Long running transactions create all sorts of problems, so transactions are generally expected to be short. The actual work behind "create payment" or "create order" is generally fairly trivial - more or less insert a row in a table. There's no good reason to make the API complicated... you either "win" at concurrency or you lose, and the difference is generally sub-millisecond. The only meaningful thing you need to communicate to the client is "you're done" (for both the win and lose cases) or "you need to try again" (for the "something unexpected went wrong" case).

Complicated workflows can certainly have multiple steps, with "fetch the current status" calls in between. But somewhere near the beginning of every complicated workflow there will be a call to "create workflow" and it will need to have sort of mechanism which allows clients to call it idempotently. Otherwise you end up with multiple starts.

I've literally received duplicate products in the mail because of this kind of problem. I've also sent multiple products in the mail because services I relied on didn't offer the necessary idempotency mechanisms.

When I let myself ruminate, it irks me that we all let HTTP become the defacto "internet protocol" just because of firewalls. Because there was a cargo cult idea that HTTP is benign and so one of few ports allowed almost everywhere, we do stupid contortions to squeeze every protocol through an HTTP tunnel.

These short-sighted acts of laziness accumulate into HTTP everywhere. And of course, the firewall is nearly pointless when "everything" is going through that one hole anyway.