New time related JMESPath filters in Kyverno!

Use time in your filters now!.

The v1.9 release of Kyverno added several time related JMESPath filters. With this addition, users now can add time based rules in their Kyverno policies. This blog post aims to describe those new additions.

What is “JMESPath”?

JMESPath (pronounced “James path”) is a JSON query language that allows you to declaratively specify how to extract elements from a JSON document. It is similar to JSONPath in Kubernetes. It can be used almost anywhere in Kyverno.

Users may note that, many policies can be written with simple overlay patterns, others require more detailed selection and transformation. The latter is where JMESPath is useful.

Kyverno supports all the JMESPath filters in the JMESPath specifications page and also adds several custom filters. Refer to Kyverno’s JMESPath docs for a comprehensive deep dive.

Why did we need these new filters?

Kyverno already supports all the availible filters in the upstream JMESpath library and some custom filters as well. But there was a lack of time related filters that come in useful at several places in Kyverno, such as

  1. When we want to add custom labels like last sync time.
  2. Enforce a policy that checks whether the current time is after or before a specified time.
  3. Perform job scheduling using cron expressions.

What has been added?

In v1.9 release, Kyverno added 11 new time related filters to fulfill the above requirements and many more as well.

  1. Time_add: We have added a filter to do arithmetic addition of a duration to a given time. The time_add() filter takes a time and a duration and returns a time. time_add('2023-01-12T12:37:56-05:00','6h') results in the value "2023-01-12T18:37:56-05:00". It can come in useful purposes like adding some time to the current time and passing it as an argument for other time related filters.
  2. Time_after: This is used to verify whether a time is after another time and returns a boolean. It can be used directly as a validation filter, where we can check if current time is greater than a specific time or not and make decisions based on that information.
  3. Time_before: time_before() filter works similarly to the time_after() filter. This is used to verify whether a time is before another time and returns a boolean. time_before('2023-01-12T19:05:59Z','2023-01-13T19:05:59Z') results in the value true. It can be used directly as a validation filter, where we can check if current time is less than a specific time or not and make decisions based on that information.
  4. Time_between: The time_between() filter completes the collection of time comparision filters. This filter allows us to check whether a time occurs between a start time and an end time. It is used in the cases that require both time_before() and time_after() filter. For example, time_between('2023-01-12T19:05:59Z','2023-01-01T19:05:59Z','2023-01-15T19:05:59Z') results in the value true. It can be used directly as a validation filter, where we can check if current time is between two times or not and make decisions based on that information.
  5. Time_diff: The time_diff() filter is used to find how much time has passed between two time period. For example, time_diff('2023-01-10T00:00:00Z','2023-01-11T00:00:00Z') results in the value "24h0m0s". It has been added for a common usecase where we want to check, whether a certain amount of time has been passed since an important event or not, such as to check whether 4 hours have passed since last scan was finished, we can use time_diff() filter like this "{{ time_diff('{{metadata.scanFinishedOn}}','{{ time_now_utc() }}') }}"
  6. Time_now: The time_now() filter, as the name suggests returns the current time. It has been introduced for the common case where we need current time as an argument for other filters. We require current time in several filters like time_add() and this is added for such cases.
  7. Time_now_utc: The time_now_utc() filter works similar to time_now() where it returns the current time according to UTC. It has been introduced for the common case where we need current time as an argument for other filters.
  8. Time_parse: The Time_parse() filter is an interesting addition where we can pass any time string in any format to convert it ino the Kyverno standard RFC 3339 format. It takes 2 arguments, the time format and the time string. For example, time_parse('Mon Jan 02 2006 15:04:05 -0700', 'Fri Jun 22 2022 17:45:00 +0100') results in the output of "2022-06-22T17:45:00+01:00". This field has been added to allow standardization such that all the timestamps added as metadata and annotations follow the RFC 3339 format.
  9. Time_to_cron: In kyverno, we use cron in several places to perform job scheduling. The cron expression enables users to schedule tasks to run periodically at a specified date/time. And it’s naturally a great tool for automating lots of process runs, which otherwise would require human intervention. We have added a time_to_cron() filter to generate a cron expression from a time. For example, time_to_cron('2022-04-11T03:14:05-07:00') results in the output "14 3 11 4 1". This filter can be used to generate things like ClusterCleanupPolicy.
  10. Time_truncate: The time_truncate() filter is used to round the given time to the nearest duration which is specified in the arguments. For example, time_truncate('2023-01-12T17:37:00Z','1h') results in the output "2023-01-12T17:00:00Z".
  11. Time_utc: The time_utc() filter takes in a time in RFC 3339 format with a time offset and presents the same time in UTC/Zulu. It can be used in pair with time_parse() to convert it to UTC.

How to use it?

An in-depth guide on how to use JMESPath filters in kyverno is present in the Kyverno’s JMESPath docs.

Summary

JMESPath is a powerful and robust tool for selecting, extracting and manipulating time in JSON. With the addition of the new time based filters, we have extended the capabilitites of JMES filters in kyverno and have opened new usecases.