render-oss/render-cron-jobs

Render Cron Jobs

This skill covers Cron Job services on Render: how schedules run, what the platform guarantees, and how they differ from workers and workflows. Pair it with Blueprint and deploy skills when authoring render.yaml or Dashboard settings.

When to Use

• Scheduled work that starts on a cron, runs a command, and exits when finished
• Choosing between cron, background worker, or workflow for periodic or long-running jobs
• Blueprint fields for type: cron, schedule, and commands
• Constraints: no disk, single concurrent run, 12-hour max duration, private-network outbound only
• UTC scheduling pitfalls (expressions are not local time)

Expression cheat sheets, framework startCommand examples, and Heroku Scheduler migration mapping live under references/.

Configuration

• Schedule: a cron expression evaluated in UTC, not the team’s local timezone. All times in the Dashboard and Blueprints are UTC.
• Command: any valid Linux shell command or bash script path. The process must exit when work is done—billing is based on run duration (prorated by the second).
• Source:
  • Git repository — Render builds on push (same deploy model as other repo-backed services); the built artifact runs on each scheduled invocation.
  • Prebuilt Docker image — the image is pulled before each run and is not retained between runs (no warm cache of the image layer set across invocations in the same way as a long-lived service).

Constraints

• No persistent disk — cron job services cannot provision or attach Render persistent disks; plan for object storage or databases instead.
• Single-run guarantee — at most one active run per cron service at a time. A new scheduled tick does not start a second overlapping instance.
• Maximum run length: 12 hours per invocation.
• Pricing: $1/month minimum per cron job service; usage is prorated by the second beyond plan/minimum rules that apply to your account.
• Private network: cron jobs can send traffic to other services on the private network; they cannot receive inbound private-network connections (no internal hostname for accepting traffic from other services).

Execution Behavior

• Manual “Trigger Run” while a run is active: Render cancels the active run, then starts a new one.
• New Git build / deploy does not affect a run already in progress—the in-flight process keeps using the revision it started with until it exits.
• Docker-based crons: the image is pulled fresh for each run; do not assume layer or image reuse across invocations like a continuously running container.
• UTC everywhere: cron expressions and “midnight” in docs mean UTC. A common mistake is copying a local-time schedule into the expression without converting to UTC.

Cron vs Worker vs Workflow

| Need | Use | Why | |------|-----|-----| | Periodic task under 12h | Cron Job | Scheduled, simple, exits when done | | Continuous job processing | Background Worker | Always running, polls a queue | | Periodic but over 12h | Background Worker | No 12h cron run ceiling | | Scheduled parallel compute | Cron Job + Workflow | Cron triggers workflow runs on a schedule; workflows fan out or orchestrate parallel steps |

Blueprint Configuration

Cron services use type: cron with a schedule and the usual build/start and env wiring:

services:
  - type: cron
    name: nightly-cleanup
    schedule: "0 * * * *" # hourly at minute 0 — must be quoted in YAML
    buildCommand: pip install -r requirements.txt
    startCommand: python cleanup.py
    envVars:
      - key: DATABASE_URL
        fromDatabase:
          name: my-db
          property: connectionString

• schedule: standard five-field cron (minute hour day-of-month month day-of-week), UTC.
• buildCommand / startCommand: same roles as other non-Docker services; Docker images use image + start command as configured for image-backed crons.
• envVars: same patterns as web services and workers (secrets, linked databases, etc.).

YAML note: the schedule value must be quoted so characters like * are not parsed as YAML aliases or flow syntax.

Common Patterns

• Database cleanup — archive or delete stale rows on a schedule
• Report generation — build CSV/PDF and upload to object storage or email
• External API sync — pull or push batches on an interval
• Cache warming — hit endpoints or rebuild caches before peak traffic
• Scheduled emails — digest or reminder sends driven by cron + mail/API

References

| Topic | File | |--------|------| | Expression examples, framework commands, errors, env vars | references/cron-patterns.md | | Heroku Scheduler → Render mapping, blueprint example | references/migration-from-scheduler.md |

Related Skills

• render-deploy — First-time deploy, service creation, Dashboard flow
• render-blueprints — Full render.yaml schema, previews, common mistakes
• render-background-workers — Long-lived processes, queues, no 12h cap
• render-workflows — Orchestrated and parallel jobs, often triggered on a schedule from cron