Validate that a CloudWatch Events ScheduleExpression value is valid? - amazon-web-services

Question

I have an internal API, backed by CloudWatch Events, that lets developers schedule cronjobs. The user provided schedule expression must be a valid value for CloudWatch Events. Is there a utility/library for validating the rate and cron schedule expression values before making the API call to AWS to create the rule?

Answer 1

https://ap-southeast-1.console.aws.amazon.com/cloudwatch/home?region=ap-southeast-1#rules:action=create
There is a undocumented API when using the above, you provide a cron expressiaon and it will validate the expression and return the next 10 dates that this cron will run.
The console does a POST to here
https://ap-southeast-1.console.aws.amazon.com/cloudwatch/CloudWatch/data/jetstream.TestScheduleExpression/20191203005825803-2287194097589902
With this payload:
{"Expression":"cron(0 19 ? * MON-FRI *)","Limit":10}
!CW-Client-Metrics!
{"clientMetrics":{"cwdbSetWizardRuleScheduleExpressionAct":1,"cwdbSaveCronAndGetTriggerDatesAct":1}}
Returns the next 10 runtimes as epoch in the NextTriggerDates response:
I am afraid I have not seen any good regex to handle the AWS style so far.

Answer 2

You can use following regex to validate it:
// Minutes
/^([*]|([0-5]?\d)|((([0-5]?\d)|(\*))\/([0-5]?\d))|(([0-5]?\d)-([0-5]?\d))|((([0-5]?\d)|(\*))(,(([0-5]?\d)|(\*)))*))$/
// Hours
/^([*]|[01]?\d|2[0-3]|((([01]?\d|2[0-3]?)|(\*))\/([01]?\d|2[0-3]?))|(([01]?\d|2[0-3]?)-([01]?\d|2[0-3]?))|((([01]?\d|2[0-3]?)|(\*))((,)(([01]?\d|2[0-3]?)|(\*))){0,23}))$/
// Day of months
/^([*]|[?]|(([1-9]|[12]\d|3[01])[LW]?)|(([1-9]|[12]\d|3[01])-([1-9]|[12]\d|3[01]))|((([1-9]|[12]\d|3[01])|(\*))(\/)([1-9]|[12]\d|3[01]))|((([1-9]|[12]\d|3[01])|(\*))((,)(([1-9]|[12]\d|3[01])|(\*)))*))$/
// Months
/^([*]|([2-9]|1[0-2]?)|(JAN|FEB|MAR|APR|MAY|JUN|JUL|AUG|SEP|OCT|NOV|DEC)|((([2-9]|1[0-2]?)|(JAN|FEB|MAR|APR|MAY|JUN|JUL|AUG|SEP|OCT|NOV|DEC)|(\*))\/(([2-9]|1[0-2]?)|(JAN|FEB|MAR|APR|MAY|JUN|JUL|AUG|SEP|OCT|NOV|DEC)))|((([2-9]|1[0-2]?)|(JAN|FEB|MAR|APR|MAY|JUN|JUL|AUG|SEP|OCT|NOV|DEC))-(([2-9]|1[0-2]?)|(JAN|FEB|MAR|APR|MAY|JUN|JUL|AUG|SEP|OCT|NOV|DEC)))|((([2-9]|1[0-2]?)|(JAN|FEB|MAR|APR|MAY|JUN|JUL|AUG|SEP|OCT|NOV|DEC)|(\*))((,)(([2-9]|1[0-2]?)|(JAN|FEB|MAR|APR|MAY|JUN|JUL|AUG|SEP|OCT|NOV|DEC)|(\*)))*))$/
// Day of Week
/^([*]|[?]|([1-7]L?)|(SUN|MON|TUE|WED|THU|FRI|SAT)|((([1-7])|(SUN|MON|TUE|WED|THU|FRI|SAT))(-|,|#)(([1-7])|(SUN|MON|TUE|WED|THU|FRI|SAT)))|((([1-7])|(SUN|MON|TUE|WED|THU|FRI|SAT)|(\*))\/(([1-7])|(SUN|MON|TUE|WED|THU|FRI|SAT)))|((([1-7])|(SUN|MON|TUE|WED|THU|FRI|SAT)|(\*))((,)(([1-7])|(SUN|MON|TUE|WED|THU|FRI|SAT)|(\*)))*))$/
// Year
/^([*]|([1-2]\d{3})|(((([1-2]\d{3})|(\*)))\/((\d{0,4})))|(([1-2]\d{3})-([1-2]\d{0,3}))|((([1-2]\d{3})|(\*))((,)(([1-2]\d{3})|(\*)))*))$/
note: years regex don't check range (1970 - 2199) but it can be easily done with code.

Answer 3

I'm not sure if it helps, but I created a AWS cron expression validator in Python and published it on PyPI here https://pypi.org/project/aws-cron-expression-validator/
pip install aws-cron-expression-validator
Usage;
from aws_cron_expression_validator.validator import AWSCronExpressionValidator
my_expression = "0 18 ? * MON-FRIbad *"
try:
AWSCronExpressionValidator.validate(my_expression)
except ValueError as e:
print(f"Oh no! My expression was invalid: {e}")
# Returns; Oh no! My expression was invalid: Invalid day-of-week value 'MON-FRIbad'.