[GitHub] [pulsar] kimcs opened a new issue #4928: [java-client] ConsumerBuilder should have option for defaultSubscriptionInitialPosition

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kimcs opened a new issue #4928: [java-client] ConsumerBuilder should have option for defaultSubscriptionInitialPosition
URL: https://github.com/apache/pulsar/issues/4928
 
 
   **Is your feature request related to a problem? Please describe.**
   Using the java client, I am unable to continue a subscription from where it was last acknowledged, or from the very beginning of the topic if no such ack is recorded or the subscription is new. If I provide a earliest or latest value to the subscriptionInitialPosition then that value is used instead of continuing from where the subscription was last acked, hence the need for to provide a default when subscribing.
   
   **Describe the solution you'd like**
   Add an option defaultSubscriptionInitialPosition(SubscriptionInitialPosition) to ConsumerBuilder that controls where the subscription should start reading if the subscription is new or has never received an ack.
   
   **Describe alternatives you've considered**
   1. The default setting of starting a subscription after the most-recent-message (end) of the topic could be controlled at the broker, tenant, namespace or topic level. Any of these alternatives provide less control and easy-of-use than the suggested solution.
   
   1. Provide a method to read out the last acked message-id directly from the PulsarClient, and combine this with consumer building options similar to the startMessageId and startMessageIdInclusive options of the ReaderBuilder interface. I think this option is less intuitive than having a default initial position option.
   
   1. Change the behaviour of the current ConsumerBuilder.subscriptionInitialPosition option to use the provided value as a default, but to always continue where the subscription last left-off if an ack has been made on this subscription earlier. This alternative will change the behaviour of existing clients, and is therefore not backwards-compatible, so I would not recommend it.

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