Monitoring mongodb connection to a replicaset in NodeJS

Question

I want to connect to a MongoDB replica set (only one instance to works with change streams) while being able to be notified of connection lost/reconnect. I followed what described here :

const { MongoClient } = require("mongodb");

// Replace the following with your MongoDB deployment's connection
// string.
const uri =
  "mongodb+srv://<clusterUrl>/?replicaSet=rs&writeConcern=majority";

const client = new MongoClient(uri);

// Replace <event name> with the name of the event you are subscribing to.
const eventName = "<event name>";
client.on(eventName, event => {
  console.log(`received ${eventName}: ${JSON.stringify(event, null, 2)}`);
});

async function run() {
  try {
    await client.connect();

    // Establish and verify connection
    await client.db("admin").command({ ping: 1 });
    console.log("Connected successfully");
  } finally {
    // Ensures that the client will close when you finish/error
    await client.close();
  }
}
run().catch(console.dir);

I tried subscribing to events:

• serverOpening and works fine
• serverClosed and I can't understand why but it does not work!!!

No "reconnect" event, any solution?

Answer 1

You are mixing monitoring connections and application connections. Unfortunately the documentation you referenced doesn't talk about this and doesn't document CMAP events so the confusion is understandable. See the Ruby driver docs for a more in-depth explanation of the events that drivers publish (including the Node driver).

Monitoring connections are established by the driver to figure out what server(s) exist in the deployment it was instructed to work with. One (or two depending on driver and server version) such connection is established per known server. You don't control when these connections are established. These connections are NOT used for operations you initiate (inserts/finds etc.). They are only used internally by the driver.

The events published for monitoring connections are server opened, server closed, server heartbeat - the ones listed here . You are going to get these events when the client is instantiated (assuming a spec-compliant client, which the Node one is not in it default configuration as you are using it) without any operations being issued like creating a change stream.

Application connections are established by the driver to satisfy the application's operations like finds and inserts. One of these would be needed for your change stream. The events relevant to these connections are CMAP ones and start with "Connection" or "Pool", eg ConnectionCreated. These connections aren't established until you issue an operation, unless you have the min pool size on the client set to a value greater than zero.

If you want to "monitor connections", you can subscribe to either category of events or both.

With that said, both types of connections are managed internally by the driver. You don't get a say in when they are created or destroyed (other than setting min pool size and idle timeouts). So if your goal is to have a working, continuously-running, resuming change stream, you don't need any of this and instead you should be using the proper change stream consumption patterns like the one described here in Ruby syntax although all spec-compliant drivers should provide the equivalent interface.

Lastly, there isn't a "reconnect" event defined in any driver specification. If you have a question specifically about this event you should reference the driver documentation where it is described and read that documentation carefully to ascertain the implemented behavior.