schemas.md

Schemas

Some output plugins does not have a pre-defined output structure.

Consider when writing to kafka, you probably have a pre-defined internal data structure for the messages being consumed from the queue.

For this purpose ffwd provides schemas.

These are pluggable components that knows how to dump the internal data structures of ffwd into a specific structure of a specific content type.

The use of a specific schema looks like the following in the configuration file.

:output:
  - :type: "kafka"
    :schema: "my-schema-v01"
    :content_type: "application/json"

To list all availble schemas and content types, use ffwd --schemas.

This assumes that my-schema-v01 is provided by a component which has been loaded.

Writing a schema is very straight forward, it's only a matter of placing a component under lib/ffwd/schema/my_schema.rb in your own ruby gem.

require_relative 'ffwd/schema'

require 'json'

module FFWD::Schema
  module MySchema
    include FFWD::Schema

    module ApplicationJSON01
      def self.dump_metric m
        JSON.dump m.to_h
      end

      def self.dump_event e
        JSON.dump e.to_h
      end
    end

    module ApplicationJSON02
      def self.dump_metric m
        h = m.to_h
        h[:my_field] = 42
        JSON.dump h
      end

      def self.dump_event e
        JSON.dump e.to_h
      end
    end

    register_schema 'my-schema-v01', 'application/json', ApplicationJSON01
    register_schema 'my-schema-v02', 'application/json', ApplicationJSON02
  end
end