grssam commented on PR #23398:
URL: https://github.com/apache/pulsar/pull/23398#issuecomment-2456268584
> Sure, I was suggesting to send a failure `ServerError.Throttled` to
producer as it is just a notification from server to client. and let client
receive this error during publish and do necessary backoff.
Ah got it now. So there are a few reasons for this not being a "response" to
a send request :
* `CommandSendError` command doesn't have the provision to notify the time
for which the producer is throttled. Adding that optional field just for one of
the `ServerError` enums doesn't seem right.
* Due to the distributed nature of a producer, relying on communication as a
response to a `Send` request might not work out. Suppose 10 producers connected
to a topic, one of them might not even have produced yet but the quota might
have breached.. So we need to send communication to all 10 producers to
throttle themselves.
* Throttling doesn't actually lead to failures - If the client has
reasonable timeouts (default being 30s is more than reasonable) then even with
throttling, the message produce will pass. Moreover, every producer could be
joining in with a different client sentTimeout value.. Thus, it is not possible
in the current protocol approach to manage or figure out whethar a throttling
on the broker side would actually lead to client side error at all. Thus, this
is decoupled from send response.
> Umm.. That should not be true because we can increase this version when we
want to check client's compatibility and this should be the example of this
usecase.
I suspect this is done because not every client wanted to implement a
feature present in protocol version X, but still wanted to implement something
present in version Y where X < Y.. thus this independent approach of "supported
feature flag" was introduced.
[This](https://apache-pulsar.slack.com/archives/C5ZSVEN4E/p1726845929631179) is
the slack discussion thread if you want to join in the conversation.
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