> For the complete documentation index, see [llms.txt](https://0xgosu.gitbook.io/monolith-framework/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://0xgosu.gitbook.io/monolith-framework/examples/cron-scheduler.md).

# Cron Scheduler

Demonstrates **cron-scheduled services** — the framework's 5th service type — backed by the embedded NATS JetStream message scheduler (nats-server v2.14+, ADR-51).

A module registers a single cron service with `RegisterCronService`. The schedule is registered **server-side**, so in a multi-node deployment exactly one message fires per occurrence (no client-side ticker, no leader election). Each occurrence is delivered to the handler through a durable pull consumer with explicit acknowledgement.

```go
func (m *HeartbeatModule) RegisterServices(c mono.ServiceContainer) error {
    return c.RegisterCronService("emit", mono.CronServiceConfig{
        Schedule: "@every 2s",
        Payload:  []byte(`{"event":"heartbeat"}`),
    }, func(ctx context.Context, msg *mono.Msg) error {
        // ... do periodic work ...
        return nil // framework Acks on nil, Naks on error
    })
}
```

## Key points

* **Requires JetStream**: enable it via `WithJetStreamStorageDir(...)`. Registering a cron service without JetStream fails fast at startup.
* **Schedule formats**: a cron expression in the standard five-field format (`"0 0 * * *"` = daily at midnight) or the six-field seconds-first format (`"0 0 0 * * *"`; five-field expressions are normalized by prepending a `"0"` seconds field), a named alias (`"@daily"`, `"@hourly"`, …), or an interval (`"@every 5m"`, minimum 1s). A `TimeZone` may only be used with cron expressions (not `@every`).
* **Acknowledgement is framework-owned**: return `nil` to Ack the occurrence, or a non-nil error to Nak it (redelivered up to the consumer's `MaxDeliver`). Do **not** call `msg.Ack()` yourself.
* **Idempotent**: the schedule is (re)published on every startup, so changing the `Schedule`, `Payload`, `TimeZone`, or `TTL` and redeploying overwrites the live schedule in place.
* **Retiring a cron service**: set `Deprecated: true` and deploy to purge the schedule (keeping the code), then remove the `RegisterCronService` call in a later release.

## Run

```bash
make run-example-4
# or
cd examples/cron-scheduler && go run .
```

You should see a heartbeat printed every 2 seconds, then a clean shutdown on Ctrl+C.


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