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HCS-13 Standard: Schema Registry and Validation

Status: Draft​

Table of Contents​

Authors​

Primary Author​

Abstract​

The HCS-13 standard introduces a comprehensive framework for defining, registering, and referencing schemas within the Hedera ecosystem. By providing a standardized approach to schema management, this specification enables type-safe data exchange, improved interoperability, and enhanced validation capabilities across decentralized applications.

Motivation​

As the Hedera ecosystem grows, the need for structured, validated data becomes increasingly important. Without a standardized schema system, developers face challenges in ensuring data consistency, validating message formats, and enabling interoperability between applications. The HCS-13 standard addresses these challenges by providing a robust schema registry system that leverages the Hedera Consensus Service (HCS) to store, manage, and reference schemas in a decentralized manner.

Key motivations include:

  1. Enabling type-safe data exchange between applications
  2. Providing a standardized way to validate message structures
  3. Improving interoperability through shared schema definitions
  4. Supporting schema evolution and versioning
  5. Enhancing NFT metadata with structured, validated data
  6. Creating a foundation for advanced data processing and analytics

Specification​

The HCS-13 standard builds upon the HCS-2 registry system and introduces specific mechanisms for schema management. It defines how schemas are created, registered, referenced, and used for validation.

Architecture Overview​

The HCS-13 standard involves three distinct types of topics:

  1. Schema Topics: Topics that store JSON Schema definitions as HCS-1 files. Each schema is stored in an HCS-1 topic, and versioning is managed through HCS-2 operations.
  2. Message Registry Topics: Topics that store validated messages conforming to a specific schema.
  3. Schema Discovery Registry: An HCS-2 guarded registry that enables discoverability of schemas across the ecosystem.

This architecture allows for clear separation of concerns while enabling powerful schema validation and discovery capabilities.

Schema Definition​

Schemas in HCS-13 are defined using JSON Schema, a widely adopted standard for describing JSON data structures. JSON Schema provides a rich set of validation rules and is supported by numerous libraries across programming languages.

Schema Format​

A schema definition must be a valid JSON Schema document (draft-07 or later) and should be stored as an HCS-1 file. The schema must include:

  1. A $schema property indicating the JSON Schema version
  2. A title property providing a human-readable name
  3. A description property explaining the schema's purpose
  4. A type property specifying the root data type

Additional metadata may be included to provide context and documentation.

Schema Example​

{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "title": "UserProfile",
  "description": "Schema for user profile data",
  "type": "object",
  "properties": {
    "username": {
      "type": "string",
      "minLength": 3,
      "maxLength": 50
    },
    "email": {
      "type": "string",
      "format": "email"
    },
    "age": {
      "type": "integer",
      "minimum": 13
    },
    "interests": {
      "type": "array",
      "items": {
        "type": "string"
      }
    }
  },
  "required": ["username", "email"]
}

Schema Topics​

Schema topics store the actual JSON Schema definitions as HCS-1 files. The workflow for creating and managing a schema topic is:

  1. Create an HCS-1 file containing the JSON Schema definition
  2. Create an HCS-2 topic to manage versions of the schema
  3. Register the HCS-1 file in the HCS-2 topic using the register operation

This approach leverages:

  • HCS-1 for storing the schema content (which may exceed the 1KB message limit)
  • HCS-2 for versioning and managing references to the schema

Example schema registration in an HCS-2 topic:

{
  "p": "hcs-2",
  "op": "register",
  "t_id": "0.0.123456", // Topic ID of the HCS-1 file containing the schema
  "metadata": {
    "name": "UserProfile",
    "description": "Standard user profile schema"
  },
  "m": "Initial schema definition"
}

Message Registry Topics​

Message registry topics store validated messages that conform to a specific schema. These topics use a modified HCS-2 format with a memo that references the schema topic ID.

Registry Creation​

A message registry topic is created as an HCS topic with a specific memo format that indicates its purpose and configuration:

hcs-13:[indexed]:[ttl]:[schemaTopicId]

FieldDescriptionExample Value
hcs-13Indicates this is a message registry following the HCS-13 standardhcs-13
indexedEnum value (0 or 1) indicating if all messages need to be processed or only the latest0
ttlTime-to-live in seconds for caching purposes86400
schemaTopicIdTopic ID of the HCS-2 topic managing the schema0.0.123456

Example memo: hcs-13:0:86400:0.0.123456

Schema Discovery Registry​

The schema discovery registry is an HCS-2 guarded registry that enables discoverability of schemas across the Hedera ecosystem. This registry allows developers to publish their schemas and make them available for others to use.

