NestJS @Cron Transactional Email with Rovela (2026): @nestjs/schedule + Idempotency-Key per Window

NestJS Schedule owns the schedule — Rovela delivers the mail

NestJS Schedule (@nestjs/schedule) brings cron, interval, and timeout decorators into a Nest application. You annotate a provider method with @Cron('0 9 * * *') (or CronExpression.*), import ScheduleModule.forRoot(), and keep the Nest process running. When the expression matches, Nest invokes the method through the DI container — perfect for digests whose HTML depends on live database state at fire time rather than a body frozen earlier with Rovela scheduled_at.

Rovela queues, DKIM-signs, and delivers when your job method calls POST https://api.rovela.dev/emails (Send Email API, Quick Start, Authentication). NestJS Schedule owns the cadence and “is due?” logic. Your job method owns aggregation, secrets for sending_access, and a stable Idempotency-Key per schedule window. Rovela only delivers.

This guide wires NestJS Schedule → job method → Rovela that:

1. Declares @Cron("0 9 * * *", { name: "daily-digest-email", timeZone: "UTC" }) on a provider method
2. Keeps ROVELA_API_KEY (sending_access), MAIL_FROM, and recipients in server-only config — never in browsers, SPA bundles, or a public Git tree
3. Aggregates a digest / report for the current schedule window inside the job method
4. Sends via Rovela with a window-scoped Idempotency-Key
5. Survives retries, restarts, and multi-worker fleets without duplicate mail

Rovela does not create NestJS Schedule schedules, workers, or storage. NestJS Schedule owns schedule → enqueue/fire → execute. Your job method owns query → map fields → POST /emails. Rovela owns delivery.

Distinct from:

• Quartz Scheduler Transactional Email — Java Quartz
• node-cron Transactional Email — node-cron
• Laravel Task Scheduler Email — Laravel Schedule::
• APScheduler Cron Email — APScheduler
• Spring @Scheduled Email — Spring @Scheduled
• Resque-scheduler Email — Resque-scheduler
• RQ Scheduler Email — RQ Scheduler
• delayed_job Recurring Email — delayed_job
• Solid Queue Recurring Email — Solid Queue
• Hangfire Recurring Jobs Email — Hangfire
• Sidekiq-Cron Transactional Email — Sidekiq-Cron
• Celery Beat Transactional Email — Celery Beat
• Agenda.js Scheduled Jobs Email — agenda
• BullMQ Repeatable Email — BullMQ
• Temporal Schedule Email — Temporal
• Trigger.dev Cron Email — Trigger.dev
• Inngest Cron Transactional Email — Inngest
• Kubernetes CronJob Email — K8s CronJob
• Vercel Cron Digest Email — Vercel cron
• Scheduled Email Sending — scheduled_at

Also distinct from plain node-cron outside Nest, and from one-shot Nest mailers (OTP / magic link) — this post is the recurring @Cron path only.

What NestJS Schedule schedules vs what Rovela sends

Concern	Owner
Cron expression, name, timezone	@Cron / CronExpression on a Nest provider
Process must stay running	Nest application host (PM2 / K8s / systemd)
DI for HttpService / repositories	Nest IoC
Server secrets for ROVELA_API_KEY (sending_access)	Process / worker env / secret store
Query DB / APIs for the window	Your job method
Render HTML + text	Your job method
POST /emails, DKIM, queue, bounces	Rovela
Idempotency-Key = nest-cron-daily-digest:{yyyy-mm-dd}	Your job method

A 200 from Rovela means accepted and queued, not “already in the inbox” (Quick Start).

NestJS Schedule constraints that shape the email job

• The Nest process must stay up. Schedules live in-process with @nestjs/schedule — they are not a separate Beat service.

• Multiple replicas double-fire unless you run cron only on a worker-role deployment or add a distributed lock.

• Use name + timeZone options on @Cron for operable schedules.

• Five-field cron by default (minute hour day month weekday); confirm package docs for seconds support.

• Prefer a dedicated worker process for mail crons rather than attaching them to every API replica.

