Standards-Based Data Architecture.
Reduced ETL Complexity.

┌─────────────────────────────────────────────────────┐
│           Layer 2: Namespace Versioning             │
│  (Cross-Version Coexistence: SDC4 ↔ SDC5 ↔ SDC6)   │
├─────────────────────────────────────────────────────┤
│  https://semanticdatacharter.com/ns/sdc4/          │
│  https://semanticdatacharter.com/ns/sdc5/          │
│  https://semanticdatacharter.com/ns/sdc6/          │
└─────────────────────────────────────────────────────┘
         ▲                 ▲                 ▲
         │                 │                 │
┌────────┴─────┐  ┌────────┴─────┐  ┌────────┴─────┐
│ Layer 1:     │  │ Layer 1:     │  │ Layer 1:     │
│ CUID-Based   │  │ CUID-Based   │  │ CUID-Based   │
│ Immutability │  │ Immutability │  │ Immutability │
│ (Within V4)  │  │ (Within V5)  │  │ (Within V6)  │
└──────────────┘  └──────────────┘  └──────────────┘

The Mechanism

SDC4 decouples structure from semantics using CUID2 identifiers:

• mc-<cuid>: Model Component type definition (structure)
• ms-<cuid>: Model Structure element (instance)
• label: Fixed semantic meaning (e.g., "Systolic Blood Pressure")

Schema Example: Immutable Blood Pressure Component

<!-- Defined once in 2025, designed for long-term persistence -->
<xsd:complexType name="mc-clj5x1g8f000008l09j7f6c3d">
  <xsd:complexContent>
    <xsd:restriction base="sdc4:ClusterType">
      <xsd:sequence>
        <!-- Semantic meaning is FIXED -->
        <xsd:element name="label" type="xsd:string"
                     fixed="Blood Pressure" />

        <!-- Structure references immutable systolic component -->
        <xsd:element ref="sdc4:ms-clj5x2p4k000108l01a2b3c4d"
                     minOccurs="1" maxOccurs="1" />

        <!-- Structure references immutable diastolic component -->
        <xsd:element ref="sdc4:ms-clj5x2p4k000108l01a2b3c4e"
                     minOccurs="1" maxOccurs="1" />
      </xsd:sequence>
    </xsd:restriction>
  </xsd:complexContent>
</xsd:complexType>

<!-- Element declaration for substitution -->
<xsd:element name="ms-clj5x1g8f000008l09j7f6c3d"
             type="mc-clj5x1g8f000008l09j7f6c3d"
             substitutionGroup="sdc4:Item" />

Why This Prevents Migration

1. Permanent Structure: mc-clj5x1g8f000008l09j7f6c3d always means the same physical structure
2. Fixed Semantics: label="Blood Pressure" is immutable in the schema
3. Queryable by CUID: Applications query ms-clj5x1g8f000008l09j7f6c3d, guaranteed to find the same structure
4. No Name Dependency: Unlike traditional XML where <bloodPressure> → <bp> breaks compatibility, CUID remains stable

Instance Data (2025)

<dm-h7k2x... xmlns:sdc4="https://semanticdatacharter.com/ns/sdc4/">
  <sdc4:ms-clj5x1g8f000008l09j7f6c3d>
    <sdc4:ms-clj5x2p4k000108l01a2b3c4d>
      <sdc4:xdquantity-value>120</sdc4:xdquantity-value>
      <sdc4:xdquantity-units>
        <sdc4:xdstring-value>mmHg</sdc4:xdstring-value>
      </sdc4:xdquantity-units>
    </sdc4:ms-clj5x2p4k000108l01a2b3c4d>
  </sdc4:ms-clj5x1g8f000008l09j7f6c3d>
</dm-h7k2x...>

Same Data, Still Valid Against SDC4 Schemas in 2035

An application in 2035 can query 2025 SDC4 data using the SDC4 namespace:

# Query by CUID - guaranteed stable structure
bp_elements = doc.findall(".//sdc4:ms-clj5x1g8f000008l09j7f6c3d",
                           namespaces={'sdc4': 'https://semanticdatacharter.com/ns/sdc4/'})

for bp in bp_elements:
    # Structure is immutable, so this always works
    systolic = bp.find(".//sdc4:ms-clj5x2p4k000108l01a2b3c4d//sdc4:xdquantity-value")
    print(f"Systolic: {systolic.text} mmHg")