Discovery Registry Operations​

The schema discovery registry supports operations for managing schema discoverability.

Register Schema​

To register a schema in the discovery registry:

{
  "p": "hcs-13",
  "op": "register",
  "t_id": "0.0.123456", // Topic ID of the HCS-2 topic managing the schema
  "metadata": {
    "author": "0.0.789012",
    "description": "Standard user profile schema",
    "tags": ["profile", "user", "identity"]
  },
  "m": "Initial schema registration"
}
Deprecate Schema​

To mark a schema as deprecated in the discovery registry:

{
  "p": "hcs-13",
  "op": "deprecate",
  "t_id": "0.0.123456", // Topic ID of the schema being deprecated
  "replacement": "0.0.789012", // Optional Topic ID of replacement schema
  "metadata": {
    "reason": "Security vulnerability in validation logic"
  },
  "m": "Schema deprecated due to security concerns"
}

Schema Referencing​

Schemas can be referenced in various contexts using Hedera Resource Locators (HRLs).

HRL Format​

The HRL format for referencing a schema follows the pattern:

hcs://13/[topicId]

Where topicId is the topic ID of the HCS-2 topic managing the schema.

To reference a specific version of a schema, append a fragment identifier with the sequence number:

hcs://13/[topicId]#[sequenceNumber]

Examples:

  • hcs://13/0.0.123456 - References the latest version of the schema
  • hcs://13/0.0.123456#42 - References version 42 of the schema (sequence number 42)

Referencing in Messages​

When a message conforms to a specific schema, it can include a reference to that schema:

{
  "p": "hcs-13",
  "op": "data",
  "schema": "hcs://13/0.0.123456",
  "t_id": "0.0.789012", // Topic ID of the HCS-1 file containing the data
  "m": "User profile data"
}

For smaller data that fits within the 1KB limit, the data can be included directly:

{
  "p": "hcs-13",
  "op": "data",
  "schema": "hcs://13/0.0.123456",
  "data": {
    "username": "alice",
    "email": "[email protected]",
    "age": 25,
    "interests": ["blockchain", "programming"]
  },
  "m": "Inline user profile data"
}

Referencing in NFTs​

There are two distinct ways to use schemas with NFTs:

  1. Reference a schema in NFT metadata: Include a schema reference in the NFT's metadata to indicate the structure of its attributes
  2. Tokenize a schema: Use the schema's HRL as the NFT's metadata to create a token representing the schema itself
Referencing a Schema in NFT Metadata​

When creating an NFT with structured attributes, include a schema reference in the metadata:

{
  "name": "Digital Identity Card",
  "description": "A digital identity card following the standard schema",
  "image": "hcs://1/0.0.456789",
  "schema": "hcs://13/0.0.123456",
  "attributes": {
    "username": "bob",
    "email": "[email protected]",
    "age": 30,
    "interests": ["art", "music", "technology"]
  }
}
Tokenizing a Schema​

To create an NFT that represents a schema itself, use the schema's HRL as the NFT's metadata:

hcs://13/0.0.123456

NFT Integration​

The HCS-13 standard enhances NFTs with structured, validated data, enabling type-safe attributes and improved interoperability.

Benefits of Schema-Validated NFTs​

  1. Data Validation: Attributes conform to a defined structure with validation rules
  2. Type Safety: Applications can rely on consistent attribute formats
  3. Enhanced Metadata: Support for complex, nested data structures
  4. Interoperability: Different applications can process NFT data consistently

Example Use Cases​

  • Gaming Items: Define schemas for different item types with specific attributes
  • Digital Identity: Create verifiable credentials with standardized fields
  • Collectibles: Ensure consistent attribute formats across collections
  • Real Estate: Standardize property metadata for marketplace interoperability

Schema Validation​

The validation process involves:

  1. Retrieving the schema from the referenced HCS-1 file (via the HCS-2 topic)
  2. Parsing the schema as a JSON Schema document
  3. Retrieving the data to be validated
  4. Validating the data against the schema
  5. Reporting validation results

Applications should validate data against schemas before storing it to ensure only valid data is registered.

