Key Shared Subscription type guarantees a key will be processed by a single consumer at any given time. When a new consumer is connected, some keys will change their mapping from existing consumers to the new consumer. Once the connection has been established, the broker will record the current read position and associate it with the new consumer. The read position is a marker indicating that messages have been dispatched to the consumers up to this point, and after it, no messages have been dispatched yet. The broker will start delivering messages to the new consumer only when all messages up to the read position have been acknowledged. This will guarantee that a certain key is processed by a single consumer at any given time. The trade-off is that if one of the existing consumers is stuck and no time-out was defined (acknowledging for you), the new consumer won't receive any messages until the stuck consumer resumes or gets disconnected.