Data Contract Overview

Data contracts define the schema (structure) of data an application will store on Dash Platform. Contracts are described using JSON Schema which allows the platform to validate the contract-related data submitted to it.

The following sections provide details that developers need to construct valid contracts: documents and definitions. All data contracts must define one or more documents, whereas definitions are optional and may not be used for simple contracts.

General Constraints

Note: There are a variety of constraints currently defined for performance and security reasons. The following constraints are applicable to all aspects of data contracts. Unless otherwise noted, these constraints are defined in the platform's JSON Schema rules (e.g. js-dpp data contract meta schema).

Keyword

Keyword	Constraint
`default`	Restricted - cannot be used (defined in DPP logic)
`propertyNames`	Restricted - cannot be used (defined in DPP logic)
`uniqueItems: true`	`maxItems` must be defined (maximum: 100000)
`pattern: <something>`	`maxLength` must be defined (maximum: 50000)
`format: <something>`	`maxLength` must be defined (maximum: 50000)
`$ref: <something>`	`$ref` can only reference `definitions` - remote references not supported
`if`, `then`, `else`, `allOf`, `anyOf`, `oneOf`, `not`	Disabled for data contracts

Data Size

Note: These constraints are defined in the Dash Platform Protocol logic (not in JSON Schema).

All serialized data (including state transitions) is limited to a maximum size of 16 KB.

Additional Properties

Although JSON Schema allows additional, undefined properties by default, they are not allowed in Dash Platform data contracts. Data contract validation will fail if they are not explicitly forbidden using the additionalProperties keyword anywhere properties are defined (including within document properties of type object).

Include the following at the same level as the properties keyword to ensure proper validation:

"additionalProperties": false

Data Contract Object

The data contract object consists of the following fields as defined in the JavaScript reference client (js-dpp):

Property	Type	Required	Description
protocolVersion	integer	Yes	The platform protocol version (currently `0`)
$schema	string	Yes	A valid URL (default: https://schema.dash.org/dpp-0-4-0/meta/data-contract)
$id	array of bytes	Yes	Contract ID generated from `ownerId` and entropy (32 bytes; content media type: `application/x.dash.dpp.identifier`)
ownerId	array of bytes	Yes	Identity that registered the data contract defining the document (32 bytes; content media type: `application/x.dash.dpp.identifier`
documents	object	Yes	Document definitions (see Documents for details)
definitions	object	No	Definitions for `$ref` references used in the `documents` object (if present, must be a non-empty object with <= 100 valid properties)

Data Contract Schema

Details regarding the data contract object may be found in the js-dpp data contract meta schema. A truncated version is shown below for reference:

{
  "$schema": "http://json-schema.org/draft-07/schema",
  "$id": "https://schema.dash.org/dpp-0-4-0/meta/data-contract",
  "type": "object",
  "definitions": {
    // Truncated ...
  },
  "properties": {
    "protocolVersion": {
      "type": "integer",
      "minimum": 0,
      "maximum": 0,
      "$comment": "Maximum is the latest Data Contract protocol version"
    },
    "$schema": {
      "type": "string",
      "const": "https://schema.dash.org/dpp-0-4-0/meta/data-contract"
    },
    "$id":{
      "type": "array",
      "byteArray": true,
      "minItems": 32,
      "maxItems": 32,
      "contentMediaType": "application/x.dash.dpp.identifier"
    },
    "ownerId":{
      "type": "array",
      "byteArray": true,
      "minItems": 32,
      "maxItems": 32,
      "contentMediaType": "application/x.dash.dpp.identifier"
    },
    "documents": {
      "type": "object",
      // Truncated ...
      "propertyNames": {
        "pattern": "^((?!-|_)[a-zA-Z0-9-_]{0,62}[a-zA-Z0-9])$"
      },
      "minProperties": 1,
      "maxProperties": 100
    },
    "definitions": {
      "$ref": "#/definitions/documentProperties"
    }
  },
  "required": [
    "protocolVersion",
    "$schema",
    "$id",
    "ownerId",
    "documents"
  ],
  "additionalProperties": false
}

Example

{
  "id": "AoDzJxWSb1gUi2dSmvFeUFpSsjZQRJaqCpn7vCLkwwJj",
  "ownerId": "7NUbPf231ixt1kVBQsBvSMMBxd7AgPad8KtdtfFGhXDP",
  "schema": "https://schema.dash.org/dpp-0-4-0/meta/data-contract",
  "documents": {
    "note": {
      "properties": {
        "message": {
          "type": "string"
        }
      },
      "additionalProperties": false
    }
  }
}

Data Contract id

The data contract $id is a hash of the ownerId and entropy as shown here.

