Cloudflare Cron Triggers

# Cloudflare Cron Triggers

**Status**: Production Ready ✅
**Last Updated**: 2025-11-25
**Dependencies**: cloudflare-worker-base (for Worker setup)
**Latest Versions**: wrangler@4.50.0, @cloudflare/workers-types@4.20251125.0

---

## Quick Start (5 Minutes)

### 1. Add Scheduled Handler to Your Worker

**src/index.ts:**

```typescript
export default {
  async scheduled(
    controller: ScheduledController,
    env: Env,
    ctx: ExecutionContext
  ): Promise<void> {
    console.log('Cron job executed at:', new Date(controller.scheduledTime));
    console.log('Triggered by cron:', controller.cron);

    // Your scheduled task logic here
    await doPeriodicTask(env);
  },
};
```

**Why this matters:**
- Handler must be named exactly `scheduled` (not `scheduledHandler` or `onScheduled`)
- Must be exported in default export object
- Must use ES modules format (not Service Worker format)

### 2. Configure Cron Trigger in Wrangler

**wrangler.jsonc:**

```jsonc
{
  "name": "my-scheduled-worker",
  "main": "src/index.ts",
  "compatibility_date": "2025-10-23",
  "triggers": {
    "crons": [
      "0 * * * *"  // Every hour at minute 0
    ]
  }
}
```

**CRITICAL:**
- Cron expressions use 5 fields: `minute hour day-of-month month day-of-week`
- All times are **UTC only** (no timezone conversion)
- Changes take **up to 15 minutes** to propagate globally

### 3. Test Locally

```bash
# Enable scheduled testing
bunx wrangler dev --test-scheduled

# In another terminal, trigger the scheduled handler
curl "http://localhost:8787/__scheduled?cron=0+*+*+*+*"

# View output in wrangler dev terminal
```

**Testing tips:**
- `/__scheduled` endpoint is only available with `--test-scheduled` flag
- Can pass any cron expression in query parameter
- Python Workers use `/cdn-cgi/handler/scheduled` instead

### 4. Deploy

```bash
npm run deploy
# or
bunx wrangler deploy
```

**After deployment:**
- Changes may take up to 15 minutes to propagate
- Check dashboard: Workers & Pages > [Your Worker] > **Cron Triggers**
- View past executions in **Logs** tab

---

## When to Load References

**Load immediately when user mentions**:

- `cron-expressions-reference.md` → "cron syntax", "schedule format", "expression", "minute hour day", "every X minutes"
- `common-patterns.md` → "examples", "use cases", "patterns", "real-world", "database cleanup", "report generation", "how to"
- `integration-patterns.md` → "implement", "Hono", "multiple triggers", "bindings", "workflows", "error handling"
- `wrangler-config.md` → "configuration", "wrangler.jsonc", "multiple crons", "environment-specific", "dev staging production"
- `testing-guide.md` → "test", "local development", "__scheduled", "unit test", "curl", "debugging"

**Load proactively when**:
- Building new scheduled task → Load `integration-patterns.md`
- Configuring wrangler.jsonc → Load `wrangler-config.md`
- Debugging cron expression → Load `cron-expressions-reference.md`
- Testing locally → Load `testing-guide.md`
- Looking for examples → Load `common-patterns.md`

---

## Cron Expression Syntax

### Five-Field Format

```
* * * * *
│ │ │ │ │
│ │ │ │ └─── Day of Week (0-6, Sunday=0)
│ │ │ └───── Month (1-12)
│ │ └─────── Day of Month (1-31)
│ └───────── Hour (0-23)
└─────────── Minute (0-59)
```

### Special Characters

| Character | Meaning | Example |
|-----------|---------|---------|
| `*` | Every | `* * * * *` = every minute |
| `,` | List | `0,30 * * * *` = every hour at :00 and :30 |
| `-` | Range | `0 9-17 * * *` = every hour from 9am-5pm |
| `/` | Step | `*/15 * * * *` = every 15 minutes |

### Common Patterns

```bash
# Every minute
* * * * *

# Every 5 minutes
*/5 * * * *

# Every 15 minutes
*/15 * * * *

# Every hour at minute 0
0 * * * *

# Every hour at minute 30
30 * * * *

# Every 6 hours
0 */6 * * *

# Every day at midnight (00:00 UTC)
0 0 * * *

# Every day at noon (12:00 UTC)
0 12 * * *

# Every day at 3:30am UTC
30 3 * * *

# Every Monday at 9am UTC
0 9 * * 1

# Every weekday at 9am UTC
0 9 * * 1-5

# Every Sunday at midnight UTC
0 0 * * 0

# First day of every month at midnight UTC
0 0 1 * *

# Twice a day (6am and 6pm UTC)
0 6,18 * * *

# Every 30 minutes during business hours (9am-5pm UTC, weekdays)
*/30 9-17 * * 1-5
```

