EventSourcingV2.java

package com.example.demo;

import lombok.Value;

/**
 * This example implements event sourcing using with versioning. It uses
 *
 * Each version has its own event interface (e.g. {@link EventV1} and {@link EventV2}) that extends the {@link Event}.
 *
 * In order to apply {@link Event} on a {@link State} the visitor pattern is used. Each version also define its own visitor interface (e.g. {@link VisitorV1} and {@link VisitorV2}). All visitors are aggregated with the {@link Visitor} interface.
 *
 * Each versioned visitor has one method for each {@link Event} of the same version.
 *
 * For 2 versioned events this solution has :
 * <ol>
 *     <li>12 types</li>
 *     <li>2 types for the state (not important for the solution)</li>
 * </ol>
 *
 * The main drawback is that it is a heavy solution. Lot of code and mutable state (if we want to keep the visitor reusable)!
 * The main advantage is that there is a compile time check when creating new event (impossible to miss something).
 */
public class EventSourcingV2 {

    interface Event {
        String getLoanApplicationReference();

        void accept(Visitor visitor);

        default String getVersion() {
            return "V1";
        }
    }

    interface EventV1 extends Event {
        @Override
        default String getVersion() {
            return "V1";
        }
    }

    @Value
    static class CreatedV1 implements EventV1 {

        private String loanApplicationReference;

        @Override
        public void accept(Visitor visitor) {
            visitor.visit(this);
        }
    }

    @Value
    static class GrantedV1 implements EventV1 {

        private String loanApplicationReference;

        @Override
        public void accept(Visitor visitor) {
            visitor.visit(this);
        }
    }

    interface EventV2 extends Event {
        @Override
        default String getVersion() {
            return "V2";
        }
    }

    @Value
    static class CreatedV2 implements EventV2 {

        private String loanApplicationReference;

        @Override
        public void accept(Visitor visitor) {
            visitor.visit(this);
        }

    }

    @Value
    static class GrantedV2 implements EventV2 {

        private String loanApplicationReference;

        @Override
        public void accept(Visitor visitor) {
            visitor.visit(this);
        }
    }

    interface VisitorV1 {
        void visit(CreatedV1 e);

        void visit(GrantedV1 e);

        void visitAll(EventV1 eventV1);
    }
    interface VisitorV2 {
        void visit(CreatedV2 e);

        void visit(GrantedV2 e);

        void visitAll(EventV2 eventV2);
    }

    interface Visitor extends VisitorV1, VisitorV2 {

    }

    interface State {
        boolean isGranted();
    }

    @Value
    static class Data implements State {

        boolean isGranted;

    }

    static void apply(Event e, State s) {
        e.accept(new Visitor() {

            private VisitorImplV1 v1 = new EventSourcingV2.VisitorImplV1();

            private VisitorImplV2 v2 = new EventSourcingV2.VisitorImplV2();

            @Override
            public void visit(CreatedV1 e) {
                v1.visit(e);
            }

            @Override
            public void visit(GrantedV1 e) {
                v1.visit(e);
            }

            @Override
            public void visitAll(EventV1 e) {
                v1.visitAll(e);
            }

            @Override
            public void visit(CreatedV2 e) {
                v2.visit(e);
            }

            @Override
            public void visit(GrantedV2 e) {
                v2.visit(e);
            }

            @Override
            public void visitAll(EventV2 e) {
                v2.visitAll(e);
            }
        });
    }

    static class VisitorImplV1 implements VisitorV1 {

        @Override
        public void visit(CreatedV1 e) {

        }

        @Override
        public void visit(GrantedV1 e) {

        }

        @Override
        public void visitAll(EventV1 eventV1) {

        }
    }

    static class VisitorImplV2 implements VisitorV2 {

        @Override
        public void visit(CreatedV2 e) {

        }

        @Override
        public void visit(GrantedV2 e) {

        }

        @Override
        public void visitAll(EventV2 eventV2) {

        }
    }

}