// From the JavaScript reference implementation (js-dpp)
// generateDataContractId.js
function generateDataContractId(ownerId, entropy) {
  return hash(
    Buffer.concat([
      ownerId,
      entropy,
    ]),
  );
}

Data Contract Documents

The documents object defines each type of document required by the data contract. At a minimum, a document must consist of 1 or more properties. Documents may also define indices and a list of required properties. The additionalProperties properties keyword must be included as described in the constraints section.

The following example shows a minimal documents object defining a single document (note) that has one property (message).

{
  "note": {
    "properties": {
      "message": {
        "type": "string"
      }
    },
    "additionalProperties": false
  }
}

Document Properties

The properties object defines each field that will be used by a document. Each field consists of an object that, at a minimum, must define its data type (string, number, integer, boolean, array, object). Fields may also apply a variety of optional JSON Schema constraints related to the format, range, length, etc. of the data.

Note: The object type is required to have properties defined either directly or via the data contract's definitions. For example, the body property shown below is an object containing a single string property (objectProperty):

const contractDocuments = {
  message: {
    properties: {
      body: {
        type: "object",
        properties: {
          objectProperty: {
            type: "string"
          },
        },
        additionalProperties: false,
      },
      header: {
        type: "string"
      }
    },
    additionalProperties: false
  }
};

Note: A full explanation of the capabilities of JSON Schema is beyond the scope of this document. For more information regarding its data types and the constraints that can be applied, please refer to the JSON Schema reference documentation.

Property Constraints

There are a variety of constraints currently defined for performance and security reasons.

Description	Value
Minimum number of properties	1
Maximum number of properties	100
Minimum property name length	1
Maximum property name length	63
Property name first/last characters	Alphanumeric only (`A-Z`, `a-z`, `0-9`)
Property name characters	Alphanumeric (`A-Z`, `a-z`, `0-9`) Hypen (`-`) Underscore (`_`)

Required Properties (Optional)

Each document may have some fields that are required for the document to be valid and other fields that are optional. Required fields are defined via the required array which consists of a list of the field names from the document that must be present. The required object should be excluded for documents without any required properties.

"required": [
  "<field name a>",
  "<field name b>"
]

Example
The following example (excerpt from the DPNS contract's domain document) demonstrates a document that has 6 required fields:

"required": [
  "label",
  "normalizedLabel",
  "normalizedParentDomainName",
  "preorderSalt",
  "records",
  "subdomainRules"
]

Document Indices

Note: The indices object should be excluded for documents that do not require indices.

Document indices may be defined if indexing on document fields is required.

The indices array consists of:

One or more objects that each contain:
A properties array composed of a <field name: sort order> object for each document field that is part of the index (sort order: asc or desc)
An (optional) unique element that determines if duplicate values are allowed for the document

Note: When defining an index with multiple properties (i.e a compound index), the order in which the properties are listed is important. Refer to the mongoDB documentation for details regarding the significance of the order as it relates to querying capabilities.

"indices": [
  {
    "properties": [
      { "<field name a>": "<asc"|"desc>" },
      { "<field name b>": "<asc"|"desc>" }
    ],
    "unique": true|false
  },
  {
    "properties": [
      { "<field name c>": "<asc"|"desc>" },
    ],
  }
]

Index Constraints

Note: For performance and security reasons, Evonet places the following constraints on indices. These constraints are subject to change over time.

Description	Value
Maximum number of indices	10
Maximum number of unique indices	3
Maximum number of properties in a single index	10
Maximum length of indexed string property	1024

Example
The following example (excerpt from the DPNS contract's preorder document) creates an index on saltedDomainHash that also enforces uniqueness across all documents of that type:

"indices": [
  {
    "properties": [
      {
        "saltedDomainHash": "asc"
      }
    ],
    "unique": true
  }
]

Full Document Syntax

This example syntax shows the structure of a documents object that defines two documents, an index, and a required field.

