Docs clarification needed for concepts-messaging/#retry-letter-topic

I find the Retry letter topic docs confusing. Here are some questions:

• Is it really the producer’s responsibility to send the message to the retry topic? The docs say “you can configure the producer to send messages to both the business topic and the retry letter topic”. If this is accurate, should the producer send to both topics at the same time or only push to retry topic after business topic fails?
• In the code example with .retryLetterTopic, does the consumer get both business and retry messages in the same stream? Are retried messages delivered interleaved with business messages based on timestamps, or does the subscription prioritize one topic over the other?
• What is the relationship between retryLetterTopic and enableRetry? (Is retryLetterTopic ignored when enableRetry is false? Is it an error to specify a retryLetterTopic when enableRetry is false?)
• What is “retrial queue” (reconsumeLater) ? Is it different than a retry topic?
• Is reconsumeLater different from throwing an exception that causes the message to go to the retryLetterTopic?
• What is maxRedeliverCount – is “redeliver” different from “retry”? Is it the number of times it tries to redeliver immediately before putting it in the retry queue, or the number of times it gets put back into the retry queue before going to the dead letter topic?
• The sentence starting “To configure the delay time” does not actually say how or where to specify a delay time. But I assume it is a global setting on the retry topic?

[alexsapps]

I was able to answer most of the questions myself after reading the original PR that introduced the feature - 6449.

Is it really the producer’s responsibility to send the message to the retry topic? …

No. It’s the consumer’s responsibility to call reconsumeLater to effectively move the message to the retry queue upon failing to process a message.

… does the consumer get both business and retry messages in the same stream …

Yes, the consumer implementation automatically subscribes itself to the retry topic as well as the original topic. Still not sure about prioritization or interleaving.

What is the relationship between retryLetterTopic and enableRetry?

enableRetry must be turned on explicitly to enable retries. retryLetterTopic is only used for customizing the retry topic name; if not set a default retry letter topic name is used. Similarly for dead letter topic name.

What is “retrial queue” (reconsumeLater) ? Is it different than a retry topic?

Pretty sure it’s the same. Didn’t see any special “retrial queue” in the code.

Is reconsumeLater different from throwing an exception that causes the message to go to the retryLetterTopic?

Still not sure.

What is maxRedeliverCount – is “redeliver” different from “retry”? …

Redeliver and retry seem to be interchangeable here. It’s the number of times it goes into the retry queue before dead letter topic.

… configure the delay time …

reconsumeLater accepts a delay time that can be different each time it’s called.

[Huanli-Meng]

@alexsapps thanks for pointing these questions out and I will ping related engineers to see if they can clarify these issues?