Pebble32 opened a new pull request, #69705: URL: https://github.com/apache/airflow/pull/69705
Add `JitteredCronTimetable`, an opt-in `CronTriggerTimetable` subclass that shifts each DAG's fire time by a deterministic, per-DAG offset drawn from `[0, max_jitter)`. This spreads out DAGs that share a cron expression so they no longer all fire at the same instant, without changing `logical_date` / `data_interval` semantics. ### Why `@daily` expands to `0 0 * * *`, so **every** daily DAG in a deployment is scheduled at exactly midnight. In large deployments this "thundering herd" at the cron boundary overloads the scheduler and workers — enough to cause task failures when dozens of DAGs are born at the same instant. The existing ways to deal with this don't actually de-collide the schedule: | Existing option | Why it doesn't solve the problem | | --- | --- | | Hand-pick a unique minute per DAG | Manual, doesn't scale, drifts and re-collides as DAG count grows, and throws away the `@daily` intent | | Hash the DAG id into a literal cron string | Same loss of intent; not reusable across DAGs/teams; every author re-implements it ad hoc | | Pools / concurrency limits (`parallelism`, `max_active_tasks_per_dag`, pool slots) | These **limit or queue execution** — the runs are still *scheduled* at the same instant. They cap contention downstream but never spread the fire times apart at the source. | `JitteredCronTimetable` is the only approach that moves the fire times themselves, deterministically: the same `seed` (e.g. the DAG id) always maps to the same offset, so runs stay stable and predictable across scheduler restarts and timetable serialization. Motivated by the discussion in https://github.com/apache/airflow/discussions/69027. ### What - New `JitteredCronTimetable(CronTriggerTimetable)` in both the Task SDK (author-facing, attrs-based) and airflow-core (scheduler-side), plus the serialization wiring (`BUILTIN_TIMETABLES` mapping, `serialize`/`deserialize`, encode/decode across the SDK↔core boundary). - Two extra kw-only params on top of `CronTriggerTimetable`: `seed: str` and `max_jitter: timedelta`. - The offset is `md5(seed) % max_jitter.total_seconds()`, applied as a "strip → cron → apply" coordinate shift so cron/DST alignment is fully delegated to the parent and only the wall-clock fire time is shifted. - **Fully opt-in and safe by default**: with the defaults (`seed=""`, `max_jitter=timedelta(0)`) the offset is zero and it behaves *identically* to `CronTriggerTimetable`. Nothing changes for anyone who doesn't use it. ### Tests `airflow-core/tests/unit/timetables/test_jittered_cron_timetable.py`, modeled on the existing `test_trigger_timetable.py`: - jittered runs equal base cron runs shifted by the fixed offset, across a catchup sequence including a DST spring-forward (`America/New_York`); - zero `max_jitter` reproduces `CronTriggerTimetable` exactly (catchup on/off); - offsets are deterministic for a given seed, bounded to `[0, max_jitter)`, and spread across distinct seeds; - `serialize`/`deserialize` round-trips seed + window + derived offset; - full encode → decode round-trip across the SDK/core layers rebuilds the core class. --- ##### Was generative AI tooling used to co-author this PR? - [X] Yes (please specify the tool below) Generated-by: Claude (Claude Code), following [the guidelines](https://github.com/apache/airflow/blob/main/contributing-docs/05_pull_requests.rst#gen-ai-assisted-contributions) --- related: https://github.com/apache/airflow/discussions/69027 -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. To unsubscribe, e-mail: [email protected] For queries about this service, please contact Infrastructure at: [email protected]
