oznitecki opened a new issue #9352:
URL: https://github.com/apache/pulsar/issues/9352


   I would like to suggest a new subscription mode that blocks a message key 
from being delivered to consumers while it has a message that did not get ack 
or timeout yet.
   
   The use case is:
   You have many messages with different keys on the same topic. The keys are 
repeating like another update on the same issue, and when they do the order 
needs to be guaranteed. Example topic:
   Key:message
   1:hello
   2:good
   1:world
   2:bye
   And we have many consumers, still we would like that 'hello' would be 
processed successfully before 'world' and 'good' would be processed 
successfully before 'bye' but we do not care for the order between 'hello' and 
'good' as they have different keys.
   
   From the currently available subscription modes:
   Key_shared would stick all the 1 keys to one consumer which can cause 
overload of unprocessed messages with key 1 on the topic.
   Shared would not guarantee the order of messages at all (not even with the 
same key).
   


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