Amazon CloudWatch Event周日10点UTC触发配置及Cron、rate疑问

Hey there! Let's break down your questions step by step to get your CloudWatch Events rule configured exactly how you need it.

1. Cron Expression: Understanding ? vs * #

First, let's recap CloudWatch Events' cron format:
cron(Minutes Hours Day-of-Month Month Day-of-Week Year)

The critical rule here is that Day-of-Month and Day-of-Week are mutually exclusive fields. You can't use * (meaning "all possible values") for both—this creates a logical conflict because you can't simultaneously specify "every day of the month" and "every Sunday" for a trigger that only needs one of those conditions.

• * = "match all values" for the field. For example, * in Day-of-Month would mean "every single day of the month".
• ? = "I don't care about this field". It's the way to opt out of specifying one of the two mutually exclusive fields (Day-of-Month or Day-of-Week) when you only care about the other.

Since your goal is to trigger every Sunday at 10 UTC, you only care about the Day-of-Week (SUN)—the specific day of the month doesn't matter. That means you should use ? for Day-of-Month.

So the correct expression is:
cron(0 10 ? * SUN *)

The cron(0 10 * * SUN *) version is technically problematic. While it might seem to work, it's not following CloudWatch's intended syntax—you're telling it to trigger on "every day of the month AND every Sunday", which reduces to Sundays but is confusing and could lead to unexpected behavior if you modify the rule later. Stick with ? for the field you don't care about when working with weekly calendar-based triggers.

2. rate() vs Scheduled (Cron) Expressions #

Let's clarify the core differences between these two scheduling methods:

Rate Expressions #

• rate(X units) is a fixed-interval trigger. It starts counting from the exact moment your rule is enabled (or deployed), and triggers every X units of time after that.
• For your example, rate(7 days) will indeed run every 7 days starting from deployment. If you activate it on a Wednesday at 2 PM UTC, it'll trigger next on the following Wednesday at 2 PM, then the next Wednesday, etc.—no regard for calendar dates or days of the week.
• Ideal when you need consistent, interval-based runs (e.g., every 3 hours, every 7 days) that aren't tied to calendar events.

Scheduled (Cron) Expressions #

• Cron expressions are calendar-based triggers. They run based on specific calendar criteria (like every Sunday at 10 UTC, or the 1st of every month at midnight).
• They don't care about when you deploy the rule—they follow the calendar schedule you define. Even if you set up the rule on a Tuesday, it'll wait until the next Sunday at 10 UTC to fire.
• Perfect when you need to align with specific calendar milestones (weekly, monthly, yearly schedules).

Quick Recap #

• Use rate(7 days) if you want a strict 7-day cycle starting from when you deploy the rule.
• Use cron(0 10 ? * SUN *) if you need it to run every Sunday at 10 UTC, no matter how many days apart those Sundays fall (they're almost always 7 days, but this ensures you're tied to the calendar's Sunday schedule).

内容的提问来源于stack exchange，提问作者deadcoder0904