Hi, thanks for the feedback, it's nice to see that other are thinking about
CQRS in PHP :)
> ... you’re assuming that we would use DDD as the domain ..
I assumed DDD as the approach, not the "domain". The domain is the business
domain and probably any (complex enough) domain would work (we are not
talking about CRUD implementations, we are assuming that the problem CQRS
is trying to solve is not solvable by CRUD). I assumed DDD because it is
one of the nicest approaches used to solve complex domains and CQRS is one
of the DDD implementations.
2017-05-19 16:38 GMT+03:00 Rivera, John <rive...@buffalo.edu>:
> Hello!
>
> Thanks for bringing this topic up — I’m a big fan of CQRS/ES!
>
> But I’ll begin with a caveat — I do not believe it’s suitable to a PSR.
> That is my opinion, and I am definitely open to debate.
>
> Continuing with the assumption that I was convinced that it’s a worthy
> PSR… I think your interfaces are definitely excessive. Here’s my minimum
> requirements for a working CQRS application:
>
> Interface Command {}
>
> interface Query {}
>
> interface Event {}
>
> interface Mediator
> {
> public function send(Command $command): void;
> public function request(Query $query): mixed;
> public function raise(Event $event): void;
> }
>
> That’s it.
>
> You’re probably asking ‘wait a minute, what about the CommandHandler and
> EventHandler?” — we could have interfaces for each, as follows:
>
> interface CommandHandler
> {
> public function __invoke(Command $command): Event;
> }
>
> interface EventHandler
> {
> public function __invoke(Event $event): void;
> }
>
> However, I eliminated those interfaces in my application because without
> them, I can write in the concrete Command object as a type hint instead of
> being forced to do a ‘if ($command instanceof ConcreteCommand) { … }”.
> Requiring the handlers to be invokable is sufficient. Perhaps someday we’ll
> get generics in PHP…
>
> Another typical way of defining the Handler interfaces is to replace
> __invoke(…) with handle(…), but I liked the elegance of working with
> invokable classes in the Mediator, and that I could enforce an sort of
> ‘interface’ without defining one.
>
> One more thing — requiring that the Command interface has a
> getAggregateId() method is a no-no — you’re assuming that we would use DDD
> as the domain. Don’t assume this — in fact, I’m not using DDD in my
> application at all, only CQRS/ES.
>
> Your Subscriber stuff seems intriguing, however — I’ll take a closer look
> at your application to see how they work.
>
> Thanks for bringing up the topic — I hope this will lead to a illuminating
> debate!
> John
>
> On May 19, 2017, at 2:53 AM, Constantin Galbenu <xpr...@gmail.com> wrote:
>
> What do you think about introducing some standards regarding CQRS and
> Event sourcing?
>
> I have a working/in production web based CRM application that uses one of
> my micro-frameworks <https://github.com/xprt64/cqrs-es> . It uses the
> command handler style described on cqrs.nu, where command handlers are
> aggregate methods. The aggregate is identified by the ID (extracted from
> Command) and class (manually/automatically subscribed).
>
> The normal flow is this (the flow implemented by my
> DefaultCommandDispatcher):
> - the client code (i.e. a REST endpoint) creates a new Command and it
> sends the command to the CommandDispatcher
> - the CommandDispatcher identifies the aggregate class (using
> CommandSubscriber) and ID (using Command::getAggregateId) then creates a
> new aggregate instance
> - it rehydrates the aggregate from the event store (by loading all the
> prior events and applying them to the aggregate instance)
> - it calls the aggregate's command method (identified by using
> CommandSubscriber) and collects the yielded events (also it applies them
> onto the aggregate itself, one by one, at collect time - a kind of reactive
> aggregate);
> - it persists the collected events to the EventStore, FutureEventsStore
> or ScheduledCommandStore depending on the yielded message type
> - it notifies the event handlers using EventDispatcher
> - it discards the aggregate
>
> The aggregate should not inherit from anything and nothing should be
> injected into it. It should remain pure, side effect free. Any side effect
> should be observed by its yielded events.
>
> I like this style because it reduces a lot of code duplication found in
> the other style (command handler being in the Application layer) where
> almost every command handler looks the same: it loads the Events from the
> store using a Repository, it factories a new Aggregate, it rehydrates the
> Aggregate (by applying the events to the Aggregate), it calls a method on
> the aggregate, it collects the events, it persists the events and then
> notify all subscribers (event handlers). In cqrs.nu's style this entire
> algorithm can be extracted in a CommandDispatcher.
>
> So, I propose (at least) the following interfaces (in the namespace Cqrs)
>
> //events
>
> interface Event;
>
>
> interface EventDispatcher;
>
>
> interface EventSubscriber;
>
>
> interface MetadataFactory;
>
>
> interface EventStore;
>
>
> interface AggregateEventStream extends EventStream;
>
>
> interface EventStream extends \IteratorAggregate;
>
>
> interface EventStreamGroupedByCommit extends EventStream;
>
>
> //commands
>
>
> interface Command;
>
>
> interface CommandDispatcher; //can be decorated by a
> CommandDispatcherWithCommandValidation
>
>
> interface CommandSubscriber;
>
>
> interface MetadataWrapper;
>
>
> interface CommandValidatorSubscriber; //used by
> CommandDispatcherWithCommandValidation
>
>
> //scheduling
>
>
> interface FutureEventsStore
>
>
> interface ScheduledCommand extends Command, ScheduledMessage;
>
>
> interface ScheduledEvent extends ScheduledMessage
>
>
> interface ScheduledMessage extends IdentifiedMessage;
>
>
> interface IdentifiedMessage;
>
>
> interface CommandScheduler;
>
>
> interface ScheduledCommandStore;
>
>
> //read models and sagas (process managers)
>
>
> interface ReadModelInterface;
>
>
> interface SagaEventTrackerRepository;
>
>
>
> I know that there are a lot of interfaces, but in fact, in a working
> application, more interfaces are needed.
>
> In order for this to work, the minimal requirements from the code in the
> Domain layer is the Command interface (for its getAggregateId method).
> The Event interface should be used only to detect event handlers by
> reflection. So, the domain code remains (almost) pure.
>
> I have a todo <https://github.com/xprt64/todosample-cqrs-es> example on
> github if you want to learn more.
>
> I know that my question mixes the proposed standard with my implementation
> but I don't know how to start the discussion otherwise.
>
> So, what do you think?
>
>
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--
Best regards,
Constantin Galbenu,
Tel: +40728247366
Skype: xprt64
Twitter: @gicagalbenu <https://twitter.com/gicagalbenu>
Github: https://github.com/xprt64
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