**CRITICAL: UTC Timezone Only**
- All cron triggers execute on **UTC time**
- No timezone conversion available
- Convert your local time to UTC manually
- Example: 9am PST = 5pm UTC (next day during DST)

---

## ScheduledController Interface

```typescript
interface ScheduledController {
  readonly cron: string;           // The cron expression that triggered this execution
  readonly type: string;           // Always "scheduled"
  readonly scheduledTime: number;  // Unix timestamp (ms) when scheduled
}
```

### Properties

#### `controller.cron` (string)

The cron expression that triggered this execution.

```typescript
export default {
  async scheduled(controller: ScheduledController, env: Env): Promise<void> {
    console.log(`Triggered by: ${controller.cron}`);
    // Output: "Triggered by: 0 * * * *"
  },
};
```

**Use case:** Differentiate between multiple cron schedules (see Multiple Cron Triggers pattern).

#### `controller.type` (string)

Always returns `"scheduled"` for cron-triggered executions.

```typescript
if (controller.type === 'scheduled') {
  // This is a cron-triggered execution
}
```

#### `controller.scheduledTime` (number)

Unix timestamp (milliseconds since epoch) when this execution was scheduled to run.

```typescript
export default {
  async scheduled(controller: ScheduledController): Promise<void> {
    const scheduledDate = new Date(controller.scheduledTime);
    console.log(`Scheduled for: ${scheduledDate.toISOString()}`);
    // Output: "Scheduled for: 2025-10-23T15:00:00.000Z"
  },
};
```

**Note:** This is the **scheduled** time, not the actual execution time. Due to system load, actual execution may be slightly delayed (usually <1 second).

---

## Execution Context

```typescript
export default {
  async scheduled(
    controller: ScheduledController,
    env: Env,
    ctx: ExecutionContext  // ← Execution context
  ): Promise<void> {
    // Use ctx.waitUntil() for async operations that should complete
    ctx.waitUntil(logToAnalytics(env));
  },
};
```

### `ctx.waitUntil(promise: Promise<any>)`

Extends the execution context to wait for async operations to complete after the handler returns.

**Use cases:**
- Logging to external services
- Analytics tracking
- Cleanup operations
- Non-critical background tasks

```typescript
export default {
  async scheduled(controller: ScheduledController, env: Env, ctx: ExecutionContext): Promise<void> {
    // Critical task - must complete before handler exits
    await processData(env);

    // Non-critical tasks - can complete in background
    ctx.waitUntil(sendMetrics(env));
    ctx.waitUntil(cleanupOldData(env));
    ctx.waitUntil(notifySlack({ message: 'Cron completed' }));
  },
};
```

**Important:** First `waitUntil()` that fails will be reported as the status in dashboard logs.

---

## Integration Patterns

**6 production-ready cron patterns**:

1. **Standalone Worker with Cron** - Single scheduled function for background tasks (database cleanup, report generation)
2. **Hono + Cron Combination** - HTTP endpoints + scheduled tasks in one Worker, sharing bindings and reducing costs
3. **Multiple Cron Triggers** - Different schedules for different tasks using `controller.cron` to route execution
4. **Accessing Bindings** - Use D1, KV, R2, AI, Vectorize, Queues, Workflows, Durable Objects in scheduled functions
5. **Integrating with Workflows** - Trigger complex, long-running multi-step workflows on schedule
6. **Error Handling Best Practices** - Comprehensive error handling with retry logic, alerting (Slack/email), failure logging, and monitoring

**Load `references/integration-patterns.md` for complete implementations with code examples, configuration details, and best practices.**

---

## Wrangler Configuration

Add cron triggers to `wrangler.jsonc` in the `triggers.crons` array. Each trigger requires a `cron` expression. Supports multiple crons (Free: 3 max, Paid: higher limits) and environment-specific configurations for dev/staging/production deployments.

**Load `references/wrangler-config.md` for complete configuration examples including multiple triggers, environment-specific schedules, timezone handling, and removal procedures.**

---

## Testing & Development

Test scheduled functions locally using the `/__scheduled` endpoint by running `bunx wrangler dev --test-scheduled`, then triggering handlers with `curl "http://localhost:8787/__scheduled?cron=0+*+*+*+*"` (use `+` instead of spaces in cron expressions).

