Spine Event Engine

Step 1. Define rich, type‑safe domain model

Describe commands, events, and state of entities using Protobuf.

// A command to craete a new task.
message CreateTask {
    TaskId id = 1;
    string name = 2 [(required) = true];
    string description = 3;
}
...

// A new task has been created.
message TaskCreated {
    TaskId id = 1;
    string name = 2 [(required) = true];
    string description = 3;
}
...

// A state of the Task aggregate.
message Task {
    TaskId id = 1;
    string name = 2 [(required) = true];
    string description = 3;
    DeveloperId assignee = 4;
}

Step 3. Code business logic in straight and testable way

Focus on the business logic rather than “plumbing”. A Command will be delivered to only one Aggregate. Projections will get all Events they need. ProcessManagers will cover more complex scenarios. Storage, message delivery, and other environment matters are isolated from the main code.

final class TaskAggregate
    extends Aggregate<TaskId, Task, TaskVBuilder> {
    ...
    @Assign
    TaskCreated handle(CreateTask cmd, CommandContext ctx) {
        return TaskCreated
                .newBuilder()
                .setId(cmd.getId())
                .setName(cmd.getName())
                .setOwner(ctx.getActor())
                .build();
    }
    ...
}

final class TaskProjection
    extends Projection<TaskId, TaskItem, TaskItemVBuilder> {
    ...
    @Subscribe
    void on(TaskCreated e) {
        getBuilder().setId(e.getId())
                    .setName(e.getName())
    }

    @Subscribe
    void on(TaskCompleted e, EventContext ctx) {
        getBuilder().setWhenDone(ctx.getTimestamp());
    }
}