• Keep sending_access server-only. Store ROVELA_API_KEY in environment / vault — never in client apps or public Git (Authentication).

• Prefer HTTPS POST /emails. Outbound SMTP from a worker is a poor fit (Email API vs SMTP).

When to use NestJS Schedule + POST /emails (not scheduled_at)

Use NestJS Schedule → live send when:

• The HTML must include fresh metrics at fire time
• You already run NestJS Schedule and want schedules next to the same workers
• You need cron-like digests without adopting a separate SaaS scheduler
• One (or a few) transactional messages go to ops / founder / customer addresses looked up at run time

Use Rovela scheduled_at when the body is already correct and you only need “deliver later” without a running scheduler (Scheduled Email Sending). Prefer plain node-cron when you are not on Nest; prefer BullMQ when you need persistent repeatable jobs across many workers.

Prerequisites

1. A verified sending domain in Rovela. Unverified from domains are refused with 400 (Quick Start).
2. An API key shaped like re_…. Prefer sending_access for the worker (Authentication).
3. NestJS app with @nestjs/schedule and ScheduleModule.forRoot().
4. Config: ROVELA_API_KEY, MAIL_FROM, DIGEST_TO.
5. A @Injectable() provider Nest can instantiate (inject HttpService or fetch).

Register the schedule

npm install @nestjs/schedule

// app.module.ts
import { Module } from "@nestjs/common";
import { ScheduleModule } from "@nestjs/schedule";
import { DigestService } from "./digest.service";

@Module({
  imports: [ScheduleModule.forRoot()],
  providers: [DigestService],
})
export class AppModule {}

// digest.service.ts
import { Injectable, Logger } from "@nestjs/common";
import { Cron, CronExpression } from "@nestjs/schedule";

@Injectable()
export class DigestService {
  private readonly log = new Logger(DigestService.name);

  @Cron("0 9 * * *", { name: "daily-digest-email", timeZone: "UTC" })
  async handleDailyDigest() {
    await this.sendDigest();
  }

  @Cron(CronExpression.EVERY_15_MINUTES, { name: "ops-pulse-email", timeZone: "UTC" })
  async handleOpsPulse() {
    await this.sendOpsPulse();
  }

  // ... sendDigest / sendOpsPulse below
}

Job method that calls Rovela

private async sendDigest() {
  const day = new Date().toISOString().slice(0, 10);
  const idempotencyKey = `nest-cron-daily-digest:${day}`;
  const stats = await this.loadStats(day);
  const to = (process.env.DIGEST_TO || "").split(",").map((s) => s.trim()).filter(Boolean);
  const subject = `Acme daily digest — ${day}`;
  const text = `Signups: ${stats.signups}\nPaid: ${stats.paid}`;
  const html = `<h1>Daily digest — ${day}</h1><ul><li>Signups: <b>${stats.signups}</b></li><li>Paid: <b>${stats.paid}</b></li></ul>`;

  this.log.log(`nest digest day=${day} key=${idempotencyKey}`); // never log ROVELA_API_KEY

  const apiKey = process.env.ROVELA_API_KEY;
  const from = process.env.MAIL_FROM;
  if (!apiKey || !from) throw new Error("missing ROVELA_API_KEY or MAIL_FROM");

  const res = await fetch("https://api.rovela.dev/emails", {
    method: "POST",
    headers: {
      Authorization: `Bearer ${apiKey}`,
      "Content-Type": "application/json",
      "Idempotency-Key": idempotencyKey,
    },
    body: JSON.stringify({
      from, to, subject, html, text,
      tags: { kind: "nest-cron-daily-digest", day, platform: "nestjs-schedule" },
    }),
  });
  const body = await res.text();
  if (!res.ok) throw new Error(`Rovela ${res.status}: ${body}`);
  const parsed = JSON.parse(body);
  this.log.log(`rovela accepted emailId=${parsed.id}`);
}

Window-scoped Idempotency-Key

Rovela’s Idempotency-Key (up to 256 characters) makes a retry of the same send within 24 hours return the original response instead of mailing again (Send Email API). That matters because:

