Webhookで金・銀の価格アラートを自動化する

POST /webhooks/metalapi HTTP/1.1
Content-Type: application/json
User-Agent: MetalAPI-Webhooks/1.0
X-MetalAPI-Timestamp: 1792765800
X-MetalAPI-Signature: t=1792765800,v1=5f0c1e…9a2b

{
  "event": "alert.triggered",
  "id": 18342,
  "created": 1792765800,
  "alert": {
    "id": "7c9e6679-7425-40de-944b-e07fc1f90ae7",
    "symbol": "XAU",
    "base": "USD",
    "condition": "above",
    "threshold": 4000,
    "note": "Gold above 4k"
  },
  "price": 4001.25,
  "unit": "troy_oz"
}

// webhook-server.mjs — Node.js 18以降、npm install express
import crypto from "node:crypto";
import express from "express";

// カンマ区切り：現在のシークレットと、ローテーション中は以前のシークレット。
const SECRETS = (process.env.METALAPI_WEBHOOK_SECRETS ?? "").split(",").filter(Boolean);
const TOLERANCE_S = 300; // 5分より古い署名は拒否する

export function verifySignature(rawBody, header) {
  const parts = {};
  for (const item of (header ?? "").split(",")) {
    const i = item.indexOf("=");
    if (i > 0) parts[item.slice(0, i).trim()] = item.slice(i + 1).trim();
  }
  const ts = parts.t ?? "";
  if (!/^\d+$/.test(ts) || Math.abs(Date.now() / 1000 - Number(ts)) > TOLERANCE_S) return false;
  if (!/^[0-9a-f]{64}$/.test(parts.v1 ?? "")) return false;
  const given = Buffer.from(parts.v1, "hex");
  return SECRETS.some((secret) => {
    const expected = crypto.createHmac("sha256", secret).update(`${ts}.`).update(rawBody).digest();
    return crypto.timingSafeEqual(expected, given);
  });
}

async function handleAlert(event) {
  const { alert, price, unit } = event;
  console.log(`${alert.symbol}/${alert.base} ${alert.condition} ${alert.threshold}: ${price} per ${unit}`);
}

const app = express();

app.post("/webhooks/metalapi", express.raw({ type: "application/json", limit: "64kb" }), (req, res) => {
  if (!Buffer.isBuffer(req.body) || !verifySignature(req.body, req.get("X-MetalAPI-Signature"))) {
    return res.status(401).send("invalid signature");
  }
  const event = JSON.parse(req.body.toString("utf8"));
  res.sendStatus(204); // 8秒以内に応答してから処理を行う
  if (event.event === "alert.triggered") {
    handleAlert(event).catch((err) => console.error("alert handler failed:", err));
  }
});

app.listen(3000);

# webhook_app.py — pip install flask の後、flask --app webhook_app run --port 3000 で起動
import hashlib
import hmac
import json
import os
import time

from flask import Flask, abort, request

# カンマ区切り：現在のシークレットと、ローテーション中は以前のシークレット。
SECRETS = [s for s in os.environ.get("METALAPI_WEBHOOK_SECRETS", "").split(",") if s]
TOLERANCE_S = 300  # 5分より古い署名は拒否する

app = Flask(__name__)


def verify_signature(raw: bytes, header: str) -> bool:
    parts = dict(p.strip().split("=", 1) for p in header.split(",") if "=" in p)
    ts, given = parts.get("t", ""), parts.get("v1", "")
    if not ts.isdigit() or abs(time.time() - int(ts)) > TOLERANCE_S:
        return False
    signed = ts.encode() + b"." + raw
    return any(
        hmac.compare_digest(hmac.new(s.encode(), signed, hashlib.sha256).hexdigest(), given)
        for s in SECRETS
    )


@app.post("/webhooks/metalapi")
def metalapi_webhook():
    raw = request.get_data()  # JSONをパースする前の生のバイト列
    if not verify_signature(raw, request.headers.get("X-MetalAPI-Signature", "")):
        abort(401)
    event = json.loads(raw)
    if event.get("event") == "alert.triggered":
        alert = event["alert"]
        # ここでイベントをジョブキューに渡し、すぐに応答を返す。
        app.logger.warning("%s/%s %s %s: %s", alert["symbol"], alert["base"],
                           alert["condition"], alert["threshold"], event["price"])
    return "", 204

create table metalapi_events (
  event_id     bigint primary key,      -- ペイロードの "id"、リトライ間で同一
  alert_id     uuid not null,
  payload      jsonb not null,
  received_at  timestamptz not null default now(),
  processed_at timestamptz
);

-- Webhookハンドラー内で、署名の検証後、応答を返す前に実行：
insert into metalapi_events (event_id, alert_id, payload)
values ($1, $2, $3)
on conflict (event_id) do nothing;

const VERBS = { above: "rose above", below: "fell below", change_pct_up: "is up", change_pct_down: "is down" };

async function handleAlert({ alert, price, unit }) {
  const level = alert.condition.startsWith("change_pct")
    ? `${alert.threshold}% since the previous close`
    : `${alert.threshold} ${alert.base}`;
  const per = unit && unit !== "currency" ? ` per ${unit}` : "";
  const text = `${alert.symbol} ${VERBS[alert.condition]} ${level}: now ${price} ${alert.base}${per}` +
    (alert.note ? ` (${alert.note})` : "");

  // Slackの受信Webhookは {"text": ...}、DiscordのWebhookは {"content": ...} を受け付ける。
  const body = process.env.CHAT_KIND === "discord" ? { content: text } : { text };
  const res = await fetch(process.env.CHAT_WEBHOOK_URL, {
    method: "POST",
    headers: { "Content-Type": "application/json" },
    body: JSON.stringify(body),
    signal: AbortSignal.timeout(10_000),
  });
  if (!res.ok) throw new Error(`chat webhook returned ${res.status}`);
}

SECRET="whsec_your_secret"
BODY='{"event":"alert.triggered","id":1,"created":1792765800,"alert":{"id":"00000000-0000-0000-0000-000000000000","symbol":"XAU","base":"USD","condition":"above","threshold":4000,"note":"test"},"price":4001.25,"unit":"troy_oz"}'
TS=$(date +%s)
SIG=$(printf '%s.%s' "$TS" "$BODY" | openssl dgst -sha256 -hmac "$SECRET" -hex | sed 's/^.* //')

curl -i -X POST http://localhost:3000/webhooks/metalapi \
  -H "Content-Type: application/json" \
  -H "X-MetalAPI-Timestamp: $TS" \
  -H "X-MetalAPI-Signature: t=$TS,v1=$SIG" \
  --data-raw "$BODY"