server that the driver is connected to shuts down. The db  handle the driver gives you emits several events, including 'close' when it loses connection and 'reconnect' when it reconnects. The driver attempts to reconnect by default, but this is a tuneable option. Note that these events only get fired for a single server or mongos, not when you're connected to a replica set (more on that later). Here's a script that uses the topology manager to stop/start a MongoDB instance so you can see these events actually fire:

const { Server } = require('mongodb-topology-manager');
const mongodb = require('mongodb');

run().catch(error => console.error(error));

async function run() {
  // Start a single server on port 27000 whose `dbpath` is in the
  // data directory in the current directory
  // Note that this requires a `mongod` executable in your PATH variable
  const server = new Server('mongod', {
    dbpath: `${__dirname}/data/db/27000`,
    port: 27000
  });
  await server.start();
  console.log('Server started...');

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