[Messenger][Mailer] Ability to choose the Message class used by Mailer::send

[Seldaek]

Description
To allow routing different emails to different messenger transports (not mailer transport), it would be good if there was a way to choose the message class used, instead of having Mailer::send hardcode it to Symfony\Component\Mailer\Messenger\SendEmailMessage.

P.S: I found the docs a little confusing as I tried setting up Mailer + Messenger together for the first time, and as both use "transport", the Mailer docs say to send to another transport you can use an X-Transport header, so I tried setting that to my messenger transport I wanted to route to, and then got an error that the transport didn't exist. Maybe the docs should be more clear there.

Example

I had to copy the entire Mailer class to do this, as there is no easy way to change the message class used, and as anyway it is final. But anyway I hope it illustrates what I am trying to get to.

<?php

namespace App;

use Symfony\Component\Mailer\Messenger\SendEmailMessage as BaseSendEmailMessage;

class SendEmailMessage extends BaseSendEmailMessage
{
}

class SendUrgentEmailMessage extends BaseSendEmailMessage
{
}

class Mailer implements MailerInterface
{
    public function send(RawMessage $message, Envelope $envelope = null): void
    {
        $this->doSend(SendEmailMessage::class, $message, $envelope);
    }

    public function sendUrgently(RawMessage $message, Envelope $envelope = null): void
    {
        $this->doSend(SendUrgentEmailMessage::class, $message, $envelope);
    }

    private function doSend(string $messageClass, RawMessage $message, Envelope $envelope = null)
    {
        if (null === $this->bus) {
            $this->transport->send($message, $envelope);

            return;
        }

        if (null !== $this->dispatcher) {
            $message = clone $message;
            $envelope = null !== $envelope ? clone $envelope : Envelope::create($message);
            $event = new MessageEvent($message, $envelope, (string) $this->transport, true);
            $this->dispatcher->dispatch($event);
        }

        $this->bus->dispatch(new $messageClass($message, $envelope));
    }
}

Now with this I am able to route urgent emails to a different messenger queue which is processed faster.

[weaverryan]

I had talked to Jordi about this. Obviously, because the message is wrapped in MessageEvent, routing different emails to different Messenger transports isn't possible. I'm not sure what Mailer can do about that, it feels like the fix will have to be in Messenger itself - #36349.

[Seldaek]

Might help if messages had an interface they can implement to tell Messenger where they should be routed? Just thinking out loud here.

[weaverryan]

I think that would be cool too - but there's no decent way (at the moment) for Mailer to communicate that over to Messenger - all the routing config is just the class => transport config sitting in the user's messenger.yaml file. Unless I'm missing something, we need a hook point over on that side.

the Mailer docs say to send to another transport you can use an X-Transport header, so I tried setting that to my messenger transport I wanted to route to,

Ah, this is referring to the *Mailer * transport - not the Messenger transport. It IS confusing in the docs - this section is right after talking about Messenger transports - it confused me just now too.

[weaverryan]

Related to #35750

[Seldaek]

Yup of course this would have to be supported in Messenger too, I'm just thinking it might allow smarter routing patterns if the code/message could route itself. It's also dangerous though for sure, it's nice to remain in control in messenger.yaml of the whole routing.

[carsonbot]

Thank you for this suggestion.
There has not been a lot of activity here for a while. Would you still like to see this feature?

[Seldaek]

Yes. I do still think this would be valuable to be able to send transactional emails ASAP, while background emails could be sent at a slower pace.

It also seems like it highlight a design flaw in the Messenger/Mailer combo somehow given how hard it is to achieve, so it's probably worth investigating what could be done. I unfortunately don't have enough knowledge about Messenger internals to say.