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It depends on the subscription type you are using. Each behaves differently in this scenario

What about the exclusive subscription type?

Using the exclusive subscription type, this scenario could happen. You might want to consider using the key-shared subscription type, which does handle this scenario in a seamless manner by detecting the addition of a new partition, halting the consumption of messages and reassigning the partitions to different consumers in the group.

Interesting. I thought that only happened when consumers are added!

In general, the addition of a partition doesn’t impact key assignment to the consumers unless the producers are using the round-robin mapping strategy. Do you know which strategy your producers are using?

Do you mean the MessageRoutingMode? It's using the RoundRobinPartition policy, but I did not think there's a difference between RoundRobinPartition or SinglePartition if a partition key is specified.