README

Command Query Responsibility Segregation (CQRS) is an established design pattern. This library aims to implement the plumbing needed to create CQRS commands and queries.

• Command Bus Usage
  • Creating a command
  • Creating a command handler
  • Dispatching a command
• Query Bus Usage
  • Creating a query handler
  • Dispatching a query
• Providers
  • Class provider
  • Method provider
  • Simple dependency provider
  • Create a custom provider
• Middleware
  • Logger middleware
  • Custom middleware

Command Bus Usage

The command bus allows commands with handlers to be dispatched with the result being either a Success or Failure.

By design, the command must not return any data.

Creating a command

Creating a command DTO to dispatch to the command bus requires us to extend the abstract Command class.

<?php

namespace My\Command;

use Zumba\CQRS\Command\Command;
use Zumba\CQRS\Command\WithProperties;

final class MyCommand extends Command implements WithProperties
{
    protected string $message;

    public static function fromArray(array $props): Command
    {
        $command = new static();
        $command->message = $props['message'];
        return $command;
    }
}

Creating a command handler

With the MyCommand created from the Creating a command section, we need to handle this command to perform an action.

Tip: The command handler is a great place to introduce any dependencies to be injected.

<?php

namespace My\Command;

use Zumba\CQRS\Command\Command;
use Zumba\CQRS\Command\CommandResponse;
use Zumba\CQRS\Command\Handler;
use Zumba\CQRS\CommandService;

final class MyCommandHandler implements Handler
{
    public function handle(Command $command, CommandService $commandService): CommandResponse
    {
        echo $command->message;
        return CommandResponse::fromSuccess();
    }
}

Dispatching a command

With both a command and handler for that command created, we're ready to dispatch our command to the command bus.

A default batteries-included command bus can be instanced that we can dispatch our command DTOs to the appropriate handlers.

<?php

use My\Command\MyCommand;
use Zumba\CQRS\CommandBus;
use Zumba\CQRS\Response\Success;

$commandBus = CommandBus::defaultBus();
$command = MyCommand::fromProperties([
    'message' => 'Hello!',
]);

$result = $commandBus->dispatch($command);
// Hello! is echoed
var_dump($result instanceof Success);
// true

Query Bus Usage

Similar to the command bus, the query bus operates with a query DTO with an accompanying handler dispatched to the query bus in order to get resulting data.

Creating a query

Creating a query DTO to dispatch to the query bus requires us to extend the abstract Query class.

<?php

namespace My\Query;

use Zumba\CQRS\Query\Query;
use Zumba\CQRS\Query\WithProperties;

final class MyQuery extends Query implements WithProperties
{
    protected string $ID;

    public static function fromArray(array $props): Query
    {
        $query = new static();
        $query->ID = $props['id'];
        return $query;
    }
}

Create a query handler

Query handlers are designed to return a query response depending on the kind of data to return.

In the above created MyQuery DTO, let's assume that this is retrieving a key-value result for a specific entity. We would build our response using the Map response type:

<?php

namespace My\Query;

use Zumba\CQRS\Query\Handler;
use Zumba\CQRS\Query\Query;
use Zumba\CQRS\Query\QueryResponse;

final class MyQueryHandler implements Handler
{
    public function handle(Query $query): QueryResponse
    {
        // Do appropriate lookups
        return QueryResponse::fromMap([
            'id' => 'example-id',
            'name' => 'example-name',
        ]);
    }
}

Dispatching a query

As with dispatching a command, we can use the batteries-included query bus which can be included using the QueryBusTrait.

namespace My\Query;

use Zumba\CQRS\QueryBusTrait;

class Example {
    use QueryBusTrait;

    public function query(): array
    {
        $response = $this->queryBus()->dispatch(new MyQuery([
            'id' => 'example-id',
        ]));
        return [
            'id' => $response['id'],
            'name' => $response['name'],
        ];
    }

}

var_dump((new Example)->query());
// [
//     'id' => 'example-id',
//     'name' => 'example-name',
// ]

Providers

Providers offer the ability for the command bus to be able to supply a handler for a DTO dispatched to the bus.

zumba\cqrs provides several providers out of the box.

Class Provider

The Zumba\CQRS\Provider\ClassProvider can be used to construct handlers for a DTO via a handler factory. This can be used when the handlers have complex dependencies (ie dependencies that require configuration).

To utilize this, create a handler factory in the same namespace as the DTO. Assume our DTO from the create a command section of MyCommand, the handler factory would look like this:

<?php

namespace My\Command;

use Zumba\CQRS\Command\HandlerFactory;
use Zumba\CQRS\Command\Handler;

class MyCommandHandlerFactory implements HandlerFactory
{
    public static function make(): Handler
    {
        return new MyCommandHandler();
    }
}

Method Provider

The Zumba\CQRS\Provider\MethodProvider is similar to the ClassProvider except the handler itself can serve as its own factory.

The MyCommandHandler can be adapted from the create a command handler section:

<?php

namespace My\Command;

use Zumba\CQRS\Command\Command;
use Zumba\CQRS\Command\CommandResponse;
use Zumba\CQRS\Command\Handler;
use Zumba\CQRS\Command\HandlerFactory;
use Zumba\CQRS\CommandService;

final class MyCommandHandler implements Handler, HandlerFactory
{
    public function handle(Command $command, CommandService $commandService): CommandResponse
    {
        echo $command->message;
        return CommandResponse::fromSuccess();
    }

    public static function make(): Handler
    {
        return new static();
    }
}

Simple Dependency Provider

In cases where all dependencies can be instantiated without parameters, the Zumba\CQRS\Provider\SimpleDependencyProvider can be used to construct the handler without the need of a HandlerFactory.

Create a custom provider

To create your own custom provider, implement the Zumba\CQRS\Provider interface and create the bus with the custom provider. You can also provide additional providers to fallback on if your CustomProvider can't accommodate the DTO.

<?php

use My\Provider\CustomProvider;

$bus = CommandBus::fromProviders(
    new CustomProvider(),
    new ClassProvider(),
);

Middleware

The CQRS library is equipped to allow for middleware to be attached to a command or query bus to allow for generic actions to occur for any or all DTOs.

Logger middleware

The logger middleware allows logging all DTOs that are dispatched to through a bus.

<?php

use Psr\Log\LogLevel;
use Psr\Log\NullLogger;
use Zumba\CQRS\CommandBus;
use Zumba\CQRS\MiddlewarePipeline;
use Zumba\CQRS\Middleware\Logger;

$bus = CommandBus::defaultBus();
// Substitute with a real Psr logger.
$logger = new NullLogger();

$middlewarePipeline = MiddlewarePipeline::fromMiddleware(
    Logger::fromLoggerAndLevel(
        $logger,
        LogLevel::INFO,
    )
);

$bus = $bus->withMiddleware($middlewarePipeline);

Custom middleware

Custom middleware can be included in the MiddlewarePipeline as long as it implements the Middleware interface.

The handle method will accept a DTO and a callable to which to continue the process.

See the Logger middleware as an example.