Declarative resolvers

Declarative resolvers use changes to the mesh configuration to customize resolved values. To make these changes programmatically using JavaScript, see Programmatic resolvers.

You can also use custom resolvers to batch repeated queries and get better response times. For more information, see Batching.

Declare a resolver to the new additionalTypeDefs by using additionalResolvers

The additionalResolvers field will make our new field executable in the unified schema:

{
  "sources": [
    {
      "name": "StackExchange",
      "handler": {
        "openapi": {
          "source": "https://raw.githubusercontent.com/grokify/api-specs/master/stackexchange/stackexchange-api-v2.2_openapi-v3.0.yaml"
        }
      }
    }
  ],
  "additionalTypeDefs": "extend type Query {\n  listQuestionsFromStackOverflow(first: Int!): [Question]\n}\n",
  "additionalResolvers": [
    {
      "targetTypeName": "Query",
      "targetFieldName": "listQuestionsFromStackOverflow",
      "sourceName": "StackExchange",
      "sourceTypeName": "Query",
      "sourceFieldName": "listQuestions",
      "sourceArgs": {
        "pagesize": "{args.first}"
      },
      "result": "items"
    }
  ]
}

Configuration options

• The target (targetTypeName, targetFieldName) - describes the queried field.
• The source (sourceName, sourceTypeName, sourceFieldName) - describes where the data is resolved for the target field.
• requiredSelectionSet fetches the required arguments.
• sourceArgs maps the requiredSelectionSet argument to the source.
• keysArg provides the name of the primary key argument. For this example, the keysArg field is the argument name used when sending an array of SKUs to fetch multiple reviews.
• keyField provides the key-value for each item in the batched query. For this example, the keyField indicates which Product field provides the SKU value to the review service.

Combining schemas using declarative API

We learned that we can combine multiple APIs in a mesh using additionalTypeDefs and additionalResolvers.

The following example has two different OpenAPI sources. We will add two new fields to the Cities type, and those fields have return types from the Weather API.

To achieve this, we will use additionalResolvers inside the mesh configuration file.

{
  "sources": [
    {
      "name": "Cities",
      "handler": {
        "openapi": {
          "source": "https://api.apis.guru/v2/specs/mashape.com/geodb/1.0.0/swagger.json",
          "operationHeaders": {
            "X-RapidAPI-Key": "a12b3c456defg78hij9kl0123m4no5pqr6stuv789wxyz01a23"
          }
        }
      }
    },
    {
      "name": "Weather",
      "handler": {
        "openapi": {
          "source": "https://api.apis.guru/v2/specs/weatherbit.io/2.0.0/swagger.json"
        }
      }
    }
  ],
  "additionalTypeDefs": "extend type PopulatedPlaceSummary {\n  dailyForecast: [Forecast]\n  todayForecast: Forecast\n}\n",
  "additionalResolvers": [
    {
      "targetTypeName": "PopulatedPlaceSummary",
      "targetFieldName": "dailyForecast",
      "requiredSelectionSet": "{\n  latitude\n  longitude\n}\n",
      "sourceName": "Weather",
      "sourceTypeName": "Query",
      "sourceFieldName": "getForecastDailyLatLatLonLon",
      "sourceArgs": {
        "lat": "{root.latitude}",
        "lon": "{root.longitude}",
        "key": "{context.headers['x-weather-api-key']}"
      },
      "result": "data"
    },
    {
      "type": "PopulatedPlaceSummary",
      "field": "todayForecast",
      "requiredSelectionSet": "{\n  latitude\n  longitude\n}\n",
      "sourceName": "Weather",
      "sourceTypeName": "Query",
      "sourceFieldName": "getForecastDailyLatLatLonLon",
      "sourceArgs": {
        "lat": "{root.latitude}",
        "lon": "{root.longitude}",
        "key": "{context.headers['x-weather-api-key']}"
      },
      "result": "data[0]"
    }
  ]
}