Motivation

In Amazon EventBridge Rules and EventBridge Scheduler, the precision of time is one minute. This is the same as the usual cron jobs.

... the minimum precision for a schedule is one minute. Your scheduled rule runs within that minute, but not on the precise 0th second. (aws-doc)

Sometimes we want to trigger precisely at zero seconds, for example, at 00:00:00.

In this post, I share the way to trigger precisely using the Lambda function with the EventBridge Schedule.

Architecture

1. EventBridge invokes the Lambda function 1 minute before the desired trigger time 00:00:00. That means the EventBridge Schedule is set to invoke the Lambda function at 23:59. As written above, we don't know what seconds in 23:59 the time Lambda function will be invoked. (It might be 23:59:16 or 23:59:38...)

2. The Lambda function calculates the time left to 00:00:00 and sleeps until then.

3. At 00:00:00, the Lambda function triggers some actions, events or resources.

Lambda function

Here is the example code for the Python Lambda function. You can just create a new function and paste this.

Note that the timeout of the Lambda function should be more than 1 minute.

import time
from datetime import datetime, timedelta


def lambda_handler(event, context):

    time_called = datetime.now()
    print("time_called", time_called)

    # ceiling minute
    target = time_called - timedelta(
        minutes=-1,
        seconds=time_called.second,
        microseconds=time_called.microsecond
    )

    diff = target - time_called
    wait_sec = diff.seconds + diff.microseconds / 1000000
    time.sleep(wait_sec)

    time_zero_sec = datetime.now()
    print("time_zero_sec", time_zero_sec)

    # Write here what you want to trigger at xx:xx:00.

Two print() are used to compare the times before and after wait.

Result

Let's try and see the result. At this time, desired time is 23:00. EventBridge Scheduler was set to invoke the Lambda function at 22:59.

Here is the capture of CloudWatch logs of the Lambda function.

• 22:59:16.345072 is from first print(), when EventBridge triggered the Lambda function.
• 2022-11-25 23:00:00.044251 is from second print(), when the Lambda function waited until 23:00:00 as intended.

Good! This is what I wanted to do here.

Summary

I've shared how to trigger events precisely in second order using EventBridge and Lambda.

More to read

Dennis Traub wrote an interesting post on the same topic using StepFunctions.

Note

There is a parameter at expression at(yyyy-mm-ddThh:mm:ss) in EventBridge Scheduler cli/boto3, but ss part does not work.