Cron

Cron — Scheduled Tasks & Recurring Job Orchestration

Activate When

• User invokes /godmode:cron
• User says "schedule a job", "cron job", "recurring task", "run every hour"
• User says "background scheduler", "periodic task", "timed execution"
• User says "node-cron", "BullMQ repeat", "Celery beat", "Sidekiq-cron", "APScheduler", "Hangfire", "Quartz"
• User needs a task to fire on a schedule (daily reports, cleanup, syncs, heartbeats)
• User asks about cron syntax, timezone handling, or overlap protection
• User needs distributed scheduling across multiple instances
• User needs rate-limited or priority-aware recurring execution
• Godmode orchestrator detects hardcoded setInterval or setTimeout loops that need conversion to proper scheduled jobs

Workflow

Step 1: Scheduling Requirements Assessment

Evaluate what needs to run, how often, and under what constraints:

SCHEDULING REQUIREMENTS ASSESSMENT:
|  Dimension            | Value                             |
|  Task type            | <cleanup | report | sync | alert |
|                       |  billing | digest | health check> |
|  Frequency            | <seconds | minutes | hourly |     |

Cron Expression Quick Reference

CRON EXPRESSION SYNTAX:
|  Field        | Values          | Special chars           |
|  Minute       | 0-59            | * , - /                 |
|  Hour         | 0-23            | * , - /                 |
|  Day of month | 1-31            | * , - / ? L W           |

Common patterns:
    and 7 = Sunday) — check your library

Scheduler Technology Selection Matrix

SCHEDULER TECHNOLOGY SELECTION:
| Technology       | Language    | Backend  | Distributed| Persistence| Ops cost |
|--|--|--|--|--|--|
| node-cron        | Node.js     | In-proc  | No         | None      | Minimal  |
|                  | Simple cron | (memory) | (single)   | (restart  |          |
|                  | schedules   |          |            | loses)    |          |
|                  |             |          |            |           |          |
| BullMQ           | Node.js     | Redis    | Yes        | Redis     | Low      |
  ...

Step 2: Schedule Architecture Design

Design the scheduling topology, job registry, and execution flow:

SCHEDULE ARCHITECTURE:
|  Scheduler              Job Registry               Executors           |
|  ---------              ------------               ---------           |
|                     +-- daily-digest -------------- Worker Pool A (2)  |
|  Cron Engine ------+-- cleanup-expired ----------- Worker Pool B (3)  |

BullMQ Repeatable Jobs (Node.js — Production)

import { Queue, Worker, QueueEvents } from 'bullmq';
import Redis from 'ioredis';

const connection = new Redis({
  host: process.env.REDIS_HOST || 'localhost',
  port: parseInt(process.env.REDIS_PORT || '6379'),

Celery Beat (Python — Production)

from celery import Celery
from celery.schedules import crontab, solar
from datetime import timedelta

app = Celery('scheduler', broker='redis://localhost:6379/0', backend='redis://localhost:6379/1')

APScheduler (Python — Standalone)

from apscheduler.schedulers.background import BackgroundScheduler
from apscheduler.jobstores.redis import RedisJobStore
from apscheduler.executors.pool import ThreadPoolExecutor, ProcessPoolExecutor
from apscheduler.triggers.cron import CronTrigger
import pytz

Sidekiq-Cron (Ruby): Sidekiq::Cron::Job.load_from_hash with cron expression, class, and queue. Hangfire (.NET): RecurringJob.AddOrUpdate with cron expression, SQL Server or Redis storage. Quartz (Java): @Configuration with JobDetail bean and CronTrigger schedule.

Step 3: Idempotency for Retries

Every scheduled job MUST safely run more than once for the same logical execution window:

IDEMPOTENCY STRATEGIES FOR SCHEDULED JOBS:
|  Strategy             | Use case                          |
|  Date-based key       | "digest:2025-03-15" — one per day |
|  Window-based key     | "sync:2025-03-15T10:00Z" — one   |
|                       | per 15-min window                 |

Principle: derive the idempotency key from the SCHEDULE, not the job ID.
  - The system re-creates the job on restart with a new ID
  ...

Idempotent Scheduled Job Implementation

// Idempotent wrapper for scheduled jobs
class ScheduledJobRunner {
  constructor(private redis: Redis) {}

  async runOnce(
    jobName: string,

Step 4: Distributed Job Locking

Prevent multiple scheduler instances from firing the same job simultaneously:

DISTRIBUTED LOCKING STRATEGIES:
| Method            | Backend | Pros                | Cons             |
| Redis SETNX + EX  | Redis   | Fast, atomic acquire | Not CP-safe      |
| Redlock           | Redis   | Multi-node consensus | Controversial    |
| DB advisory lock  | Postgres| No extra infra       | DB-coupled       |
| ZooKeeper/etcd    | ZK/etcd | CP-safe, reliable    | Ops overhead     |

Redis Distributed Lock

class DistributedSchedulerLock {
  constructor(private redis: Redis) {}

  async acquireLeader(schedulerId: string, ttl: number = 30): Promise<boolean> {
    // Only one scheduler instance becomes leader
    const acquired = await this.redis.set(