The Mechanism

Each SDC major version gets its own XML namespace URI:

• SDC4: https://semanticdatacharter.com/ns/sdc4/
• SDC5: https://semanticdatacharter.com/ns/sdc5/ (when released)
• SDC6: https://semanticdatacharter.com/ns/sdc6/ (future)

Coexistence in XML Schema

<!-- SDC4 Schema (permanent) -->
<xs:schema
  xmlns:sdc4="https://semanticdatacharter.com/ns/sdc4/"
  targetNamespace="https://semanticdatacharter.com/ns/sdc4/"
  xmlns:xs="http://www.w3.org/2001/XMLSchema">

  <xs:complexType name="XdQuantityType">
    <!-- SDC4 definition -->
  </xs:complexType>
</xs:schema>

<!-- SDC5 Schema (coexists with SDC4) -->
<xs:schema
  xmlns:sdc5="https://semanticdatacharter.com/ns/sdc5/"
  targetNamespace="https://semanticdatacharter.com/ns/sdc5/"
  xmlns:xs="http://www.w3.org/2001/XMLSchema">

  <xs:complexType name="XdQuantityType">
    <!-- SDC5 definition (enhanced) -->
  </xs:complexType>
</xs:schema>

<!-- Application schema can import BOTH -->
<xs:schema
  xmlns:sdc4="https://semanticdatacharter.com/ns/sdc4/"
  xmlns:sdc5="https://semanticdatacharter.com/ns/sdc5/"
  xmlns:xs="http://www.w3.org/2001/XMLSchema">

  <xs:import namespace="https://semanticdatacharter.com/ns/sdc4/"
             schemaLocation="sdc4.xsd"/>
  <xs:import namespace="https://semanticdatacharter.com/ns/sdc5/"
             schemaLocation="sdc5.xsd"/>

  <!-- Use SDC4 components for legacy data -->
  <xs:element name="LegacyPatient" type="sdc4:DMType"/>

  <!-- Use SDC5 components for new data -->
  <xs:element name="EnhancedPatient" type="sdc5:DMType"/>
</xs:schema>

RDF Graph Partitioning

# Query SDC4 data only
SELECT ?patient ?age
FROM NAMED <https://semanticdatacharter.com/ns/sdc4/>
WHERE {
  GRAPH <https://semanticdatacharter.com/ns/sdc4/> {
    ?patient sdc4:hasAge ?age .
  }
}

# Federated query across SDC4 AND SDC5
SELECT ?patient ?ageSource ?age
WHERE {
  {
    GRAPH <https://semanticdatacharter.com/ns/sdc4/> {
      ?patient sdc4:hasAge ?age .
      BIND("SDC4" AS ?ageSource)
    }
  } UNION {
    GRAPH <https://semanticdatacharter.com/ns/sdc5/> {
      ?patient sdc5:hasAge ?age .
      BIND("SDC5" AS ?ageSource)
    }
  }
}

# Aggregate across all versions
SELECT ?ageSource (COUNT(?patient) AS ?count)
WHERE {
  {
    GRAPH <https://semanticdatacharter.com/ns/sdc4/> {
      ?patient sdc4:hasAge ?age .
      BIND("SDC4" AS ?ageSource)
    }
  } UNION {
    GRAPH <https://semanticdatacharter.com/ns/sdc5/> {
      ?patient sdc5:hasAge ?age .
      BIND("SDC5" AS ?ageSource)
    }
  } UNION {
    GRAPH <https://semanticdatacharter.com/ns/sdc6/> {
      ?patient sdc6:hasAge ?age .
      BIND("SDC6" AS ?ageSource)
    }
  }
}
GROUP BY ?ageSource

Why This Prevents Cross-Version Migration

1. Separate Namespaces: SDC4 and SDC5 have distinct URIs - no name collision
2. XML Schema Imports: Applications can import both schemas simultaneously
3. RDF Named Graphs: Triplestores partition by namespace, enabling federated queries
4. W3C Standards Foundation: XML namespaces are a core W3C standard providing long-term stability