**Load `references/testing-guide.md` for complete testing strategies, local development setup, unit testing examples, integration testing patterns, and production monitoring techniques.**

---

## Green Compute

Run cron triggers only in data centers powered by renewable energy.

### Enable Green Compute

**Via Dashboard:**

1. Go to [Workers & Pages](https://dash.cloudflare.com/?to=/:account/workers-and-pages)
2. In **Account details** section, find **Compute Setting**
3. Click **Change**
4. Select **Green Compute**
5. Click **Confirm**

**Applies to:**
- All cron triggers in your account
- Reduces carbon footprint
- No additional cost
- May introduce slight delays in some regions

**How it works:**
- Cloudflare routes cron executions to green-powered data centers
- Uses renewable energy: wind, solar, hydroelectric
- Verified through Power Purchase Agreements (PPAs) and Renewable Energy Credits (RECs)

---

## Known Issues Prevention

This skill prevents **6** documented issues:

### Issue #1: Cron Changes Not Propagating

**Error:** Cron triggers updated in wrangler.jsonc but not executing

**Source:** [Cloudflare Docs - Cron Triggers](https://developers.cloudflare.com/workers/configuration/cron-triggers/)

**Why It Happens:**
- Changes to cron triggers take up to **15 minutes** to propagate globally
- Cloudflare network needs time to update edge nodes
- No instant propagation like regular deploys

**Prevention:**
- Wait 15 minutes after deploy before expecting execution
- Check dashboard: Workers & Pages > [Worker] > Cron Triggers
- Use `wrangler triggers deploy` for trigger-only changes

```bash
# If you only changed triggers (not code), use:
bunx wrangler triggers deploy

# Wait 15 minutes, then verify in dashboard
```

---

### Issue #2: Handler Does Not Export

**Error:** `Handler does not export a 'scheduled' method`

**Source:** Common deployment error

**Why It Happens:**
- Handler not named exactly `scheduled`
- Handler not exported in default export object
- Using Service Worker format instead of ES modules

**Prevention:**

```typescript
// ❌ Wrong: Incorrect handler name
export default {
  async scheduledHandler(controller, env, ctx) { }
};

// ❌ Wrong: Not in default export
export async function scheduled(controller, env, ctx) { }

// ✅ Correct: Named 'scheduled' in default export
export default {
  async scheduled(controller, env, ctx) { }
};
```

---

### Issue #3: UTC Timezone Confusion

**Error:** Cron runs at wrong time

**Source:** User expectation vs. reality

**Why It Happens:**
- All cron triggers run on **UTC time only**
- No timezone conversion available
- Users expect local timezone

**Prevention:**

Convert your local time to UTC manually:

```typescript
// Want to run at 9am PST (UTC-8)?
// 9am PST = 5pm UTC (17:00)
{
  "triggers": {
    "crons": ["0 17 * * *"]  // 9am PST = 5pm UTC
  }
}

// Want to run at 6pm EST (UTC-5)?
// 6pm EST = 11pm UTC (23:00)
{
  "triggers": {
    "crons": ["0 23 * * *"]  // 6pm EST = 11pm UTC
  }
}

// Remember: DST changes affect conversion!
// PST is UTC-8, PDT is UTC-7
```

**Tools:**
- [Time Zone Converter](https://www.timeanddate.com/worldclock/converter.html)
- [Cron Expression Generator](https://crontab.guru/)

---

### Issue #4: Invalid Cron Expression

**Error:** Cron doesn't execute, no error shown

**Source:** Silent validation failure

**Why It Happens:**
- Invalid cron syntax silently fails
- Validation happens at deploy, but may not be obvious
- Common mistakes: wrong field order, invalid ranges

**Prevention:**

```bash
# ❌ Wrong: Too many fields (6 fields instead of 5)
"crons": ["0 0 * * * *"]  # Has seconds field - not supported

# ❌ Wrong: Invalid minute range
"crons": ["65 * * * *"]  # Minute must be 0-59

# ❌ Wrong: Invalid day of week
"crons": ["0 0 * * 7"]  # Day of week is 0-6 (use 0 for Sunday)

# ✅ Correct: 5 fields, valid ranges
"crons": ["0 0 * * 0"]  # Sunday at midnight UTC
```

**Validation:**
- Use [Crontab Guru](https://crontab.guru/) to validate expressions
- Check wrangler deploy output for errors
- Test locally with `--test-scheduled`

