External Data

Overview

Some resources may be held in external systems, typically accessed via a REST style API.

These are referred to as ‘external Dao’s and must at a minimum implement the ReaderDao.

Currently only one implementation is supported, ConfigurableRestDocDao. As the name suggest a configuration can be injected in change URL mapping and data mappings. Future enhancements may allow the use of custom implementations.

Setting up a new external DAO

Update the GraphQL schema

The type information must be added to the schema. Below is a snippet with the extra data to include starships. See ‘starwars_ex.graphqls’ for a full example.

type Query {
  starship(id: ID!): Starship
}

type Starship {
    id: ID!
    name: String!
    manufacturer : String
    model : String
    lengthInMetres : String
    costInCredits : String
}

Register the DAO

Information is stored in events. Two are required.

Register Event

The register event is enough for the application to know that the doc ‘Starship’ is now access as an external read only resource, in this case using the ‘ConfigurableRestDocDao’

{"aggregateId":"Starship",
 "creator":"graph-store",
 "payload":{"implementingClass":"ConfigurableRestDocDao"},
  "id":"8c3db09c-3043-46b3-972d-ae3237304ea8",
  "type":"ExternalDaoRegistered",
  "timestamp":1532798376422}

Configure Event(s)

These setup any configuration required. By convention all external DAOs should just accept a Map of available options, in this case:

baseUrl - the root for building a REST query
resultMapperScript - a groovy script that maps the external attributes to those needed by the GraphQL schema

{"aggregateId":"Starship",
 "creator":"graph-store",
 "payload":{"baseUrl":"https://swapi.co/api/starships/",
            "resultMapperScript":"import ianmorgan.graphstore.mapper.MapperHelper;\n\ndef helper = new MapperHelper(raw)\nhelper.copyIfExists('name')\nhelper.copyIfExists('manufacturer')\nhelper.copyIfExists('model')\nhelper.copyIfExists('length','lengthInMetres')\nhelper.copyIfExists('cost_in_credits','costInCredits')\nreturn helper.output() "},
 "id":"ba6490d2-d421-4a11-b49b-563d72ef59e9",
 "type":"ExternalDaoConfigured",
 "timestamp":1532798376423}

The ‘ExternalDaoRegistry’ class has helper methods for generating these events.

Future

The intention is to support custom implementations in the future. This would likely require the introduction of a DI framework such as Google Guava or Spring.