Just making sure I understand the "one round trip" point. If the client has chained 3 calls together, that still requires 3 messages sent from the client to the server. Correct?

That is, the client is not packaging up all its logic and sending a single blob that describes the fully-chained logic to the server on its initial request. Right?

When I first read it, I was thinking it meant 1 client message and 1 server response. But I think "one round trip" more or less message "1 server message in response to potentially many client messages". That's a fair use of "1 RTT", but took me a moment to understand.

Just to make that distinction clear from a different angle, suppose the client were _really_ _really_ slow and it did not send the second promise message to the server until AFTER the server had computed the result for promise1. Would the server have already responded to the client with the result? That would be a way to incur multiple RTTs, albeit the application wouldn't care since it's bottlenecked by the client CPU, not the network in this case.

I realize this is unlikely. I'm just using it to elucidate the system-level guarantee for my understanding.

As always, thanks for sharing this, Kenton!

To chain three calls, the client will send three messages, yes. (At least when using the WebSocket transport. With the HTTP batch transport, the entire batch is concatenated into one HTTP request body.)

But the client can send all three messages back-to-back without waiting for any replies from the server. In terms of network communications, it's effectively the same as sending one message.

> the client is not packaging up all its logic and sending a single blob that describes the fully-chained logic to the server on its initial request. Right

See "But how do we solve arrays" part:

> > .map() is special. It does not send JavaScript code to the server, but it does send something like "code", restricted to a domain-specific, non-Turing-complete language. The "code" is a list of instructions that the server should carry out for each member of the array

My understanding is that your first read is right and your current understanding is wrong.

The client sends over separate 3 calls in one message, or one message describing some computation (run this function with the result of this function) and the server responds with one payload.