---

### Issue #5: Missing ES Modules Format

**Error:** `Worker must use ES modules format`

**Source:** Legacy Service Worker format

**Why It Happens:**
- Scheduled handler requires ES modules format
- Old Service Worker format not supported
- Mixed format in codebase

**Prevention:**

```typescript
// ❌ Wrong: Service Worker format
addEventListener('scheduled', (event) => {
  event.waitUntil(handleScheduled(event));
});

// ✅ Correct: ES modules format
export default {
  async scheduled(controller, env, ctx) {
    await handleScheduled(controller, env, ctx);
  },
};
```

---

### Issue #6: CPU Time Limits Exceeded

**Error:** `CPU time limit exceeded`

**Source:** Long-running scheduled tasks

**Why It Happens:**
- Default CPU limit: 30 seconds
- Long-running tasks exceed limit
- No automatic timeout extension

**Prevention:**

**Option 1: Increase CPU limit in wrangler.jsonc**

```jsonc
{
  "limits": {
    "cpu_ms": 300000  // 5 minutes (max for Standard plan)
  }
}
```

**Option 2: Use Workflows for long-running tasks**

```typescript
// Instead of long task in cron:
export default {
  async scheduled(controller, env, ctx) {
    // Trigger Workflow that can run for hours
    await env.MY_WORKFLOW.create({
      params: { task: 'long-running-job' },
    });
  },
};
```

**Option 3: Break into smaller chunks**

```typescript
export default {
  async scheduled(controller, env, ctx) {
    // Process in batches
    const batch = await getNextBatch(env.DB);

    for (const item of batch) {
      await processItem(item);
    }

    // If more work, send to Queue for next batch
    const hasMore = await hasMoreWork(env.DB);
    if (hasMore) {
      await env.MY_QUEUE.send({ type: 'continue-processing' });
    }
  },
};
```

---

## Always Do ✅

1. **Use exact handler name** - Must be `scheduled`, not `scheduledHandler` or variants
2. **Use ES modules format** - Export in default object, not addEventListener
3. **Convert to UTC** - All cron times are UTC, convert from local timezone
4. **Wait 15 minutes** - Cron changes take up to 15 min to propagate
5. **Test locally first** - Use `wrangler dev --test-scheduled`
6. **Validate cron syntax** - Use [Crontab Guru](https://crontab.guru/)
7. **Handle errors gracefully** - Log, alert, and optionally re-throw
8. **Use ctx.waitUntil()** - For non-critical async operations
9. **Consider Workflows** - For tasks that need >30 seconds CPU time
10. **Monitor executions** - Check dashboard logs regularly

---

## Never Do ❌

1. **Never assume local timezone** - All crons run on UTC
2. **Never use 6-field cron expressions** - Cloudflare uses 5-field format (no seconds)
3. **Never rely on instant propagation** - Changes take up to 15 minutes
4. **Never use Service Worker format** - Must use ES modules format
5. **Never forget error handling** - Uncaught errors fail silently
6. **Never run CPU-intensive tasks without limit increase** - Default 30s limit
7. **Never use day-of-week 7** - Use 0 for Sunday (0-6 range only)
8. **Never deploy without testing** - Always test with `--test-scheduled` first
9. **Never ignore execution logs** - Dashboard shows past failures
10. **Never hardcode schedules for testing** - Use environment-specific configs

---

## Common Use Cases

**Load `references/common-patterns.md` for 10 real-world cron patterns including database cleanup, API data collection, daily reports generation, cache warming, monitoring & health checks, data synchronization, backup automation, sitemap generation, webhook processing, and scheduled notifications.**

---

## TypeScript Types

```typescript
// Scheduled event controller
interface ScheduledController {
  readonly cron: string;
  readonly type: string;
  readonly scheduledTime: number;
}

// Execution context
interface ExecutionContext {
  waitUntil(promise: Promise<any>): void;
  passThroughOnException(): void;
}

// Scheduled handler
export default {
  async scheduled(
    controller: ScheduledController,
    env: Env,
    ctx: ExecutionContext
  ): Promise<void>;
}
```

---

## Limits & Pricing

### Limits

| Feature | Free Plan | Paid Plan |
|---------|-----------|-----------|
| **Cron triggers per Worker** | 3 | Higher (check docs) |
| **CPU time per execution** | 10 ms (avg) | 30 seconds (default), 5 min (max) |
| **Wall clock time** | 30 seconds | 15 minutes |
| **Memory** | 128 MB | 128 MB |

### Pricing

Cron triggers use **Standard Workers pricing**:

