[java-client] ConsumerBuilder should have option for defaultSubscriptionInitialPosition

[kimcs]

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 an 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 ease-of-use than the suggested solution.

2. 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.

3. 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.

[kimcs]

I have changed my design and now rely on the pulsar client application to track positions rather than using subscriptions and acknowledgements, so I no longer need this feature for what I'm working on. Still I think this option is useful, as it allows using Pulsar subscriptions to be master of stream progress with at-least-once semantics.

[sijie]

@kimcs

ConsumerBuilder.subscriptionInitialPosition is only applied to the subscription that doesn't exist before. If there is already a subscription created, the consumer will start from where it was left. Hence the existing behavior of ConsumerBuilder.subscriptionInitialPosition already meets your requirements. Is that correct?

[tisonkun]

Closed as answered.