Alert on balance changes

curl -fsSO https://raw.githubusercontent.com/lumeusdev/docs/main/examples/verify-webhook.mjs

curl -fsSO https://raw.githubusercontent.com/lumeusdev/docs/main/examples/webhook_receiver.py

// Node 18+: save as alerts.mjs next to verify-webhook.mjs and run
//   WEBHOOK_SECRET="$(cat webhook-secret.txt)" ALERT_BELOW_XLM=100 node alerts.mjs
import { createServer } from 'node:http';
import { verifyWebhook } from './verify-webhook.mjs';

const secrets = [process.env.WEBHOOK_SECRET, process.env.WEBHOOK_NEXT_SECRET].filter(Boolean);
const threshold = process.env.ALERT_BELOW_XLM ?? '100';
const belowRaw = BigInt(Math.round(Number(threshold) * 1e7));
const seen = new Set(); // demo only: use a database UNIQUE key on (network, id)

function record(network, event) {
  if (event.type !== 'balance.changed') return;
  const key = `${network}|${event.id}`;
  if (seen.has(key)) return;
  seen.add(key);
  const now = BigInt(event.balance_raw);
  const before = BigInt(event.previous_balance_raw);
  console.log(`${event.closed_at} ${event.holder} ${event.asset} ${event.balance} (${now >= before ? '+' : '-'})`);
  if (event.asset === 'native' && now < belowRaw && before >= belowRaw) {
    console.log(`ALERT: ${event.holder} fell below ${threshold} XLM`);
  }
}

createServer((req, res) => {
  if (req.method !== 'POST') return res.writeHead(405).end();
  const chunks = [];
  req.on('data', (c) => chunks.push(c));
  req.on('end', () => {
    const rawBody = Buffer.concat(chunks);
    let context;
    try {
      ({ context } = verifyWebhook({ rawBody, headers: req.headers, secrets }));
    } catch (error) {
      console.warn('rejected:', error.message); // a short code, safe to log
      return res.writeHead(400).end();
    }
    if (context.lane !== 'test') record(context.network, JSON.parse(rawBody));
    res.writeHead(204).end(); // answer after the event is recorded
  });
}).listen(3000, () => console.log('listening on :3000'));

# Python 3.9+: save as alerts.py next to webhook_receiver.py and run
#   WEBHOOK_SECRET="$(cat webhook-secret.txt)" ALERT_BELOW_XLM=100 python3 alerts.py
import json
import os
from decimal import Decimal
from http.server import BaseHTTPRequestHandler, HTTPServer

from webhook_receiver import WebhookError, verify_webhook

SECRETS = [s for s in (os.environ.get("WEBHOOK_SECRET"), os.environ.get("WEBHOOK_NEXT_SECRET")) if s]
THRESHOLD = os.environ.get("ALERT_BELOW_XLM", "100")
BELOW_RAW = int(Decimal(THRESHOLD) * 10**7)
seen = set()  # demo only: use a database UNIQUE key on (network, id)


def record(network, event):
    if event.get("type") != "balance.changed" or (network, event["id"]) in seen:
        return
    seen.add((network, event["id"]))
    now, before = int(event["balance_raw"]), int(event["previous_balance_raw"])
    sign = "+" if now >= before else "-"
    print(f"{event['closed_at']} {event['holder']} {event['asset']} {event.get('balance')} ({sign})", flush=True)
    if event["asset"] == "native" and now < BELOW_RAW <= before:
        print(f"ALERT: {event['holder']} fell below {THRESHOLD} XLM", flush=True)


class Receiver(BaseHTTPRequestHandler):
    def answer(self, status):
        self.send_response(status)
        self.end_headers()

    def do_POST(self):
        try:
            raw_body = self.rfile.read(int(self.headers.get("Content-Length") or 0))
        except ValueError:
            return self.answer(400)  # malformed Content-Length
        try:
            context = verify_webhook(raw_body, self.headers, SECRETS)["context"]
        except WebhookError as error:
            print("rejected:", error, flush=True)  # a short code, safe to log
            return self.answer(400)
        if context["lane"] != "test":
            record(context["network"], json.loads(raw_body))
        self.answer(204)  # answer after the event is recorded

    def log_message(self, *args):
        pass


print("listening on :3000", flush=True)
HTTPServer(("", 3000), Receiver).serve_forever()