Airflow每月Cron表达式触发异常求助：运行逻辑与预期不符疑问

Understanding Your Airflow Cron & Backfill Behavior #

Let's break down exactly what's happening with your cron schedule and backfill setup step by step:

1. First, Decode Your Cron Expression

Your cron string 43 10 3,8,12 */3 * translates to:

• Run at 10:43 UTC
• On the 3rd, 8th, and 12th day of the month
• Only in months that are every 3 months starting from January: January, April, July, October (this is the key part you might have misunderstood)

Cron's */3 in the month field doesn't start counting from your DAG's start_date (November 2019) — it’s an absolute interval based on the calendar year. So any month number that fits 1 + 3*n (where n is 0,1,2,3) is included, which gives 1,4,7,10. That’s why October 2020 is a valid month for your schedule.

2. Why 2020-10-03 and 2020-10-08 Were Triggered

With backfill enabled, Airflow automatically runs all task instances where the execution_date falls:

• After your DAG’s start_date (2019-11-18 UTC)
• Before the current date (2020-10-19 UTC)

Both 2020-10-03 10:43 UTC and 2020-10-08 10:43 UTC meet these criteria: they’re in a valid month (October), valid day, and well within the date range. That’s why backfill triggered them — this is expected behavior, not a bug.

3. Why 2020-10-12 Wasn’t Triggered

There are a few likely reasons for this:

• Scheduler Delay: Airflow’s scheduler might have a backlog or hasn’t yet processed the 2020-10-12 10:43 UTC execution date. Check your Airflow UI’s "Browse > Task Instances" page to see if this instance exists in a scheduled or queued state (it might just be waiting to run).
• Timezone Mismatch: If your current date 2020-10-19 is in a local timezone (not UTC), double-check what UTC time that corresponds to. For example, if you’re in a timezone 8 hours ahead of UTC, local 2020-10-19 is UTC 2020-10-18 — which means the 2020-10-12 10:43 UTC execution date is still within the backfill window, but the scheduler might need an extra nudge.
• Accidental Backfill Limits: Even though your DAG’s end_date is None, if you ran a backfill command with a --end-date parameter (e.g., airflow dags backfill -s 2019-11-18 -e 2020-10-09 your_dag_id), that would cap the backfill at October 9, excluding the 12th.
• Croniter Edge Case: Airflow uses the croniter library to parse cron schedules. While rare, there could be an edge case if there’s a date overlap issue, but this is unlikely given your straightforward schedule.

If you don’t see the 2020-10-12 instance in the UI at all, try manually triggering a backfill for that specific date range to test:

airflow dags backfill -s 2020-10-12 -e 2020-10-12 your_dag_id

内容的提问来源于stack exchange，提问作者Mehul