EventFabric Core

The core package of the EventFabric framework — a small, CloudEvents-based CQRS toolkit for TypeScript. It provides typed Commands, Queries and Events, a validating Router and an in-process EventBus with retries and OpenTelemetry instrumentation.

Refer to the EventFabric main repository or the EventFabric documentation for more information about the EventFabric framework.

Install

# Deno
deno add npm:@eventfabric-cqrs/core

# NPM
npm install @eventfabric-cqrs/core

# Bun
bun add @eventfabric-cqrs/core

Examples

The snippets below walk through a tiny "todo" domain so you can see how the core building blocks fit together. Each example is runnable on its own.

For detailed documentation, please refer to the EventFabric documentation.

Command

A Command asks the system to do something (a write). You declare a Zod schema that extends Nimbus' built-in commandSchema, write a handler, and register both on the router. Incoming messages are validated against the schema before they reach your handler.

import { commandSchema, createCommand, getRouter } from "@eventfabric-cqrs/core";
import { z } from "zod";

const ADD_TODO = "com.example.todo.add";

const addTodoSchema = commandSchema.extend({
    type: z.literal(ADD_TODO),
    data: z.object({
        title: z.string().min(1),
    }),
});
type AddTodoCommand = z.infer<typeof addTodoSchema>;

const handleAddTodo = async (cmd: AddTodoCommand) => {
    // ...persist the todo here...
    return { id: "todo-1", title: cmd.data.title, status: "open" };
};

const router = getRouter();
router.register(ADD_TODO, handleAddTodo, addTodoSchema);

const result = await router.route(
    createCommand<AddTodoCommand>({
        type: ADD_TODO,
        source: "https://app.example.com",
        data: { title: "Write the README" },
    })
);

console.log(result);
// { id: 'todo-1', title: 'Write the README', status: 'open' }

createCommand fills in the boilerplate CloudEvents fields (id, correlationid, time, …) for you, so you only specify the parts that matter to your domain.

Query

A Query asks the system to read something. Mechanically it is identical to a Command — same router, same validation, same shape — only the intent differs.

import { createQuery, getRouter, querySchema } from "@eventfabric-cqrs/core";
import { z } from "zod";

const GET_TODO = "com.example.todo.get";

const getTodoSchema = querySchema.extend({
    type: z.literal(GET_TODO),
    data: z.object({ id: z.string() }),
});
type GetTodoQuery = z.infer<typeof getTodoSchema>;

const handleGetTodo = async (q: GetTodoQuery) => {
    // ...load the todo from your read model...
    return { id: q.data.id, title: "Write the README", status: "open" };
};

const router = getRouter();
router.register(GET_TODO, handleGetTodo, getTodoSchema);

const todo = await router.route(
    createQuery<GetTodoQuery>({
        type: GET_TODO,
        source: "https://app.example.com",
        data: { id: "todo-1" },
    })
);

Event

An Event announces that something has happened. Events are published to the in-process EventBus, which delivers them to every matching subscriber asynchronously, with exponential-backoff retries on handler errors and built-in OpenTelemetry traces and metrics.

import { createEvent, eventSchema, getEventBus } from "@eventfabric-cqrs/core";
import { z } from "zod";

const TODO_ADDED = "com.example.todo.added";

const todoAddedSchema = eventSchema.extend({
    type: z.literal(TODO_ADDED),
    data: z.object({
        id: z.string(),
        title: z.string(),
    }),
});
type TodoAddedEvent = z.infer<typeof todoAddedSchema>;

const eventBus = getEventBus();

eventBus.subscribeEvent<TodoAddedEvent>({
    type: TODO_ADDED,
    handler: async (event) => {
        console.log(`New todo added: ${event.data.title}`);
    },
    onError: (error, event) => {
        console.error(`Failed to handle ${event.id} after retries`, error);
    },
});

eventBus.putEvent(
    createEvent<TodoAddedEvent>({
        type: TODO_ADDED,
        source: "https://app.example.com",
        subject: "/todos/todo-1",
        data: { id: "todo-1", title: "Write the README" },
    })
);

A typical flow is to publish an event from inside a command handler once the write has succeeded — that's how reads, side effects and integrations stay decoupled from the command path.

Router

A typical app configures a single named router at startup with cross-cutting concerns (logging, correlation IDs, …) and then resolves it from anywhere via getRouter().

import { getLogger, getRouter, setupRouter } from "@eventfabric-cqrs/core";

setupRouter("default", {
    logInput: (input) => {
        getLogger().debug({
            category: "Router",
            message: `Routing ${input.type}`,
            correlationId: input.correlationid,
        });
    },
    logOutput: (output) => {
        getLogger().debug({
            category: "Router",
            message: "Handler completed",
            data: { output },
        });
    },
});

// Anywhere else in your app:
const router = getRouter("default");
router.register(/* type, handler, schema */);
await router.route(/* command or query */);

You can have multiple named routers (for example one per transport) by calling setupRouter with different names and resolving them with getRouter('<name>').

License

Copyright 2026 devn.ch

Copyright 2024 Overlap GmbH & Co KG (https://overlap.at)

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.