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Preview of ytt Validations

by John Ryan — May 26, 2022

Announcing! 📣

We are excited to announce that in ytt v0.41.0, we are including a preview of a powerful new feature:

ytt Validations!

What are ytt Validations? 🤔

Validations are constraints that you can define on Data Values via an annotation. A Validation ensures that a Data Value is in the expected range of values.

Like this:

    #@schema/validation one_of=["aws", "azure", "vsphere"]
    provider: ""

That one new annotation — @schema/validation — will:

  • validate the final data value against that rule (provider: is set to one of the three values),
  • when exporting schema, translate the rule to OpenAPI v3 / JSON Schema validation.
  • automatically include documentation describing the rule.

While we plan on delivering a host of useful named rules, out of the box, there is a way to define custom validation rules, yourself.

For all the juicy details, check out the Validations Proposal/Specification.

ytt Validations are very useful for situations where a mis-configured deployment could waste a lot of resources and be much more difficult to troubleshoot than getting an immediate error message that the configuration value was wrong.

It’s an Experiment! 🧪

To make a preview possible, we’re also introducing a feature for enabling “experiments” in v0.41.0.

An “experiment” is a feature that is still in development. By default, all experiments are off.

When you switch on an experiment, it lights-up the named feature, allowing you to take it for a spin. Of course, like any early access software, using ytt with any experiments enabled means it’s not suitable for production environments. All APIs introduced with the experiment are unstable and will change as we improve the UX and behavior of the feature.

So, ytt Validations is the first of these such experiments.

We’re doing this so that you have a voice in making sure features like this meet your needs.

How do I get started? 🏇

ytt v0.41.0 will be released soon. When it does, grab yourself the latest.

Then, to enable Validations, start including validation rules in a copy of your Data Values Schema:

  #@schema/validation ("non-empty value", lambda v: len(v) > 1)
  username: ""
  #@schema/validation ("non-empty value", lambda v: len(v) > 1), ("not 'default'", lambda v: v != "default")
  namespace: ""
  #@schema/validation ("one of: aws, azure, vsphere", lambda v: v in ["aws", "azure", "vsphere"])
  provider: ""

… and set the OS environment variable, YTTEXPERIMENTS

$ YTTEXPERIMENTS=validations ytt -f schema.yml -v dex.namespace=default
ytt: Error: One or more data values were invalid:
- "username" (schema.yml:5) requires "non-empty value" (by schema.yml:4)
- "namespace" ((data-value arg):1) requires "not 'default'" (by schema.yml:6)
- "provider" (schema.yml:9) requires "one of: aws, azure, vsphere" (by schema.yml:8)

You’ll notice that with the validations experiment turned off, ytt doesn’t recognize those annotations.

$  ytt -f schema.yml -v dex.namespace=default --data-values-inspect
  username: ""
  namespace: default
  provider: ""

If you want to verify you’ve switched on the right experiment(s), use the version command:

$ YTTEXPERIMENTS=validations ytt version
ytt version 0.41.0
- experiment "validations" enabled.

Experiments on ytt Playground

The hosted Playground instance ( is based on the latest version of ytt. However, all experiments are switched off.

To get the REPL-like Playground experience while exploring experimental features, launch it locally:

$ YTTEXPERIMENTS=validations ytt website
Listening on http://localhost:8080

What is implemented so far? 🛠

As of v0.41.0, we’ve implemented the core behavior of recognizing, parsing, and checking validations in schema.

In essence, we’ve introduced the @schema/validation annotation and wired it in to the ytt pipeline — specifically, at the end of the “Calculate Data Values” step.

We’re including light documentation as we go, so check out Data Values Schema Reference > @schema/validation for the exact details of what’s available.

You can peek into our backlog to see what we plan to do next: Schema Validation #561.

How can I give feedback? 🗣

We would love to tell you, thank you for asking! 😁

Delightfully, any format that is most convenient for you, we’re happy to hear your thoughts!

The likely easiest/best way is by popping by our channel on the Kubernetes Slack workspace: #carvel. You can fork a thread, there; request a direct message conversation; or even a video call. If you don’t yet have an account there, you can get an invite at

That said, if you prefer the long-form, feel free to create a GitHub Issue in our repo.

We would love to hear any and all thoughts you have. Here are some prompts:

  • what could be changed that would make it easier to use?
  • what could we do to improve the formatting and/or content of the error messages?
  • what validation rules do you find yourself repeatedly writing?

Why are Validations Useful? 💡

To date, ytt Library / Carvel Package authors have had to “roll their own” logic to check that customer’s inputs are valid: that data values are present/non-empty and within acceptable values.

They have had to…

  • learn Starlark, in some depth, just to get started
  • write code to collect all violations (instead of failing on first violation)
  • hand-manage ordering validation evaluation — leaves to root
  • write custom code to handle common scenarios (empty values, enums, unions, conditional)
  • hand-document validations by adding comments to their Data Values (Schema) describing those constraints.

The more time you spend doing this, the less time you spend making progress on your actual goal.

Of course the critical ingredient in all this is that the tool actually solved the problem in the way you want it to. And this is it: here’s your chance to add your perspective to the mix; a perspective we highly value. 🏆

Join the Carvel Community

We are excited to hear from you and learn with you! Here are several ways you can get involved:

  • Join Carvel’s slack channel, #carvel in Kubernetes workspace, and connect with over 1000+ Carvel users.
  • Find us on GitHub. Suggest how we can improve the project, the docs, or share any other feedback.
  • Attend our Community Meetings! Check out the Community page for full details on how to attend.

We look forward to hearing from you and hope you join us in building a strong packaging and distribution story for applications on Kubernetes!