Queries

Queries represent read operations - requests for information without changing application state.

Queries also fit perfectly into the CQRS pattern (Command Query Responsibility Segregation), where reads and writes are separated for better scalability and maintainability. But keep it simple for your use case and needs. CQRS in an option, but not required.

An in Depth Example

This guide also has an in depth example of a working application built with EventFabric. Combining DDD, CQRS and Event Sourcing.

Check out the In Depth Example page to learn how everything is connected and works out in a real-world application.

Key Characteristics

• Read Operations: Queries fetch data without modifying state
• Idempotent: Multiple executions return the same result (if data hasn’t changed)
• Type-Safe: Queries are fully typed and validated using Zod
• Optimized for Reading: Can use specialized read models or databases

Query Structure

A query in EventFabric follows the CloudEvents specification and consists of:

type Query<TData = unknown> = {
    specversion: "1.0";
    id: string;
    correlationid: string;
    time: string;
    source: string;
    type: string;
    data: TData;
    datacontenttype?: string;
    dataschema?: string;
};

Property	Description
specversion	The CloudEvents specification version (always '1.0')
id	A globally unique identifier for the query
correlationid	A unique identifier to correlate this query with related messages
time	ISO 8601 timestamp when the query was created
source	A URI reference identifying the system creating the query
type	The query type following CloudEvents naming (e.g., at.overlap.nimbus.get-user)
data	The query parameters (e.g., filters, pagination)
datacontenttype	Optional MIME type of the data (defaults to application/json)
dataschema	Optional URL to the schema the data adheres to

Query Schema

EventFabric provides a base Zod schema for validating queries:

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

// Extend the base schema with your specific query type and data
const getUserQuerySchema = querySchema.extend({
    type: z.literal("at.overlap.nimbus.get-user"),
    data: z.object({
        id: z.string().length(24),
    }),
});

type GetUserQuery = z.infer<typeof getUserQuerySchema>;

Create Queries

You can create queries using the createQuery() helper:

import { createQuery } from "@eventfabric-cqrs/core";
import { GetUserQuery } from "./getUser.query.ts";

const query = createQuery<GetUserQuery>({
    type: "at.overlap.nimbus.get-user",
    source: "nimbus.overlap.at",
    data: {
        id: "123",
    },
});

The createQuery() helper automatically generates default values for:

• id - A unique ULID
• correlationid - A unique ULID (if not provided)
• time - Current ISO timestamp
• specversion - Always '1.0'
• datacontenttype - Defaults to 'application/json'

Routing Queries

Queries are routed to handlers using the MessageRouter. See the Router documentation for details on registering handlers and routing messages.