- **Workers Paid Plan**: $5/month required
- **Requests**: $0.30 per million requests (after 10M free)
- **CPU Time**: $0.02 per million CPU-ms (after 30M free)

**Cron execution = 1 request**

**Example:**
- Cron runs every hour (24 times/day)
- 30 days × 24 executions = 720 executions/month
- Average 50ms CPU time per execution

**Cost:**
- Requests: 720 (well under 10M free)
- CPU time: 720 × 50ms = 36,000ms (under 30M free)
- **Total: $5/month (just subscription)**

**High frequency example:**
- Cron runs every minute (1440 times/day)
- 30 days × 1440 = 43,200 executions/month
- Still under free tier limits
- **Total: $5/month**

---

## Troubleshooting

### Issue: Cron not executing

**Possible causes:**
1. Changes not propagated yet (wait 15 minutes)
2. Invalid cron expression
3. Handler not exported correctly
4. Worker not deployed

**Solution:**

```bash
# Re-deploy
bunx wrangler deploy

# Wait 15 minutes

# Check dashboard
# Workers & Pages > [Worker] > Cron Triggers

# Check logs
# Workers & Pages > [Worker] > Logs > Real-time Logs
```

---

### Issue: Handler executes but fails

**Possible causes:**
1. Uncaught error in handler
2. CPU time limit exceeded
3. Missing environment bindings
4. Network timeout

**Solution:**

```typescript
export default {
  async scheduled(controller, env, ctx) {
    try {
      await yourTask(env);
    } catch (error) {
      // Log detailed error
      console.error('Handler failed:', {
        error: error.message,
        stack: error.stack,
        cron: controller.cron,
        time: new Date(controller.scheduledTime),
      });

      // Send alert
      ctx.waitUntil(sendAlert(error));

      // Re-throw to mark as failed
      throw error;
    }
  },
};
```

Check logs in dashboard for error details.

---

### Issue: Wrong execution time

**Cause:** UTC vs. local timezone confusion

**Solution:**

Convert your desired local time to UTC:

```typescript
// Want 9am PST (UTC-8)?
// 9am PST = 5pm UTC (17:00)

{
  "triggers": {
    "crons": ["0 17 * * *"]
  }
}
```

**Tools:**
- [World Clock Converter](https://www.timeanddate.com/worldclock/converter.html)
- Remember DST changes (PST vs PDT)

---

### Issue: Local testing not working

**Possible causes:**
1. Missing `--test-scheduled` flag
2. Wrong endpoint (should be `/__scheduled`)
3. Python Worker (use `/cdn-cgi/handler/scheduled`)

**Solution:**

```bash
# Correct: Start with flag
bunx wrangler dev --test-scheduled

# In another terminal
curl "http://localhost:8787/__scheduled?cron=0+*+*+*+*"
```

---

## Production Checklist

Before deploying cron triggers to production:

- [ ] Cron expression validated on [Crontab Guru](https://crontab.guru/)
- [ ] Handler named exactly `scheduled` in default export
- [ ] ES modules format used (not Service Worker)
- [ ] Local timezone converted to UTC
- [ ] Error handling implemented with logging
- [ ] Alerts configured for failures
- [ ] CPU limits increased if needed (`limits.cpu_ms`)
- [ ] Environment bindings tested
- [ ] Tested locally with `--test-scheduled`
- [ ] Deployment tested in staging environment
- [ ] Waited 15 minutes after deploy for propagation
- [ ] Verified execution in dashboard logs
- [ ] Monitoring and alerting configured
- [ ] Documentation updated with schedule details

---

## Related Documentation

- **Cloudflare Cron Triggers**: https://developers.cloudflare.com/workers/configuration/cron-triggers/
- **Scheduled Handler API**: https://developers.cloudflare.com/workers/runtime-apis/handlers/scheduled/
- **Cron Trigger Examples**: https://developers.cloudflare.com/workers/examples/cron-trigger/
- **Multiple Cron Triggers**: https://developers.cloudflare.com/workers/examples/multiple-cron-triggers/
- **Wrangler Triggers Command**: https://developers.cloudflare.com/workers/wrangler/commands/#triggers
- **Workers Pricing**: https://developers.cloudflare.com/workers/platform/pricing/
- **Workflows Integration**: https://developers.cloudflare.com/workflows/
- **Crontab Guru** (validator): https://crontab.guru/
- **Time Zone Converter**: https://www.timeanddate.com/worldclock/converter.html

---

**Last Updated**: 2025-10-23
**Version**: 1.0.0
**Maintainer**: Claude Skills Maintainers | maintainers@example.com