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META SPEC: Metadata Ontology

shortname:      DID
name:           Metadata Ontology
type:           Standard
status:         Valid
version:        0.1
editor:         Aitor Argomaniz <aitor@nevermined.io>
contributors:   

Motivation

Every asset (dataset, algorithm) in Nevermined has an associated Decentralized Identifier (DID) and DID document / DID Descriptor Object (DDO). Because assets without proper descriptive metadata have poor visibility and discoverability.

See DID SPEC for information about the overall structure of Nevermined DDOs and DIDs.This OEP is about one particular part of Nevermined DDOs: the asset metadata, a JSON object with information about the asset.

This SPEC defines the assets metadata ontology, i.e. the schema for the asset metadata. It's based on the public schema.org DataSet schema.

This SPEC doesn't detail the exact method of registering assets on-chain or storing DDOs.

The main motivations of this SPEC are to:

  • Specify the common attributes that MUST be included in any asset metadata stored in Nevermined networks
  • Normalize the attributes to use in any curation process, to provide a common structure to sort and filter the DDOs
  • Identify the recommended additional attributes that SHOULD be included in a DDO to facilitate asset search
  • Provide an example of an asset metadata object and additional links for reference

Life Cycle of Metadata

Local Metadata

Metadata is first created by the publisher of the asset. The publisher has knowledge of the file URLs, and they are stored in plaintext in the files object. This initial metadata is the local metadata.

Remote Metadata

A publisher publishes (registers) an asset using Nevermined SDKs, which might be running on their local machine or remotely. When they do, the local metadata is passed to the SDK, which makes some changes and additions in the metadata, puts it into a DDO, and sends that DDO to a metadata API.

The Metadata API may also make some changes and additions to the metadata, such as the datePublished or parts of the curation object. The metadata that finally gets stored by the Metadata API is the remote metadata.

A marketplace can and might also act as a publisher. SPEC ACCESS describes the publishing flow in more detail.

Metadata Attributes

An asset is the representation of different type of resources in Nevermined. Typically can asset could be one of the following asset types:

  • Dataset. An asset representing a dataset or data resource. It could be for example a CSV file or a multiple JPG files.
  • Algorithm. An asset representing a piece of software. It could be a python script using tensorflow, a spark job, etc.

Each kind of asset require a different subset of metadata attributes. The distinction between the type of asset (dataset, algorithm) is given by the attribute DDO.services["metadata"].main.type

A metadata object has the following attributes, all of which are objects.

Attribute Required Description
main Yes Main attributes used to calculate the service checksum
curation (remote) Curation attributes
additionalInformation No Optional attributes
encryptedFiles (remote) Encrypted string of the attributes.main.files object.
encryptedServices (remote) Encrypted string of the attributes.main.services object.

The main, curation and additionalInformation attributes are independent of the asset type, all assets have those metadata sections.

Main Attributes

This list of attributes can't be modified after creation, because these are considered as the metadata essence of the asset created. This information is used to calculate the unique checksum of the asset. If any change would be necessary in the following attributes, it would be necessary to create a new asset derived from the existing one.

The main object has the following attributes, not all are required. Some are required by only the metadata store (remote) and others are mandatory for local metadata only. If required or not by both, they are marked with Yes/No in the Required column.

Attribute Type Required Description
name Text Yes Descriptive name or title of the asset.
type Text Yes Type of the asset. Helps to filter by the type of asset. It could be for example ("dataset", "algorithm").
dateCreated DateTime Yes The date on which the asset was created by the originator. ISO 8601 format, Coordinated Universal Time, e.g. 2019-01-31T08:38:32Z.
datePublished DateTime (remote) The date on which the asset DDO is registered into the metadata store (Metadata API)
author Text Yes Name of the entity generating this data (e.g. Tfl, Disney Corp, etc.).
license Text Yes Short name referencing the license of the asset (e.g. Public Domain, CC-0, CC-BY, No License Specified, etc. ). If it's not specified, the following value will be added: "No License Specified".
price String Yes Price of the asset. It must be an integer encoded as a string, e.g. "123000000000000000000".
files Array of files object Yes Array of File objects including the encrypted file urls. Further metadata about each file is stored, see File Attributes

File Attributes

File attributes are a subset of the main section.

A file object has the following attributes, with the details necessary to consume and validate the data.

Attribute Required Description
url (local) Content URL. Omitted from the remote metadata. Supports http(s):// and ipfs:// URLs.
name no File name.
index yes Index number starting from 0 of the file.
contentType yes File format.
checksum no Checksum of the file using your preferred format (i.e. MD5). Format specified in checksumType. If it's not provided can't be validated if the file was not modified after registering.
checksumType no Format of the provided checksum. Can vary according to server (i.e Amazon vs. Azure)
contentLength no Size of the file in bytes.
encoding no File encoding (e.g. UTF-8).
compression no File compression (e.g. no, gzip, bzip2, etc).
encrypted no Boolean. Is the file encrypted? If is not set is assumed the file is not encrypted
encryptionMode no Encryption mode used. Just valid if encrypted=true
resourceId no Remote identifier of the file in the external provider. It is typically the remote id in the cloud provider.
attributes no Key-Value hash map with additional attributes describing the asset file. It could include details like the Amazon S3 bucket, region, etc.

Additional Attributes

All the additional information will be stored as part of the additionalInformation section.

