接続プールの監視

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>";
// Subscribe to the event
client.on(eventName, (event) =>
  console.log("\nreceived event:\n", event)
);
async function run() {
  try {
    // Establish and verify connection
    await client.db("admin").command({ ping: 1 });
    console.log("\nConnected successfully!\n");
  } finally {
    // Ensures that the client will close when you finish/error
    await client.close();
  }
}
run().catch(console.dir);

function connectionPoolStatus(client) {
  let checkedOut = 0;
  function onCheckout() {
    checkedOut++;
  }
  function onCheckin() {
    checkedOut--;
  }
  function onClose() {
    client.removeListener('connectionCheckedOut', onCheckout);
    client.removeListener('connectionCheckedIn', onCheckin);
    checkedOut = NaN;
  }
  // Decreases count of connections checked out of the pool when connectionCheckedIn event is triggered
  client.on('connectionCheckedIn', onCheckin);
  // Increases count of connections checked out of the pool when connectionCheckedOut event is triggered
  client.on('connectionCheckedOut', onCheckout);
  // Cleans up event listeners when client is closed
  client.on('close', onClose);
  return {
    count: () => checkedOut,
    cleanUp: onClose
  };
}