Core Architecture


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    This document gives an overview of the core architecture of Comunica, which gives us the desired modularity and flexibility.

    This core architecture has been implemented in @comunica/core.

    On top of this architecture, the more high-level SPARQL architecture has been defined.

    Core components: Actor, Mediator, and Bus

    Comunica's architecture has been designed with flexibility and loose coupling of components as main goals. For this, Comunica consists of three types of components: actors, mediators, and buses.

    All logic in Comunica is separated into different actors (Actor), following the actor model. Each actor independently performs a specific task. For example, one actor can take implement the SPARQL UNION operator, another actor can parse JSON-LD documents, and another actor can parse JSON-LD documents in a different way.

    All actors are subscribed onto task-specific buses (Bus), following the publish-subscribe pattern. For example, a SPARQL query operator bus could contain actors for UNION, SELECT, FILTER, and more. An RDF parsing bus could contain actors for JSON-LD, RDFa, Turtle, and more.

    Since multiple actors can exist for solving a specific task (for example if we have two actors for parsing JSON-LD documents), mediators (Mediator) are used for determining the "best" actor on a bus for executing a certain action, following the mediator pattern.

    To ensure loose coupling of components, actors never communicate with each other directly. Instead, they always communicate via a mediators and buses, as shown in the following figure:

    Actor Mediator Bus Architecture
    With Observers, you can passively observe actions executed by actors on a given bus.
    Optionally, actors can log certain events.

    Run and test phases for selecting an actor

    Different mediators can select actors in different ways. For this, the mediator will go through two phases:

    1. Test phase: The action is sent onto the bus to all subscribed actors. The actors return the estimated conditions under which the action could be executed, without actually executing the action.
    2. Run phase: The action is sent to a single actor for execution, where this actor is chosen by the mediator based on the returned test conditions.

    For instance, the following figure shows an example of a mediator that will always pick the fastest actor on the bus as possible.

    Run and test phase

    Therefore, an actor must at least expose the following methods:

    export interface Actor {
      test(action: IAction): Promise<IActorTest>;
      run(action: IAction): Promise<IActorOutput>;
    }

    The interfaces IAction, IActorTest, and IActorOutput depend on the bus this actor is subscribed to.

    Learn more about the actors, buses and mediators that exist in Comunica.

    Wiring of components

    All Comunica actors, buses, and mediators are implemented as separate npm packages. In order to wire these different components with each other in a single application, we make use of the dependency injection framework Components.js. Components.js allows us to wire components with each other using one or more configuration files. Plugging in different components therefore do not require any code changes, but simply a config change.

    Considering these different types of components, we make use of the following naming conventions for packages:

    • Buses: @comunica/bus-[name-of-bus-type]
    • Mediators: @comunica/mediator-[name-of-mediator]
    • Actors: @comunica/actor-[name-of-bus-type]-[name-of-actor]
    • Mediator types: @comunica/mediatortype-[name-of-mediator-type]