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Schema Language

A method of expressing the contents of a schema that is a combination of syntax (usually a unique syntax for a given schema language) with some semantics particular to writing schema.

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Deliverable

Task 43 has been evaluating different Schema Languages based on their features and the needs of the community. The comparisons table is herearrow-up-right - a fuller report with better descriptions of the features and why they're relevant is pending.

Examples of Schema Languages include:

  • JSONSchema

  • XMLSchema (XSD)

  • RDFSchema (RDFS)

  • AvroSchema,

  • English*,

  • German*,

  • and many others…

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*Note

A Schema Language could simply be a plain language like English or German - although a linguistic solution struggles to meet FAIR Principles, being difficult to search and hard for machines to parse unambiguously.

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Some examples of schema in different languages

Let's take data in JSON format:

{
  "productId": 1,
  "productName": "A green door",
  "price": 12.50,
  "tags": [ "home", "green" ]
}

While generally straightforward, the example leaves some open questions. Here are just a few of them: What is productId? Is productName required? Can the price be zero (0)? Are all of the tags string values?

JSON Schema is a Schema language that is a proposed IETF standard for how to answer such questions for data. A schema for the example above can be:

{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "$id": "https://example.com/product.schema.json",
  "title": "Product",
  "description": "A product from Acme's catalog",
  "type": "object",
  "properties": {
    "productId": {
      "description": "The unique identifier for a product",
      "type": "integer"
    },
    "productName": { "description": "Name of the product", "type": "string" },
    "price": {
      "description": "The price of the product",
      "type": "number",
      "exclusiveMinimum": 0
    },
    "tags": {
      "description": "Tags for the product",
      "type": "array",
      "items": { "type": "string" },
      "minItems": 1,
      "uniqueItems": true
    }
  },
  "required": ["productId", "productName", "price"]
}
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