Styling by facet membership

[facets=<id>] in a .krs.style sheet — style by what a set an element belongs to, not by what it is.

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System view

Source

facet-styling/index.krs

// Demonstrates: styling by facet membership with `[facets=<id>]` (#2175 — experimental).
//
// A `facet` is a set defined OUTSIDE the architecture — a regulation, a policy,
// an audit scope. A tag says what an element *is*; a facet says what
// externally-defined set it *belongs to*. Membership is written on the element
// (`facets pci`), so renaming an element never means editing a distant list.
//
// This example is about the STYLING half. Open `facets.krs.style` beside it:
// the sheet matches on membership, which is the replacement for abusing an
// arbitrary tag selector like `[pci] { … }` for the same job.
//
// Three things to look for in the rendered diagram:
//
//   1. The `[facets=pci]` rule matches both `Checkout` (a service) and
//      `Ledger` (a database): the selector asks about the set, not the shape.
//      `Ledger` wears the result as the amber border; on `Checkout` the amber
//      is overridden by a later rule (see 3), which is a cascade outcome, not
//      a matching one.
//   2. `Ledger` also gets a fill, because `database[facets=pci]` compounds the
//      membership with a kind. `Checkout` does not — it is not a database.
//   3. `Checkout` is in BOTH facets, so `[facets=pci][facets=pii]` (score 20)
//      beats either single-facet rule (score 10) and dashes its border. Its
//      border COLOUR, though, is the teal of `pii` rather than the amber of
//      `pci`: the two single-facet rules tie at 10, so the later declaration
//      wins, exactly as two tag selectors would. That tie-break is the whole
//      reason a sheet can be migrated one rule at a time.
//
// The overlay in the preview's "Facets" selector is a separate, viewer-side
// thing: it paints a temporary highlight and writes nothing. What you see here
// is the author's own styling, and it is on for every reader.

@import "facets.krs.style"

facet pci {
  label "Cardholder data"
  description "In scope for the annual PCI DSS assessment"
  link "https://example.com/policies/pci" "PCI policy"
}

facet pii {
  label "Personal data"
  description "Holds or transits data identifying a natural person"
}

system Shop {
  label "Online shop"

  user Customer [human] {
    label "Customer"
  }

  client WebApp [web] {
    label "Web storefront"
    facets pii
  }

  // In both facets: it sees the buyer and the card. Watch the dashed border.
  service Checkout {
    label "Checkout"
    description "Takes payment — sees both the buyer and the card"
    facets pci, pii
  }

  service Accounts {
    label "Accounts"
    description "Profiles, addresses, consent records"
    facets pii
  }

  // In neither facet. Holds nothing about anyone, and takes no card.
  service Catalogue {
    label "Catalogue"
    description "Products and prices"
  }

  database Ledger {
    label "Payment ledger"
    facets pci
  }

  database ProfileStore {
    label "Profile store"
    facets pii
  }

  storage ProductImages {
    label "Product images"
  }

  Customer -> WebApp "browses"
  WebApp -> Accounts "sign in"
  WebApp -> Checkout "pay"
  WebApp -> Catalogue "browse"
  Accounts -> ProfileStore "read/write"
  Checkout -> Ledger "record"
  Catalogue -> ProductImages "read"
}