回复：[Discuss] BIP-002 Runtime Settings for Limiting Fanout Scale toPersistent Sessions

[gujiaweijoe]

Hi Yonny,

Thank you for your proposal. I have a few questions regarding the behavior when 
a persistent-related throttle is triggered:


For MaxPersistentFanout and MaxPersistentFanoutBytes — will the 
system drop entire messages, or will it drop only some recipients to stay 
within the limit? From the proposal, it seems to drop recipients. Could you 
clarify what policy will be applied?



For MaxGroupFanout — if this throttle is triggered, will the system drop 
newly subscribed recipients, or is there a different expected behavior?



Since these settings are hot-reloadable, is it possible to estimate appropriate 
values based on the cluster size?





Best regards,
Gu Jiawei (Joe)


         原始邮件
         
       
发件人：Yonny Hao <popd...@gmail.com&gt;
发件时间：2025年8月13日 17:20
收件人：dev <dev@bifromq.apache.org&gt;
主题：[Discuss] BIP-002 Runtime Settings for Limiting Fanout Scale toPersistent 
Sessions



       Hello&nbsp;all,

I’d&nbsp;like&nbsp;to&nbsp;start&nbsp;a&nbsp;discussion&nbsp;on&nbsp;*BIP-002:&nbsp;Runtime&nbsp;Settings&nbsp;for&nbsp;Limiting
Fanout&nbsp;Scale&nbsp;to&nbsp;Persistent&nbsp;Sessions(&nbsp;*
https://cwiki.apache.org/confluence/display/BIFROMQ/BIP-002%3A+Runtime+Settings+for+Limiting+Fanout+Scale+to+Persistent+Sessions
&nbsp;*)*.

This&nbsp;feature&nbsp;helps&nbsp;operators&nbsp;better&nbsp;control&nbsp;persistent-session&nbsp;fanout&nbsp;in
multi-tenant&nbsp;scenarios,&nbsp;preventing&nbsp;*DistService*&nbsp;and&nbsp;*InboxService*
hotspots&nbsp;caused&nbsp;by&nbsp;large&nbsp;persistent&nbsp;subscriber&nbsp;sets&nbsp;or&nbsp;many
shared-subscription&nbsp;groups&nbsp;with&nbsp;persistent&nbsp;members.

The&nbsp;proposal&nbsp;introduces&nbsp;three&nbsp;*tenant-level,&nbsp;runtime-configurable*&nbsp;settings:


&nbsp;&nbsp;&nbsp;-

&nbsp;&nbsp;&nbsp;*MaxPersistentFanout*&nbsp;–&nbsp;caps&nbsp;the&nbsp;number&nbsp;of&nbsp;persistent&nbsp;sessions&nbsp;per
&nbsp;&nbsp;&nbsp;message.
&nbsp;&nbsp;&nbsp;-

&nbsp;&nbsp;&nbsp;*MaxPersistentFanoutBytes*&nbsp;–&nbsp;caps&nbsp;the&nbsp;total&nbsp;bytes&nbsp;fanned&nbsp;out&nbsp;to
&nbsp;&nbsp;&nbsp;persistent&nbsp;sessions&nbsp;per&nbsp;message.
&nbsp;&nbsp;&nbsp;-

&nbsp;&nbsp;&nbsp;*MaxGroupFanout*&nbsp;–&nbsp;caps&nbsp;the&nbsp;number&nbsp;of&nbsp;shared-subscription&nbsp;groups&nbsp;per
&nbsp;&nbsp;&nbsp;topic.

I’ve&nbsp;started&nbsp;the&nbsp;implementation&nbsp;in&nbsp;my&nbsp;forked&nbsp;branch:
https://github.com/popduke/bifromq/tree/feat-max-persistent-fanout-limit

Looking&nbsp;forward&nbsp;to&nbsp;your&nbsp;thoughts&nbsp;and&nbsp;suggestions.

Best&nbsp;regards,

--&nbsp;
Yonny(Yu)&nbsp;Hao