## BLOG 23: tiny toy application**—a pocket-sized traffic booth

## **tiny toy application**—a pocket-sized traffic booth—that:

* Accepts incoming “requests” and drops them into an in-memory **queue**
* Exposes a **metric** (`myapp_queue_length`) at `/metrics`
* Processes **1 request every 2 seconds** (like a sleepy clerk sipping tea between tasks)

# 🟦 **📦 Dummy App (Node.js)**

:::spoiler **app.js**

const express = require("express");
const client = require("prom-client");

const app = express();
app.use(express.json());

// Create a registry
const register = new client.Registry();

// Custom metric – current queue length
const queueLengthGauge = new client.Gauge({
  name: "myapp_queue_length",
  help: "Current number of items in the processing queue"
});

register.registerMetric(queueLengthGauge);

// Our in-memory queue
let queue = [];

// Endpoint to add an item to the queue
app.post("/enqueue", (req, res) => {
  const item = req.body.item || `job-${Date.now()}`;
  queue.push(item);
  queueLengthGauge.set(queue.length);
  console.log(`Enqueued: ${item}, queue length is now ${queue.length}`);
  res.json({ message: "Item added", queue_length: queue.length });
});

// Worker that processes 1 item every 2 seconds
setInterval(() => {
  if (queue.length > 0) {
    const item = queue.shift();
    console.log(`Processed: ${item}`);
    queueLengthGauge.set(queue.length);
  }
}, 2000);

// Expose metrics to Prometheus
app.get("/metrics", async (req, res) => {
  res.set("Content-Type", register.contentType);
  res.end(await register.metrics());
});

// Simple health endpoint
app.get("/", (req, res) => {
  res.send("Queue processor is running");
});

const port = 8080;
app.listen(port, () => {
  console.log(`Dummy queue app running on port ${port}`);
});

:::

---

:::spoiler **package.json**

{
  "name": "dummy-queue-app",
  "version": "1.0.0",
  "main": "app.js",
  "license": "MIT",
  "dependencies": {
    "express": "^4.18.2",
    "prom-client": "^14.1.0"
  }
}

:::

---

:::spoiler **Dockerfile**

FROM node:18-alpine

WORKDIR /app
COPY package.json package-lock.json* ./
RUN npm install

COPY . .

EXPOSE 8080
CMD ["node", "app.js"]

:::

---
[Optional]
# 🧪 **Test the app locally**

Start the server:

node app.js

Add items to the queue:

curl -X POST http://localhost:8080/enqueue -H "Content-Type: application/json" -d '{"item": "A"}'

Check Prometheus metrics:

curl http://localhost:8080/metrics

You'll see:

# HELP myapp_queue_length Current number of items in the processing queue
# TYPE myapp_queue_length gauge
myapp_queue_length 3

The worker will eat one item every 2 seconds 🍽️⏲️

---

# 🌿 **This app is perfect for HPA testing**

Because:

* Incoming traffic → queue grows → metric spikes
* Prometheus scrapes `myapp_queue_length`
* HPA scales when queue > 10
* More pods = more queue consumers → queue clears faster

It's like hiring extra staff when the line gets too long.

---