Attribute Type Required
categories Array of Text No
tags Array of Text No
description Text No
copyrightHolder Text No
workExample Text No
links Array of Link No
inLanguage Text No

Other Suggested Additional Attributes

These are examples of attributes that can enhance the discoverability of a resource:

Attribute Description
sla Service Level Agreement.
industry
updateFrequency An indication of update latency - i.e. How often are updates expected (seldom, annually, quarterly, etc.), or is the resource static that is never expected to get updated.
termsOfService
privacy
keyword A list of keywords/tags describing a dataset.
structuredMarkup A link to machine-readable structured markup (such as ttl/json-ld/rdf) describing the dataset.

The publisher of a DDO MAY add additional attributes or change the above object definition.

Curation Attributes

A curation object has the following attributes.

Attribute Type Required Description
rating Number (decimal) Yes Decimal value between 0 and 1. 0 is the default value.
numVotes Integer Yes Number of votes. 0 is the default value.
schema Text No Schema applied to calculate the rating.
isListed Boolean No Flag unsuitable content. False by default. If it's true, the content must not be returned.

Example of Local Metadata

{  
  "main": {
    "name": "Madrid Weather forecast",
    "dateCreated": "2019-05-16T12:36:14.535Z",
    "author": "Norwegian Meteorological Institute",
    "type": "dataset",
    "license": "Public Domain",
    "price": "123000000000000000000",
    "files": [  
      {
        "index": 0,
        "url": "https://example-url.net/weather/forecast/madrid/350750305731.xml",
        "contentLength": "0",
        "contentType": "text/xml",
        "compression": "none"
      }
    ]
  },
  "additionalInformation":{  
    "description": "Weather forecast of Europe/Madrid in XML format",
    "copyrightHolder": "Norwegian Meteorological Institute",
    "categories": ["Other"],
    "links": [],
    "tags": [],
    "updateFrequency": null,
    "structuredMarkup": []
  }
}

Example of Remote Metadata

Similarly, this is how the metadata file would look as a response to querying Metadata API (remote metadata). Note that url is removed from all objects in the files array, and encryptedFiles & curation are added.

{  
  "service": [
    {  
      "index": 0,
      "serviceEndpoint": "http://metadata:5000/api/v1/metadata/assets/ddo/{did}",
      "type": "metadata",
      "attributes": {  
        "main": {  
          "type": "dataset",
          "name": "Madrid Weather forecast",
          "dateCreated": "2019-05-16T12:36:14.535Z",
          "author": "Norwegian Meteorological Institute",
          "license": "Public Domain",
          "price": "123000000000000000000",
          "files":[  
            {  
              "contentLength": "0",
              "contentType": "text/xml",
              "compression": "none",
              "index": 0
            }
          ],          
          "datePublished": "2019-05-16T12:41:01Z"
        },
        "encryptedFiles": "0x7a0d1c66ae861…df43aa9",
        "curation":{  
          "rating": 1,
          "numVotes": 7,
          "schema": "BINARY",
          "isListed": true
        },
        "additionalInformation": {  
          "description": "Weather forecast of Europe/Madrid in XML format",
          "copyrightHolder": "Norwegian Meteorological Institute",
          "categories": ["Other"],
          "links": [],
          "tags": [],
          "updateFrequency": null,
          "structuredMarkup": []
        }
      }
    }
  ]
}

Specific attributes per asset type

Depending on the asset type (dataset, algorithm), there are different metadata attributes supported:

Algorithm attributes

An asset of type algorithm has the following additional attributes under main.algorithm:

Attribute Type Required Description
language string no Language used to implement the software
format string no Packaging format of the software.
version string no Version of the software.
container Object yes Object describing the Docker container image.

The container object has the following attributes:

Attribute Type Required Description
entrypoint string yes The command to execute, or script to run inside the Docker image.
image string yes Name of the Docker image.
tag string yes Tag of the Docker image.
{
      "index": 0,
      "serviceEndpoint": "http://localhost:5000/api/v1/metadata/assets/ddo/{did}",
      "type": "metadata",
      "attributes": {
        "main": {
          "author": "John Doe",
          "dateCreated": "2019-02-08T08:13:49Z",
          "license": "CC-BY",
          "name": "My super algorithm",
          "price": "1",
          "type": "algorithm",
          "algorithm": {
            "language": "scala",
            "format": "docker-image",
            "version": "0.1",
            "container": {
              "entrypoint": "node $ALGO",
              "image": "node",
              "tag": "10"
            }
          },
          "files": [
            {
              "name": "build_model",
              "url": "https://raw.githubusercontent.com/keyko-io/test-algorithm/master/javascript/algo.js",
              "index": 0,
              "checksum": "efb2c764274b745f5fc37f97c6b0e761",
              "contentLength": "4535431",
              "contentType": "text/plain",
              "encoding": "UTF-8",
              "compression": "zip"
            }
          ]
        },
        "additionalInformation": {
          "description": "Workflow to aggregate weather information",
          "tags": [
            "weather",
            "uk",
            "2011",
            "workflow",
            "aggregation"
          ],
          "copyrightHolder": "John Doe"
        }
      }
    }

References

Schema.org is a collaborative, community activity with a mission to create, maintain, and promote schemas for structured data on the Internet. Data types use the Schema.org primitive data types.