{
  "<document name a>": {
    "properties": {
      "<field name b>": {
        "type": "<field data type>"
      },
      "<field name c>": {
        "type": "<field data type>"
      },
    },
    "indices": [
      {
        "properties": [
          {
            "<field name c>": "<asc|desc>"
          }
        ],
        "unique": true|false
      },
    ],
    "required": [
      "<field name c>"
    ]
    "additionalProperties": false
  },
  "<document name x>": {
    "properties": {
      "<property name y>": {
        "type": "<property data type>"
      },
      "<property name z>": {
        "type": "<property data type>"
      },
    },
    "additionalProperties": false
  },    
}

Document Schema

Full document schema details may be found in this section of the js-dpp data contract meta schema.

Data Contract Definitions

The optional definitions object enables definition of aspects of a schema that are used in multiple places. This is done using the JSON Schema support for reuse. Items defined in definitions may then be referenced when defining documents through use of the $ref keyword.

Note: Properties defined in the definitions object must meet the same criteria as those defined in the documents object (e.g. the additionalProperties properties keyword must be included as described in the constraints section).

Note: Data contracts can only use the $ref keyword to reference their own definitions. Referencing external definitions is not supported by the platform protocol.

Example
The following example shows a definition for a message object consisting of two properties:

{
  // Preceding content truncated ...
  "definitions": {
    "message": {
      "type": "object",
      "properties": {
        "timestamp": {
          "type": "number"
        },
        "description": {
          "type": "string"
        }
      },
      "additionalProperties": false
    }
  }
}

Note: In the js-dpp reference implementation, definitions are added to a data contract via the .setDefinitions() method (e.g. myContract.setDefinitions({\"message\": { ... }})). This must be done prior to broadcasting the contract for registration.

Data Contract Creation

Data contracts are created on the platform by submitting the data contract object in a data contract create state transition consisting of:

Field	Type	Description
protocolVersion	integer	The platform protocol version (currently `0`)
type	integer	State transition type (`0` for data contract)
dataContract	data contract object	Object containing the data contract details
entropy	array of bytes	Entropy used to generate the data contract ID. Generated as shown here. (32 bytes)
signaturePublicKeyId	number	The `id` of the identity public key that signed the state transition
signature	array of bytes	Signature of state transition data (65 bytes)

Each data contract state transition must comply with this JSON-Schema definition established in js-dpp:

{
  "$schema": "http://json-schema.org/draft-07/schema",
  "properties": {
    "protocolVersion": {
      "type": "integer",
      "minimum": 0,
      "maximum": 0,
      "$comment": "Maximum is the latest Data Contract Create Transition protocol version"
    },
    "type": {
      "type": "integer",
      "const": 0
    },
    "dataContract": {
      "type": "object"
    },
    "entropy": {
      "type": "array",
      "byteArray": true,
      "minItems": 32,
      "maxItems": 32
    },
    "signaturePublicKeyId": {
      "type": "integer",
      "minimum": 0
    },
    "signature": {
      "type": "array",
      "byteArray": true,
      "minItems": 65,
      "maxItems": 65
    }
  },
  "additionalProperties": false,
  "required": [
    "protocolVersion",
    "type",
    "dataContract",
    "entropy",
    "signaturePublicKeyId",
    "signature"
  ]
}

Example State Transition

{
  "protocolVersion": 0,
  "type": 0,
  "signature": "IG2Tr16rS2+FNoiH71eAva94H5BLV5QNl7Fg25s8ZzWvPlR4wihupdqYupvzTXGiAqPqSK3KQE1EouATMhgHPDc=",
  "signaturePublicKeyId": 0,
  "dataContract": {
    "protocolVersion": 0,
    "$id": "AoDzJxWSb1gUi2dSmvFeUFpSsjZQRJaqCpn7vCLkwwJj",
    "$schema": "https://schema.dash.org/dpp-0-4-0/meta/data-contract",
    "ownerId": "7NUbPf231ixt1kVBQsBvSMMBxd7AgPad8KtdtfFGhXDP",
    "documents": {
      "note": {
        "properties": {
          "message": {
            "type": "string"
          }
        },
        "additionalProperties": false
      }
    }
  },
  "entropy": "ahw7IvTAYkZcPaGcvh6BCVYP9rh/KyBkoeCGk28yoAw="
}

Data Contract State Transition Signing

Data contract state transitions must be signed by a private key associated with the contract owner's identity.

The process to sign a data contract state transition consists of the following steps:

Canonical CBOR encode the state transition data - this include all ST fields except the signature and signaturePublicKeyId
Sign the encoded data with a private key associated with the ownerId
Set the state transition signature to the value of the signature created in the previous step
Set the state transitionsignaturePublicKeyId to the public key id corresponding to the key used to sign