Load Testing and Capacity Planning for WebSockets
Learn how to simulate thousands of concurrent WebSocket connections, measure server limits, and plan capacity so your realtime system scales without surprises.
Load Testing and Capacity Planning for WebSockets is a free WebSockets & Realtime Systems Programming lesson on CoddyKit — lesson 4 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the WebSockets & Realtime Systems Programming learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
Why Load Testing Matters
Benchmarking tells you how a single connection behaves, but load testing tells you what happens when thousands of clients connect at once.
- Find the breaking point before users do
- Validate horizontal scaling assumptions
- Size your infrastructure budget accurately
Connections vs Messages
Two independent dimensions stress a realtime server differently:
- Connection count drives memory and file-descriptor usage
- Message throughput drives CPU and network bandwidth
Always test both axes separately and combined.
Choosing a Load Tool
Popular WebSocket load tools include artillery, k6, and websocket-bench. They open many sockets and report latency percentiles.
# Install artillery
npm install -g artillery
artillery --versionA Basic Artillery Scenario
This YAML config ramps up to 500 new connections per second for 60 seconds and sends a join message.
config:
target: 'ws://localhost:8080'
phases:
- duration: 60
arrivalRate: 500
scenarios:
- engine: ws
flow:
- send: '{"type":"join","room":"load"}'Scripting Connections in Node
You can also script load tests manually for full control over message timing.
const WebSocket = require('ws');
const TOTAL = 1000;
let open = 0;
for (let i = 0; i < TOTAL; i++) {
const ws = new WebSocket('ws://localhost:8080');
ws.on('open', () => { open++; if (open === TOTAL) console.log('all connected'); });
}Measuring Latency Percentiles
Averages hide pain. Report p50, p95, and p99 latency. A good p50 with a terrible p99 means some users have a bad experience.
Watching Server Resources
During a test, monitor the server side too: CPU, RSS memory, open file descriptors, and event-loop lag.
# Count open sockets for a process
lsof -p $(pgrep -f node) | grep -c TCPFile Descriptor Limits
Each connection consumes a file descriptor. The default OS limit (often 1024) will cap your connections long before CPU does.
# Inspect and raise the soft limit
ulimit -n
ulimit -n 100000Finding the Breaking Point
Increase load in steps until latency spikes or connections start dropping. That inflection point is your per-node capacity.
- Record the connection count at first failure
- Leave a safety margin of 30-50%
From Capacity to Node Count
Capacity planning is arithmetic once you know per-node limits.
const peakUsers = 80000;
const perNode = 10000;
const safety = 0.6; // use 60% of measured max
const nodes = Math.ceil(peakUsers / (perNode * safety));
console.log('nodes needed:', nodes);Testing in a Realistic Environment
Run load tests against staging hardware that mirrors production, and generate load from multiple machines so the client is never the bottleneck.
Quick Check
Which OS limit most commonly caps WebSocket connection counts first?
Recap
You learned to load test and plan capacity for WebSocket systems:
- Test connection count and message throughput separately
- Report p95/p99 latency, not averages
- Raise file descriptor limits before testing
- Find the breaking point and size node count with a safety margin
Frequently asked questions
Is the “Load Testing and Capacity Planning for WebSockets” lesson free?
Yes — the full text of “Load Testing and Capacity Planning for WebSockets” is free to read here on the web, and the WebSockets & Realtime Systems Programming course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the WebSockets & Realtime Systems Programming course, upgrade to CoddyKit PRO.
What will I learn in “Load Testing and Capacity Planning for WebSockets”?
Learn how to simulate thousands of concurrent WebSocket connections, measure server limits, and plan capacity so your realtime system scales without surprises. You practise WebSockets & Realtime Systems Programming with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start WebSockets & Realtime Systems Programming?
No prior experience is required. WebSockets & Realtime Systems Programming on CoddyKit is structured for beginners through advanced learners; this is — lesson 4 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Load Testing and Capacity Planning for WebSockets” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this WebSockets & Realtime Systems Programming lesson?
Yes. Every WebSockets & Realtime Systems Programming lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Benchmarking WebSocket Performance
- Profiling and Debugging Realtime Issues
- Realtime Monitoring and Alerting
- Load Testing and Capacity Planning for WebSockets