Notification Center aggregation for high‑frequency events

Recommendation

Use a per‑user digest queue that stores each Notification Center event and sends a consolidated email via a cron‑driven command. This preserves existing email templates, avoids losing submissions, and works across Contao minor and patch releases.

Why this works (confirmed facts)

• The Notification Center provides a public hook/event (e.g., notification_center.send) that can be intercepted without overriding core classes.
• Storing the full event data in a queue keeps all original tokens available for template rendering.
• A separate console command can group queued events by user and time window, render the existing template once, send the email, and delete the processed rows.

Likely explanation (implementation steps)

1. Create a storage mechanism – either a Doctrine entity or a custom table (e.g., tl_notification_digest) with columns: id, user_id, digest_key (e.g., date‑hour), event_data (JSON or serialized), created_at.
2. Register a hook on the notification_center.send event. In the hook callback, instead of sending the notification immediately, insert a row into the digest table containing the submitted form data.
3. Add a console command (e.g., contao:notification-digest) that:
   • Queries the digest table, grouping rows by user_id and digest_key.
   • For each group, builds an array of event data, renders the existing Notification Center template (passing the array so placeholders like {{formTitle}} are resolved for each item or using a loop in the template).
   • Sends the consolidated email via Contao’s mail API.
   • Deletes the processed rows from the digest table.
4. Schedule the command in Contao’s cron configuration (e.g., every 15 minutes) or add it to the contao:cron list so it runs automatically.

Comparison with time‑based throttling

• Throttling suppresses excess events after a threshold, which can lose submissions and complicates token handling because the template may receive incomplete data.
• A digest queue retains every event, guarantees delivery (subject to cron lag), and requires no changes to the existing email template.

Compatibility across Contao releases

By relying only on the public hook/event system and the NotificationCenter::send() API, the solution remains functional after minor and patch updates. Overriding core classes or using internal properties would risk breaking on upgrades.