[GitHub] [pulsar] michaeljmarshall commented on a change in pull request #9125: [Docs]Add documentation for consumerless subscriptions and their modes

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michaeljmarshall commented on a change in pull request #9125:
URL: https://github.com/apache/pulsar/pull/9125#discussion_r552823511



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File path: site2/docs/concepts-messaging.md
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@@ -242,6 +242,12 @@ A subscription is a named configuration rule that determines how messages are de
 > * If you want to achieve "message queuing" among consumers, share the same subscription name among multiple consumers(shared, failover, key_shared).
 > * If you want to achieve both effects simultaneously, combine exclusive subscription mode with other subscription modes for consumers.
 
+### Consumerless Subscriptions and Their Corresponding Modes
+If, at any point in time, there are no consumers for a given subscription, a subscription has no specified mode. This
+includes new subscriptions and existing subscriptions that no longer have any consumers. As a consequence, there is no
+option to specify the subscription mode when creating a subscription. The mode is established when the consumer makes its
+connection, and the mode can be "changed" by restarting all consumers with a different configuration.

Review comment:
       I'd like to include something that explicitly mentions the following
   
   > if there are no consumers, a subscription has no specified mode.
   
   Based on my reading of your change, it's implicit that a subscription's mode is undefined when there are no consumers, but I'd like it to be explicit. I found the subscription mode's mutability less than intuitive when first trying to understand them. What do you think about the following:
   
   > When a subscription has no consumers, its subscription mode is undefined. A subscription's mode is defined when a consumer connects to the subscription, and the mode can be changed by restarting all consumers with a different configuration.




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