Schema Validation (MVP)

[pivotaljohn]

As a Configuration Author
I want to declare what constitutes a valid Data Value right in my schema definition
So that*

• I can state those invariants in one central place and know they will be respected throughout the execution
• those who use my ytt library and violate one of those variants will receive a helpful error message

[proposal]
[getting started]

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Backlog

(The follow is scope to provide a minimally-viable feature with a vetted API. Subsequent features are identified in #724.)

Core:

• Annotate a Data Value with validation #604
• Annotate Data Value Schema with validation #611
• Disable Data Value validations #623

Absolute Minimum OOTB validations:

• Conditionally skip validations for a Data Value #641
• Require consumer to supply a Data Value (string, scalar, maps) #642
• Validate that of a set of siblings, exactly one is not null (union structures) #692
• Validate a value is within an enumeration #691

User Validation:

• User-Test ytt Schema Validation #707

Required Adjustments for GA:

• when not_null= rule is present, run first and stop if not satisfied #711
• one_not_null= treats missing keys as null valued #722
• Encode YAML Fragments to strings in violation messages #712
• Allow @schema/nullable alongside @schema/type any=True #723
• Remove redundant when_null_skip= kwarg #732
• Allow access to contained document for conditional validations #733

Documentation:

• @ytt:assert module Reference
• Validations Cheat Sheet #726
• "Writing Schema" includes guidance on including validations #725
• Add "Writing Validations" #727

Reflections:

• Retrospect on the SDLC flow for Validations. #747

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Resources

• Schema proposal: Part 7 — Validating Documents
• Schema Validation proposal

Related Issues:

• [schema, data/values] support specifying schema for data structures #103 — originating issue
• Add support for data values schema without defaults #556 — requesting "required" use-case, explicitly.