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Notification delivery — Celery Beat + Django Background Tasks

Date

2023-12-19 (updated 2026-05-31)

Status

In-building

Context

The application needs to send time-based notifications to users (e.g. upcoming vet visits). Key constraints:

  • Avoid overwhelming the database with frequent polling.
  • Support at least one external channel (Discord is the primary target).
  • Work within the existing Django monolith (ADR-03) without adding a separate service.

Options evaluated:

  • django-crontab — OS-level cron wrapper; simple but couples scheduling to the server's cron daemon. No retry logic, no visibility into task state.
  • Celery Beat — distributed periodic task scheduler; integrates with the Celery worker (Redis broker) already used for async tasks. Supports retry, monitoring via Flower, and dynamic schedule updates.
  • Django Background Tasks API (ImmediateBackend) — lightweight in-process runner; no separate worker process needed; suitable for short, non-critical tasks.

Decision

django-crontab was chosen initially but has since been removed. The current stack uses two complementary mechanisms:

  1. Celery Beat (periodic tasks via celery_notifications/cron.py) — handles scheduled checks (e.g. scan for upcoming vet visits and enqueue notification tasks). Runs as a separate celery_beat Docker service with Redis 7 as the broker.

  2. Django Background Tasks API (ImmediateBackend) — used for lightweight in-process tasks that do not need a separate worker. Configured in settings.py.

Notification channel: Discord via discord.py. Additional channels (e-mail, SMS) are deferred.

Consequences

  • The homepage.CronJob model (the django-crontab leftover) was dropped in migration homepage/0004_delete_cronjob.py — the orphan-table follow-up this ADR originally flagged is resolved.
  • Celery Beat requires two running processes: the Celery worker (queue service) and the Beat scheduler (celery_beat service). Both are defined in docker/docker-compose.yml.
  • Task visibility is available via Celery Flower (port 5555).
  • Adding a new notification type means: (a) write a task function in celery_notifications/cron.py, (b) register it in the Celery Beat schedule in celery_notifications/config.py.

Keywords

  • Celery, Celery Beat, notifications, Discord, cron, queue, broker, background tasks

Links

[2026-05-31]
https://docs.celeryq.dev/en/stable/userguide/periodic-tasks.html

[2023-12-19]
https://docs.djangoproject.com/en/stable/topics/db/multi-db/ (database routing reference)