MongoDB topology description change listener showing server latency windows

topology.js

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

// localThresholdMS: The latency window in milliseconds (default is 15)
const client = new MongoClient("mongodb://localhost:27017,localhost:27018,localhost:27019", {
  localThresholdMS: 15 
});

client.on('topologyDescriptionChanged', (event) => {
  const servers = event.newDescription.servers;
  
  // 1. Find the "Fastest" RTT among usable servers to determine the threshold
  let minAvgRtt = Infinity;
  servers.forEach(s => {
    if (s.roundTripTime > -1 && s.type !== 'Unknown') {
      if (s.roundTripTime < minAvgRtt) minAvgRtt = s.roundTripTime;
    }
  });

  const localThresholdMS = client.options.localThresholdMS;
  const fastest = minAvgRtt === Infinity ? 'N/A' : minAvgRtt + 'ms';
  console.log(`\n--- Topology Update (Fastest Node: ${fastest}, window: ${localThresholdMS}ms) ---`);

  // 2. Evaluate each server
  const entries = [];
  servers.forEach((server, address) => {
    const avgRtt = server.roundTripTime;
    const isConnected = server.type !== 'Unknown' && server.type !== 'PossiblePrimary';
    const isWithinLatencyWindow = isConnected && (avgRtt <= (minAvgRtt + localThresholdMS));
    entries.push({ address, avgRtt, isConnected, isWithinLatencyWindow });
  });

  const group = e => !e.isConnected ? 2 : e.isWithinLatencyWindow ? 0 : 1;

  entries.sort((a, b) => {
    if (group(a) !== group(b)) return group(a) - group(b);
    if (a.avgRtt !== b.avgRtt) return a.avgRtt - b.avgRtt;
    return a.address.localeCompare(b.address);
  });

  const groupHeaders = ['suitable, within latency window', 'suitable, outside latency window', 'not suitable'];

  let lastGroup = null;
  entries.forEach(entry => {
    const { address, avgRtt, isConnected, isWithinLatencyWindow } = entry;
    const g = group(entry);
    if (g !== lastGroup) {
      if (lastGroup !== null) console.log();
      console.log(`[${groupHeaders[g]}]`);
    }
    lastGroup = g;
    const suitable = isConnected ? 'yes' : 'no';
    const rtt = avgRtt > -1 ? avgRtt + 'ms' : 'N/A';
    const withinLatencyWindow = !isConnected ? 'N/A' : isWithinLatencyWindow ? 'yes' : 'no';
    console.log(`   ${address} suitable=${suitable}, rtt=${rtt}, withinLatencyWindow=${withinLatencyWindow}`);
  });
});

client.connect();