Scheduling

Scheduling

This page covers three related systems: WP-Cron reminders, race condition protection, and recurring series.

WP-Cron Reminders

The plugin uses two mechanisms to ensure appointment reminders are reliably sent. Both are managed by the Caswell_Cron class in includes/class-cron.php.

Per-Booking Scheduled Events

When a booking is created, wp_schedule_single_event() schedules a one-time cron event to fire at the exact reminder time (e.g., 24 hours before the appointment).

Hourly Sweep

A recurring hourly cron job scans for upcoming appointments and catches any reminders that were missed — for example, if WordPress cron was delayed or the site was temporarily down.

Double-Send Prevention

Both mechanisms check for a transient flag before sending:

caswell_reminder_sent_{booking_id}

If this transient exists, the reminder is skipped. The flag is set immediately after a successful send. This ensures each client receives exactly one reminder per booking.

Related page: Notifications (email and SMS templates used by reminders)

Race Condition Protection

When a booking is submitted, a transient-based lock is set for the requested slot:

caswell_slot_lock_{timestamp}_{session_length}

This 30-second lock prevents two clients from simultaneously booking the same time slot. The lock is released after the booking completes or fails.

Related page: AJAX-Endpoints (caswell_submit_booking action)

Recurring Series

Recurring series are managed by the Caswell_Recurring class in includes/class-recurring.php, with data stored in the wp_caswell_recurring_series table (see Database-Schema).

Supported Frequencies

• Weekly — same day and time every week
• Biweekly — every two weeks
• Monthly — same day of month (with overflow handling)

Rules and Limits

• Maximum 52 occurrences per series (safety cap)
• Monthly overflow: When the target day doesn't exist in a month (e.g., January 31 → February), the plugin falls back to the last day of the month (February 28/29)
• Availability check: All occurrences are validated for availability before the series is created
• Independent cancellation: Individual bookings within a series can be cancelled without affecting other bookings in the series

Cancellation

• Single booking: Use caswell_cancel_booking to cancel one occurrence
• Entire series: Use caswell_cancel_series to cancel all future bookings in the series

See AJAX-Endpoints for both cancellation actions.