It looks great!...but we already mentioned the transaction middleware briefly in https://symfony.com/doc/current/messenger.html#using-middleware-factories where we explain middleware factories.
But having this dedicated docs really makes sense. However end-users should not have to deal with the factories themselves for this middleware as everything should be wired by the DoctrineBundle. So it could be actually simplified but doctrine/DoctrineBundle#817 isn't merged yet :'(

Users are supposed to be able to just use:

        framework:
            messenger:
                buses:
                    command_bus:
                        middleware:
                            # Using default entity manager
                            - doctrine_transaction_middleware
                            # Using "custom" entity manager
                            - doctrine_transaction_middleware: ['custom']

Oh, I've missed that entire page of documentation!

Okey, Lets wait for doctrine/DoctrineBundle#817

Now when the doctrine PR is merge, I will update these docs to the new config syntax, right?

This PR is updated.
Ping @sroze

Thank you for the review. I've updated the RP

@weaverryan

Also, correct me if I'm wrong, isn't this only relevant if you have multiple handlers that are doing flushes?

No it's relevant even with a single command bus and single handler for the message, so you don't have to deal with flushing changes yourself, nor you can commit changes partially to the database if a command handler failed at some point, rollbacking any change.

Friendly ping @Nyholm 🎶 👋

Could you please find some time and finish this PR? Thank you 🙏

I'm closing this old pull request because we already mention the doctrine_transaction in the main Messenger article (see https://symfony.com/doc/current/messenger.html#middleware-for-doctrine) so this is probably no longer needed. Thanks.