Adding a config parameter to an actor

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    In this guide, we will add a parameter to an existing actor, and show how to set values for this parameter via the config file.

    We will start from the actor we have created in the guide on contributing a new actor.

    1. Modifying the constructor

    We want to add a parameter that is set at configuration/startup time. For this, we need to make sure that our actor accepts this parameter via the constructor.

    For this, first create a new interface that is used as single argument in the constructor:

    export interface IActorQueryOperationReducedMyArgs extends IActorQueryOperationTypedMediatedArgs {
      myParam: number;

    Here, IActorQueryOperationTypedMediatedArgs is the default constructor argument for query operation actors that contains common parameters that will automatically be set behind the scenes.

    Next, replace our constructor with the following:

    public constructor(args: IActorQueryOperationReducedMyArgs) {
      super(args, 'reduced');

    In order to use the passed parameter values, add the following field in your class:

    private readonly myParam: number;

    In order to temporarily check the passed parameter value, we can add a console.log statement in the runOperation method.

    Make sure to run yarn run build in the repo root to make sure that your modifications to the TypeScript files have been compiled to JavaScript.

    2. Set values in our config file

    Everything has now been setup to define values for our parameter via the config file.

    As such, we can modify our declaration of our actor in engines/config-query-sparql/config/query-operation/actors/query/reduced.json by adding a value for "myParam":

      "@context": [
      "@id": "urn:comunica:default:Runner",
      "@type": "Runner",
      "actors": [
          "@id": "urn:comunica:default:query-operation/actors#reduced",
          "@type": "ActorQueryOperationReducedMy",
          "mediatorQueryOperation": { "@id": "urn:comunica:default:query-operation/mediators#main" }
          "myParam": 123

    As a test, you can now attempt a query execution with our config. If you placed a console.log statement in your actor, you should now see the value 123 on stdout.

    In this guide, we showed how to define an integer parameter. You can instead also define other parameter types, where parameters can even accept other components (such as mediators).
    When running yarn run build, a JSON-LD representation of your TypeScript files will be created in the components/ directory of your package. The components/context.jsonld will list all discovered parameters that you can pass within the config file.