Annotate a Data Value with validation

[pivotaljohn]

Given a Data Values document (and only a Data Values document), annotated with a validation rule:

#! values.yml

@data/values
---
#@assert/validate ("a non-empty string", lambda v: assert.fail("length = {}", len(v)) if len(v) > 0 else None)
username: ""

Scenario: Invalid Data Value

Given the final Data Value's value does not satisfies the rule:
When ytt runs
Then reports a Data Values violation error message:

$ ytt -f values.yml --data-values-inspect
ytt: Error: One or more data values were invalid:
- "adminPort" requires a valid value: a non-empty string; length = 0 (validated by values.yml:6).

Scenario: Valid Data Value

Given the final Data Value's value satisfies the rule:
Then no error message is displayed.

$ ytt -f values.yml -v username=alice --data-values-inspect
username: alice

Note: validations are checked after all overlays have been applied.

---

Notes:

• we show evaluating just one rule on one data value. We'll ultimately support evaluating multiple rules per Data Value, and evaluate validity of as many data values as possible.
• one has to use the feature to opt-in. In a near-future story, we'll implement a feature flag.
• while assertions work well with schema, a current design goal is to implement them orthogonally: this story establishes the core feature of validation; a subsequent story (Annotate Data Value Schema with validation #611) covers declaring a validation within schema.
• we anticipate users seeing "violation messages" more often and in larger volume; hence the more compact error message format being highlighted. Feel free to play with the format and verbiage — what's given in the story is a "thoughtful suggestion" not the final word.
• https://github.com/vmware-tanzu/carvel/blob/004-schema-validation/proposals/ytt/004-schema-validation/README.md#assertvalidate

[gcheadle-vmware]

Should the #@assert/validate annotation work on data values contained in a data values file?

#@ load("@ytt:assert", "assert")
@data/values
---
#@assert/validate ("a non-empty string", lambda v: assert.fail("length = {}", len(v)) if len(v) > 0 else None)
username: "andrew"

[pivotaljohn]

Should the #@assert/validate annotation work on data values contained in a data values file?

Insightful question. And I think you intuition is right: that an @assert/validate should mean the same thing everywhere (just like an @overlay/match does).

This points to a need to be able to declare the need for an @assert/validate (specifically in a document containing schema).

Folding this into the overall proposal.

🙌🏻

[pivotaljohn]

Split this story into two:

• this story covers the @assert/validate annotation (and required plumbing)
• Annotate Data Value Schema with validation #611 is scoped to emitting @assert/validate annotations, given a schema declaring validations.

[gcheadle-vmware]

Closing since follow up PR was merged.