Practical docs for using Validation

[mattyg]

Closed by #520.

Must include:

• code examples
• practical, terse explanation

Topics to include (consider splitting into sub-pages):

• Validation
  • Determinism
    • dependencies
      • must_get_* functions
        • must_get_valid_record is the only one that checks for validation receipts/warrants because an entry is only valid/invalid in the context of an action
        • this also means it trusts the word of others (1-of-n validation)
      • short-circuiting with ?
  • Operate on ops
    • flat ops convenience thingy
  • Considerations re: what should be validated on each op (IOW, what authorities should be responsible for what things)
  • Things that can be validated
    • membrane proof
    • entry structure (entry_def macro gives you deserialisation and error short-circuiting for free with ? operator)
    • permission
    • rate limiting with weight field
    • dependencies, incl source chain history
      • Inductive validation for costly dep trees (pattern?)
  • Limitations
    • Cannot must_get links or actions on a base
    • Cannot currently co-validate multiple actions (can only validate an action based on prior valid actions)
    • Cannot validate the non-existence of something, because that can always change
  • (future) source chain restructured to atomic bundle of actions, co-validated
  • sys validation
  • Lifecycle of a validation
    • At author time
    • At publish time
    • At gossip time
• Sys validation
  • What does it validate?
    • Source chain continuity
    • Timestamp monotonicity
    • Contiguity of authorship
      • (future) if DPKI enabled, sys validation checks that changes of agent ID are reflected in DPKI DNA as-at action timestamp
    • What else?