• Retries re-enter the job after 5xx / timeouts
• Crash after Rovela 200 but before the job is marked succeeded can redeliver
• Queue lag can shift wall-clock — key off the business schedule window, not millisecond now
• Multi-tenant jobs need tenant ids in the key

Recommended keys:

Key	Use when
nest-cron-daily-digest:{yyyy-mm-dd}	One ops digest per UTC day
nest-cron-ops-pulse:{yyyy-mm-dd}T{HH}-{MM}	/15 or hourly crontab (bucket to the window)
nest-cron-tokyo-ops:{yyyy-mm-dd}	Local-TZ morning digest
nest-cron-weekly-report:{yyyy}-W{ww}	One founder report per ISO week
nest-cron-tenant-digest:{tenantId}:{yyyy-mm-dd}	Per-tenant job args

If you mail many personalized digests in one job, prefer per-recipient keys (or POST /emails/batch for up to 100 non-template messages — batch rejects template / topic_id by design).

Attach secrets and verify the schedule

1. Set ROVELA_API_KEY (sending_access), MAIL_FROM, DIGEST_TO on the process that runs the scheduler/worker.
2. Confirm the schedule entry is registered with the expected cron and timezone.
3. Confirm workers are online.
4. Wait for the next tick, or trigger once in non-prod while keeping the window key stable.
5. Check logs for idempotencyKey + Rovela emailId. Optionally correlate later with Rovela webhooks.

Prefer sending_access on production senders — never full_access. Never log API keys. Rotate Rovela by create-then-revoke.

Failure modes and how to handle them

Symptom	Likely cause	What to do
Cron never fires	Scheduler/worker down	Keep the scheduling process always on
Double digests	Multiple schedulers / replicas	Single leader or distributed lock / persistent store
Wrong wall time	Assumed UTC / wrong timezone	Set timezone explicitly
Duplicate digests after retries	Unstable key / keyed by ms now	Use window-scoped keys; Idempotency-Key collapses duplicates within 24h
Rovela 401	Bad / missing ROVELA_API_KEY	Fix server env; confirm sending_access
Rovela 400 naming a domain	MAIL_FROM domain not verified	Verify the domain under Rovela Domains
Rovela 429	Rate limit	Raise/retry; keep the same Idempotency-Key
Secrets in Git	Key committed	Keep only in env / vault

Production checklist

• Verified Rovela sending domain; MAIL_FROM matches it
• ROVELA_API_KEY is sending_access, in worker env / vault only
• Scheduler + workers always running as documented for NestJS Schedule
• Documented cron + timezone
• Rovela send uses a window-scoped Idempotency-Key
• Logs show successful emailId without printing secrets
• Alerts on consecutive job failures / scheduler down
• Optional: Rovela webhooks for email.bounced / email.complained on digest recipients

FAQ

Can the scheduler call Rovela without a job method? No usefully — the scheduler only wakes your code. Your method must aggregate live data and keep sending_access local. Rovela does not host your scheduler or workers.

Should I use Rovela scheduled_at from the job? Usually no. If the scheduler already fires at send time with a live body, call POST /emails immediately. Use scheduled_at when you accept the message now and want Rovela to deliver later without another wake (Scheduled Email Sending).

What key scope does sending_access cover? Enough to queue mail via POST /emails / batch. Reading messages, rescheduling, and cancel need full_access or a dashboard session (Send Email API, Authentication). Keep the worker on sending_access.

Does a 200 mean the founder already got the digest? No — it means Rovela accepted and queued the message. Use GET /emails/:id, events, or webhooks for delivery state (Quick Start).

Why both scheduler retries and Rovela Idempotency-Key? The scheduler retries the job when your code fails. Rovela’s key collapses duplicate sends if the HTTP call succeeds twice in the same window (or times out after Rovela accepted). You want both layers.

Ship the recurring job; let Rovela deliver

Register the NestJS Schedule schedule with an explicit cron and timezone, put live aggregation and POST /emails in the job method, scope Idempotency-Key to the schedule window (and tenant), and keep sending_access server-only. NestJS Schedule wakes on time; Rovela delivers the transactional mail.