Schema Versioning​

Schema versioning is handled through the HCS-2 topic that manages references to the schema HCS-1 files.

Version Management​

Each time a schema is updated:

  1. Create a new HCS-1 file with the updated schema
  2. Submit a new register operation to the schema's HCS-2 topic referencing the new HCS-1 file
  3. The new message becomes the current version of the schema
  4. Previous versions remain accessible through their sequence numbers

Example schema update:

{
  "p": "hcs-2",
  "op": "register",
  "t_id": "0.0.567890", // Topic ID of the new HCS-1 file containing the updated schema
  "metadata": {
    "name": "UserProfile",
    "description": "User profile schema with profile picture support"
  },
  "m": "Added optional profile picture field"
}

Backward Compatibility​

When updating schemas, developers should consider backward compatibility:

  • Adding optional fields preserves compatibility
  • Removing required fields breaks compatibility
  • Relaxing validation rules preserves compatibility
  • Tightening validation rules may break compatibility

User Process for Schema Management​

The typical workflow for schema management includes:

  1. Define Schema: Create a JSON Schema document
  2. Store Schema: Store the schema as an HCS-1 file
  3. Create Schema Topic: Create an HCS-2 topic to manage schema versions
  4. Register Schema: Register the HCS-1 file in the HCS-2 topic
  5. Create Message Registry: Create a message registry topic referencing the schema topic
  6. Register in Discovery Registry: Register the schema in the discovery registry
  7. Prepare Data: Create data conforming to the schema
  8. Validate Data: Validate the data against the schema
  9. Store Data: Store the data as an HCS-1 file (if needed)
  10. Register Data: Register the data in the message registry

Schema Management Lifecycle​

The following diagram illustrates the complete lifecycle of schema management using the HCS-13 standard:

┌─────────────────┐     ┌─────────────────┐     ┌─────────────────┐
│                 │     │                 │     │                 │
│  Define Schema  │────▶│ Store Schema as │────▶│Create Schema    │
│  (JSON Schema)  │     │   HCS-1 File    │     │Topic (HCS-2)    │
│                 │     │                 │     │                 │
└─────────────────┘     └─────────────────┘     └────────┬────────┘
                                                         │
                                                         ▼
┌─────────────────┐     ┌─────────────────┐     ┌─────────────────┐
│                 │     │                 │     │                 │
│Create Message   │────▶│Register in      │────▶│  Prepare Data   │
│Registry Topic   │     │Discovery Registry│     │                 │
│                 │     │                 │     │                 │
└─────────────────┘     └─────────────────┘     └────────┬────────┘
                                                         │
                                                         ▼
┌─────────────────┐     ┌─────────────────┐     ┌─────────────────┐
│                 │     │                 │     │                 │
│  Update Schema  │◀────│ Register Data   │◀────│ Validate Data   │
│  (As Needed)    │     │ in Registry     │     │ Against Schema  │
│                 │     │                 │     │                 │
└─────────────────┘     └─────────────────┘     └─────────────────┘

This lifecycle demonstrates how schemas are created, stored, registered, and evolved over time:

  1. First, a schema is defined using JSON Schema format
  2. The schema is stored as an HCS-1 file due to potential size limitations
  3. An HCS-2 topic is created to manage versions of the schema
  4. A message registry topic is created to store data conforming to the schema
  5. The schema is registered in the discovery registry for discoverability
  6. Data is prepared, validated against the schema, and registered
  7. When updates are needed, a new version of the schema is created and registered

Conclusion​

The HCS-13 standard provides a comprehensive framework for schema management in the Hedera ecosystem. By leveraging HCS-1 for content storage and HCS-2 for versioning, it creates a robust system for defining, registering, and referencing schemas.

Key benefits of the HCS-13 standard include:

  1. Type-safe data exchange between applications
  2. Standardized validation of message structures
  3. Improved interoperability through shared schema definitions
  4. Flexible versioning that maintains backward compatibility
  5. Enhanced NFT metadata with structured, validated data

By following this standard, developers can create more reliable, interoperable applications that leverage the power of structured data validation within the Hedera ecosystem.