Avro Decorators

Typescript class decorators to model your avro schema

Getting Started

To model your avro schema as follows to generate avsc files from it:

@Record()
export class Fruit {
  @AvroInt()
  id: number

  @AvroString({ fieldDoc: 'The name of the fruit' })
  name: string
}

you need to setup the following

• install avro-decorators

  $ npm install -D avro-decorators
  #   or
  $ yarn add -D avro-decorators

• create a configuration file avro-decorators.config.ts with the following content:

  import { Config } from 'avro-decorators'
  import { Fruit } from './fruit.model'
  
  const config: Config = {
  models: [
    // reference your models here
    { class: Fruit },
    // optionally declare an avscFileName for each model
  ],
  // each referenced model will be written to an avsc file
  outDir: 'src/example/schemas',
  }
  
  export default config

• finally add the generate script to your package.json and run it

  {
  // package.json
  "scripts": {
    "generate-avro-models": "avro-decorators generate"
  }
  }

  $ npm run generate-avro-models
  #   or
  $ yarn generate-avro-models

Configuration

Avro Decorators requires a configuration file written in TypeScript, to ensure the models have applied the decorators accordingly to read the required metadata.

The models array in the config is mandatory. Each model requires class - a reference to the TypeScript class, and an optional filename avscFileName to name the schema output file.

Additionally, an output directory outDir can be declared as seen above. If it is not specified, the generated schemas will be printed to stdout instead.

Locating config not in root

By default, Avro Decorators will check the current working directory for the file avro-decorators.config.ts. If your config is located in a different folder, pass it to the program using the flag --config <path> or -c <path>.

Usage

Note that fields not decorated with Avro-decorators will not be part of the schema. This gives you full transparency and control to declare the schema.

Namespace

Declare a namespace for a record as seen in the following example. If you want to use a model name different than the class name, you can use the name property.

For enum and fixed fields you can also declare them in the field decorator.

@Record({
  namespace: 'fruits.meta',
  name: 'FruitModel',
})
export class Fruit {
  @AvroEnum({
    namespace: 'fruits.data',
    name: 'FruitType',
    symbols: fruitTypes,
  })
  fruitType: FruitType
}

Different field or record name in schema than in class

To use a different field name in the schema than in the class, you can use the decorator property fieldName:

  @AvroString({ fieldName: 'fieldNameInSchema' })
  fieldNameInClass: string

Nested Records

To use a record inside another record on a field type, you should declare both records independently and then reference it on the field. It will then be inlined in the schema avsc file:

@Record()
export class Address {
  @AvroString()
  street: string
}

@Record()
export class User {
  @AvroRecord({ ofType: () => Address })
  address: Address
}

Reference schema by name

Referencing by name works using @AvroReferenceByName:

@Record()
export class Fruit {
  @AvroReferenceByName({
    referencedTypeName: 'MyReferencedType',
  })
  field: unknown
}

This will result in the schema

{
  "name": "Fruit",
  "type": "record",
  "fields": [
    {
      "name": "field",
      "type": "MyReferencedType"
    }
  ]
}

Note that there is no validation if that referenced type actually exists anywhere.

Unions

Note that if you just want to add a null type to a field, you can always use the nullable property:

@Record()
export class Fruit {
  @AvroString({ nullable: true })
  field: string | null
}

To express more complex unions, use the @AvroUnion decorator. It requires a second argument, which is an array of all referenced union types.

For map, array, enum and fixed, the array element is an object with a single key indicating the type, e. g. like this for enums:

@AvroUnion({}, ['null', { enum: { name: 'EnglishCount', symbols: ['one', 'two', 'three'] } }])

More extensive example:

@Record()
export class Address {
  /* ... */
}

@Record()
export class Model {
  @AvroUnion(
    {
      fieldDoc:
        'Can be an int, a string, null, an address, a map of strings or an array of longs',
      fieldDefault: 0,
    },
    [
      'int',
      'string',
      'null',
      () => Address,
      { map: { values: 'string' } },
      { array: { items: 'long', default: [] } },
    ]
  )
  field: number | string | null | Address | Record<string, string> | number[]
}

Features not supported yet

• Top-level non-record (e. g. enum or fixed)
• Validation of name and namespace according to specification
• Custom tsconfig for complex model compilation

Apache Avro Version

This library was developed in accordance with the Apache Avro™ 1.10.2 Specification.