The concert that oversold

import express from 'express';
import { DynamoDBClient, PutItemCommand } from '@aws-sdk/client-dynamodb';

// Glass Garden gives every instance its own PORT, because they all share one machine here.
// In a real deployment they would each be on their own machine, and could all listen on the same port.
const port = Number(process.env.PORT);
if (!port) {
  throw new Error('PORT is not set');
}

// Set by the table this node is connected to. The endpoint and the credentials the SDK
// needs to reach the local AWS region arrive in the environment too, which is why the
// client below takes no arguments at all. You can see every one of them in the Config tab
const tableName = process.env.DYNAMODB_TABLE;
if (!tableName) {
  throw new Error('DYNAMODB_TABLE is not set. Connect this instance group to one table node.');
}

const dynamodb = new DynamoDBClient({});

// The tickets this process has left to sell. A variable lives in the memory of the process
// that declared it, and starts again at 500 whenever a process starts
let remaining = 500;

// Embedded Metric Format: the shape CloudWatch extracts metrics from a log line, and how
// anything running here puts a number on its own Metrics tab
function putMetric(name, value, unit) {
  console.log(
    JSON.stringify({
      _aws: {
        Timestamp: Date.now(),
        CloudWatchMetrics: [
          { Namespace: 'glass-garden', Dimensions: [[]], Metrics: [{ Name: name, Unit: unit }] }
        ]
      },
      [name]: value
    })
  );
}

// Once at startup, so the name is in the Metrics tab before anybody buys a ticket
putMetric('tickets sold', 0, 'Count');

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

// The load balancer asks for this before it sends any buyers here, the way an ALB checks a
// target group. An instance that stops answering it is taken out of the rotation
app.get('/', (req, res) => {
  res.type('text/plain').send('ok\n');
});

app.post('/tickets', async (req, res) => {
  const { buyer = 'nobody' } = req.body;

  // Sold out is an answer, not a failure, so it is a 409 rather than an error
  if (remaining === 0) {
    res.status(409).json({ status: 'sold out' });
    return;
  }
  remaining -= 1;

  // Every ticket sold is recorded, keyed by who bought it
  await dynamodb.send(
    new PutItemCommand({
      TableName: tableName,
      Item: { id: { S: `ticket-${buyer}` }, buyer: { S: buyer } }
    })
  );

  putMetric('tickets sold', 1, 'Count');
  res.status(201).json({ status: 'sold' });
});

app.listen(port, () => {
  console.log(`Server running on http://localhost:${port}`);
});