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Query

In the CQRS (Command Query Responsibility Segregation) architecture, a Query is the counterpart to a Command. While Commands are used to modify the state of the system, Queries are used to retrieve data.

Queries are designed to work with Projected Entities, which are the results of a Projection process. This ensures that the data returned by Queries is up-to-date and consistent with the projections.

Implementation​

  • Query Class:

    • GetAllQuery: Represents the data request, holding query parameters and ensuring their validity.

  • Query Handler:

    • GetAllQueryHandler: Processes the Query, interacts with the repository to retrieve data, and returns the result.

Query​

A Query is a Data Transfer Object (DTO) that contains the information needed to fetch data from the system. It acts as a request for data retrieval and is passed to the QueryHandler.

A Query can be found at: Task/src/useCases/read/ActiveTaskOverview/application/GetAllQuery.ts.

import { isType } from "is-what";
import { validate } from "uuid";
import { OverwriteProtectionBody, ValidationError } from "@codebricks/codebricks-framework";

export interface GetAllQueryProperties {
}

export class GetAllQuery {
    constructor(readonly properties: GetAllQueryProperties) {
        this.validate();
    }

    @OverwriteProtectionBody(false)
    validate(): void {
    }
}

The GetAllQuery class is responsible for holding query parameters and ensuring their validity through the validate method.

Query Handler​

The QueryHandler processes the Query and interacts with the repository to fetch the required data. It converts the Query into a request that the repository can handle and returns the results.

The QueryHandler can be found at: Task/src/useCases/read/ActiveTaskOverview/application/GetAllQueryHandler.ts.

import { OverwriteProtectionBody } from "@codebricks/codebricks-framework";
import { ActiveTaskOverviewEntity } from "shared/application/readmodels/ActiveTaskOverviewEntity";
import { ActiveTaskOverviewRepository } from "../infrastructure/ActiveTaskOverviewRepository";
import { GetAllQuery } from "./GetAllQuery";

export class GetAllQueryHandler {
    constructor(readonly repository: ActiveTaskOverviewRepository = new ActiveTaskOverviewRepository()) {
    }

    @OverwriteProtectionBody(false)
    async handle(query: GetAllQuery): Promise<ActiveTaskOverviewEntity[]> {
        const result: ActiveTaskOverviewEntity[] = await this.repository.getAll(
        );
        return result;
    }
}

The GetAllQueryHandler class is responsible for:

  • Receiving the GetAllQuery.
  • Using the repository to fetch the data.
  • Returning the results to the caller.

Best Practices​

  • DTO Usage: Use Query classes as DTOs to encapsulate query parameters and validation logic.
  • Separation of Concerns: Keep Query and QueryHandler responsibilities distinct to maintain clarity and manageability.
  • Error Handling: Implement robust validation and error handling within Queries and QueryHandlers to ensure data integrity and user-friendly error responses.