PostgreSQL Advisory Lock

import hashlib

def pg_advisory_lock_for_job(conn, job_name: str) -> bool:
    """Acquire a PostgreSQL advisory lock for a scheduled job.
    Returns True if lock acquired, False if another process holds it."""
    # Convert job name to a stable int64 for PG advisory lock

Step 5: Job Monitoring & Alerting

Track schedule health and detect missed or failing runs:

CRON JOB MONITORING:
|  Metric                    | Current | Alert    | Status |
|  Jobs scheduled            | 8       | —        | OK     |
|  Last run: daily-digest    | 09:00   | miss > 1h| OK     |
|  Last run: cleanup         | 06:00   | miss > 7h| OK     |

Monitoring Implementation

// Scheduled job health monitor
class CronJobMonitor {
  constructor(private redis: Redis, private alerter: Alerter) {}

  async recordRun(jobName: string, result: {
    status: 'success' | 'failure';

Step 6: Dead Letter Handling for Scheduled Jobs

Handle scheduled jobs that exhaust retries:

SCHEDULED JOB DLQ DESIGN:
|  DLQ for Scheduled Jobs                                   |
|  Queue:       scheduled-jobs-dlq                          |
|  Retention:   30 days                                     |
|  Alert:       Any entry (scheduled jobs should not fail)  |

Step 7: Priority Queues & Job Chaining

Design priority-aware scheduling and dependent job pipelines:

SCHEDULED JOB PRIORITIES:
|  Priority | Schedule         | Job               | SLA    |
|  P0       | */5 * * * *      | Health check       | < 30s  |
|  P1       | */15 * * * *     | Inventory sync     | < 2m   |
|  P1       | 0 9 * * *        | Daily digest       | < 5m   |

JOB CHAINING (dependent execution):
|  Pipeline: "end-of-day"                                   |

Job Chaining Implementation

// BullMQ Flow (parent-child dependencies)
import { FlowProducer } from 'bullmq';

const flowProducer = new FlowProducer({ connection });

// End-of-day pipeline — child jobs run first, parent last

Step 8: Rate-Limited Execution

Prevent scheduled jobs from overwhelming downstream services:

RATE-LIMITED SCHEDULING:
|  Scenario                | Strategy                       |
|  API with 100 req/min    | Token bucket limiter on worker |
|  Email provider 500/hr   | BullMQ limiter: 500 per 3600s |
|  DB batch writes         | Chunk + delay between batches  |

Rate-Limited Scheduled Job

// Scheduled job that processes in rate-limited batches
async function runDailyDigest() {
  const users = await db.users.findMany({
    where: { digestEnabled: true, lastDigestBefore: today() },
  });

Step 9: Timezone Handling

Handle timezone-aware scheduling correctly:

TIMEZONE HANDLING:
|  Rule                                                     |
|  1. Store all schedules in UTC internally                 |
|  2. Convert to user timezone for display only             |
|  3. Use IANA timezone names (America/New_York), never     |

Step 10: Database-Backed vs Redis-Backed Schedulers

Choose the right persistence layer:

SCHEDULER PERSISTENCE COMPARISON:
|  Aspect              | Redis-backed      | DB-backed      |
|  Speed               | Sub-ms operations | 1-10ms queries |
|  Durability          | RDB/AOF (config)  | Full ACID      |
|  Crash recovery      | May lose last      | No data loss   |

Use Redis-backed (BullMQ, Sidekiq-Cron) when:
  - Need complex queries on job history

Step 11: Commit and Transition

1. Save schedule configuration as `config/schedules/registry.ts`
2. Save scheduler setup as `config/schedules/scheduler.ts`
3. Save idempotency helpers as `lib/scheduled-job-runner.ts`
4. Save monitoring as `lib/cron-monitor.ts`
5. Commit: "cron: <scheduler> — <N> jobs, <frequency range>, <lock strategy>"
6. If new scheduler: "Scheduled jobs configured. <N> jobs registered. Deploy and monitor."
7. If fixing failures: "Idempotency and locking added. Missed-schedule alerting enabled."
8. If migrating: "Migrated from <old> to <new>. All schedules verified."

Key Behaviors

Never ask to continue. Loop autonomously until done.

# Validate cron expressions and test jobs
npx cron-validate "0 9 * * 1-5"
npm run test:cron -- --timeout 30000
redis-cli KEYS "bull:*:repeat:*" | head -10

IF job duration > 80% of interval: increase interval or optimize job. WHEN missed runs > 0 in 24h: alert and investigate. IF retry count > 3: move to dead letter queue.

1. Monitor every scheduled job. Missed runs, failures, duration.
2. Idempotency mandatory. Same result if called twice.
3. Distributed locking in production. No duplicate fires.
4. Overlap protection. Use max_instances: 1 or coalesce. On failure: revert with git reset --hard HEAD~1.

Flags & Options

Quality Targets

• Job success rate: >99% over 30 days
• Runtime per job: <80% of schedule interval
• Alert after: >3 consecutive failures

Keep/Discard

KEEP if: improvement verified. DISCARD if: regression or no change. Revert discards immediately.

Stop Conditions

Stop when: target reached, budget exhausted, or >5 consecutive discards.