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Resolvers

Resolvers are the functions that return the data of the fields when you send a GraphQL query to the Slicknode API. You can write your own custom resolvers as handlers in your Slicknode modules and extend any object type of your application, enrich it with data from 3rd party APIs, implement custom logic or easily embed existing legacy APIs in your Slicknode application.

This enables you to use Slicknode with pretty much any existing database and API.

Creating a Resolver

To create a new resolver in Slicknode, define in the slicknode.yml file of the module which handler should be executed for which field. The configuration for the resolvers is defined in a two dimensional map that includes the type name and the field name:

modules/my-module/slicknode.yml:

# Runtime needs to be enabled for code execution
runtime:
  engine: nodejs@8

# Resolver map
resolvers:
  # The type to add the fields to. `Query` is always the root query type
  Query:
    # The field name (prepended with the module namespace to avoid name collisions)
    MyModule_hello:
      # Path to the JS source file where the custom resolver logic is implemented (within module)
      handler: src/resolvers/Query.MyModule_hello

For the field to be exposed via the GraphQL API we have to add it to the schema.graphql file of the module and specify the return types and potential input arguments:

modules/my-module/schema.graphql:

extend type Query {
  MyModule_hello(name: String): String
}

This adds a field with the name MyModule_hello to the Query type that takes a name with input type String as an input argument.

Last we can implement the resolver as a simple handler with plain javascript:

modules/my-module/src/resolvers/Query.MyModule_hello.js:

module.exports = async function(payload, context) {
  // Call external APIs, process data etc.

  // We return the name that was provided as an input argument. 
  // If we don't have an input argument, we greet the stranger...
  return {
    data: 'Hello ' + (payload.args.name || 'Stranger')
  };
};

Querying the Resolver

With the resolver in place we can fetch the data via the Slicknode GraphQL endpoint like for any other field:

query SayHello {
  MyModule_hello(name: "John")
}